97 7.4l Irregular missfire, at my whits end

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Schurkey

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I only did the driver side bank, but I did a quick cold/dry compression test. Cylinders 1, 5, 7 showed 128-134. Cylinder 3 showed 98#. Reset and confirmed that reading a second time.
GM has changed their specs over time. At one point, "100 psi" was the lower limit for acceptable cranking compression pressure--assuming it was within 75% of the highest reading. In both cases, I "demand more"; I think GM's spec is too low, and too lax. It's set that low because they can avoid doing warranty work on marginal engines that way.

I suspect things would improve some if the engine was at operating temperature.

However, those readings--even the 130-ish ones--are lower than I'd prefer (depending on your altitude.) Around here, I dislike anything below 140; 150--155 is pretty common, and I get happier as things increase from there. I want lowest-to-highest to be within 15%; and of course more-even is better.

What is your altitude? Was the throttle at least part-way open? Did the engine crank at the normal speed?

To me it seems like that would still fire normally, be a bit down on power, or be a consistent miss if it were caused by wear or damage causing low compression - not an inconsistent miss.
Agreed.

Again, I have also not done a wet test.
I haven't done a "wet" (Oiled) test in decades. I don't believe they're valid any more. MAYBE they were valid in an era of inline engines with vertical cylinders--where you dump oil in through the plug hole, and there's some chance it'll spread around the piston. But with tilted cylinders--V8, Slant 6 or Slant 4, horizontally-opposed...the oil is going to pool on one side. Maybe piston motion stirs it up some, but it's certainly not going to be evenly spread around that top ring giving it a chemical seal.

Compression pressure increases because the oil takes up room in the combustion chamber, leading to a temporarily-higher compression ratio.
 
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Alex_m

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GM has changed their specs over time. At one point, "100 psi" was the lower limit for acceptable cranking compression pressure--assuming it was within 75% of the highest reading. In both cases, I "demand more"; I think GM's spec is too low, and too lax. It's set that low because they can avoid doing warranty work on marginal engines that way.

I suspect things would improve some if the engine was at operating temperature.

However, those readings--even the 130-ish ones--are lower than I'd prefer (depending on your altitude.) Around here, I dislike anything below 140; 150--155 is pretty common, and I get happier as things increase from there. I want lowest-to-highest to be within 15%; and of course more-even is better.

What is your altitude? Was the throttle at least part-way open? Did the engine crank at the normal speed?

We're around 1800 ft here, throttle was wide open, and I had a second battery connected to maintain consistent cranking speed.

I think in any case it would be fair to say that the engine is a little "loose in the hips". I'd agree with your assessment that 100# is a "little low", and I'm not thrilled with the results either. I know my work truck with 6.0, it lays down 165-170# across the board.
 
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Alex_m

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One other note - the ECU does not seem to detect the misfire. There has never been a misfire code set, and I have never seen the check engine light flash.

I will try to get a video to upload.
 

Schurkey

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1800 is not so high as to make your results seem adequate. Yup, it's got low compression.

The question is whether you can restore compression with chemicals; or whether an engine teardown is required. Clearly, I'd try the chemicals first.

Warm the engine, run it at ~2000 rpm. Dump a gallon of water into the throttle body as a "urine stream" so as not to overload the engine with water. If the engine misfires, stop pouring the water until it clears up. This removes the carbon from the chambers.

Engine fully warmed-up, remove spark plugs. Dump 4 oz of GM Top Engine Cleaner (TEC) into each plug hole. Put the plugs back in loosely. Turn crank by hand at least two full revolutions (more is better, but after the first two revolutions you can use the starter motor if you disable the fuel spray and the ignition) to spread the solvent around. Let it soak overnight. Turn crank at least two full revolutions by hand. Tighten plugs, replace plug wires, start engine. It's gonna smoke! When engine is warmed-up, shut it off and repeat the process of dumping TEC in the plug holes, turn the crank, and let it soak overnight.

Probably best to change oil. Gets rid of the moisture from the steam-cleaning and also removes any TEC and carbon that drizzled past the rings.

If that doesn't free-up the rings and give you better compression-test results, you'll need to perform a cylinder leakdown test to see where the compression is going--intake valve, exhaust valve, rings, or head-gasket/cracked casting.

Listen for hissing at the throttle body (throttle somewhat open) which would indicate leakage past the intake valve. Listen for hissing at the tailpipe which would indicate leakage past the exhaust valve. Look for bubbles in the radiator which indicates compression pressure getting into the water jacket via a head gasket failure, or a cracked casting.

ANY leakage past the valves is not good. ANY leakage into the cooling system is unacceptable. There WILL be leakage past the rings which you'll hear at the oil-fill opening. That's why you need the gauges on the leakdown tester to confirm whether or not the leakage is within acceptable limits.
 
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Road Trip

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One update - I was in a rush so I only did the driver side bank, but I did a quick cold/dry compression test. Cylinders 1, 5, 7 showed 128-134. Cylinder 3 showed 98#. Reset and confirmed that reading a second time. To me it seems like that would still fire normally, be a bit down on power, or be a consistent miss if it were caused by wear or damage causing low compression - not an inconsistent miss. Again, I have also not done a wet test.

Hello Alex_m,

No doubt this has been a frustrating mess. I wrote down what you've tried to date, and it's pretty comprehensive.
So we're going to have to look for something a little off the beaten path. In English, you have eliminated 7/8 of the
diagnostic pie, but the problem still persists? So let's figure out what's left, and how to sift through it all.

1) 4 cylinders on left bank compression tested. None of the cylinders are tip top. And cylinder #3 is suboptimal.

Before we get started, good on you for having an open throttle and using the 2nd battery in order to keep
the cranking speed up. (FWIW, the 3 variables that usually aren't controlled when compression test results
are shared are Elevation, closed throttle, and battery voltage dropping as the troubleshooter works their way
through all 8 cylinders.)

Even though you have only tested 4 cylinders, I would be really interested in the results of a leak-down test.
(Applying compressed air to each cylinder {at TDC on the compression stroke} via an adapter screwed into the
spark plug hole, and then listening for air hissing out the exhaust pipe, throttle body, or the crankcase.)
Note: On the 7.4, the troubleshooter will be listening to the piston ring <> cylinder wall leakage (hissing) via the oil fill tube.

Also keep in mind, piston rings *always* leak -- worn out rings a lot more than fresh, seated rings...but they
will all hiss to a certain extent.)

On the other hand, zero hissing is allowed out the tailpipe or the throttle body -- for hissing means that the
valves aren't sealing properly against their seats.

So, in order to know what we have to work with, it would help all the remote troubleshooters trying to help by
pulling all 8 plugs, perform another compression check, write down the results, and share them with us.
And if you don't possess a leak-down tester, you can rent these for free from a local auto parts store.

2) Congratulations for making the following check and sharing it with us:

Load is not necessary - it will do it in neutral.

This allows us to avoid trying to figure out if the roughness is from the engine, or a chuggling TCC, or
some other drivetrain-related engine is victim, drivetrain is perpetrator scenario.

On the assumption that the fuel delivery system is 100%, and that all of the cylinder leakdown is purely
past the piston rings, then let's take a deep dive into the Vortec Ignition system.

1) Let's start where the timing does -- the CKP sensor. I noticed that this was on the punchlist of parts
replaced. But since the Master Reference pulse that sets the timing for everything inside the VCM (including
triggering the fuel injectors and driving the coil via the Ignition Control Module) is so important, I'd ask you
to give the CKP & associated wiring a careful visual inspection.

There's been reports scattered through the forum where a CKP sensor gets too close to the spinning reluctor
underneath it to the point where there's physical rubbing. It's easy to prove/disprove that there's an issue
in this area.

2) Give the underneath of your distributor cap (and the rotor too) an extra careful visual inspection. For several
reasons, it's pretty common to see carbon tracking between adjacent spark plug terminals. And due to the
revised distributor 'crab cap' used in the Vortec design, it's pretty common to see the center terminal short out to
the path leading to the #3 cylinder's spark plug terminal:

You must be registered for see images attach


Now why would this cause only a high rpm misfire? The only informed guess would be that either
the higher PPS (pulses per second) or higher KV (needed to fire at higher engine speeds) would
cause the wounded insulation to roll over and turn into a conductor.

Disclaimer: The quickest way to be wrong in public is trying to predict what high voltage will do when
given more than 1 choice to work with, so I won't try to guess at the probability that you are suffering
from the above.

Your best bet is to personally verify that your cap didn't fail at what seems to be the Achilles heel of the
Vortec distributor cap redesign.

3) Assuming that you don't find anything wrong in step #2, then let's give some thought as to where the
rotor's exact position is when the VCM triggers the Ignition System to fire? Which spark plug terminal
is the rotor closest to? The correct spark plug terminal? Or is the distributor installed such a way that
when lots of spark advance is dialed up, now the wrong spark plug terminal presents the easiest path
for the spark to ground?

In order to answer this question quickly & easily, you need to use a scan tool to monitor the proper
synchronization of the CMP & CKP sensors. Most tools refer to this as "Cam Retard". (The factory
specs are 0°, +/- 2°, at 1000-1100 rpm.)

Note: The VCM is supposed to kick a P1345 DTC if this isn't within factory specification, but for
various reasons the test that performs this check may be precluded from running for other reasons.

Again, the best plan of attack is to check this value & verify that your distributor's housing is correctly
installed/phased to the rest of the engine. (Note that I didn't say correctly timed. In Vortec land,
all timing is straight off the front of the crankshaft snout, and the distributor just needs to be lined
up so that in a worst-case scenario, we aren't jumping the spark to the wrong terminal with lots of
spark advance dialed in by the VCM.)

By the way, how much leeway is there inside the Vortec spark plug cap for error? Surprisingly,
not that much. (See attached for a quick take on where the rotor is vis-a-vis the spark plug
terminal with the Cam Retard within spec, but the VCM has shoved the spark advance up
to the max?

You must be registered for see images attach

(Note: 3K RPM with no load will get you close to the max spark timing advance -- see attached for a 3D spark table.)


4) Steps #1, #2, & #3 all check out good. Now what?

When all else fails, check the grounds. :0)

Seriously, most folks don't realize that when the key is in either START or RUN, one side of
the injectors has a constant +12v on one side, and the injectors are actually fired by the VCM
sending them individual ground pulses on the order of milliseconds. (!)

So the higher the engine RPM, the more ground PPS (pulses per second) have to be reliably delivered
in order for the necessary current to be generated inside each injector's coil that will pull the pintle off
the seat and allow the flammable juice to flow.

Now this sounds like I'm starting to grasp at straws, but in my mind the more the ground pulses
per second the higher the duty cycle on that marginal ground. NOTE: To really prove this theory
I'd need something along the lines of a good Pico scope, and I wish they gave those away...but
unfortunately they don't.

But that's my personal challenge. All you need to do is make sure that your VCM to engine grounds
are beautious. I think they are G103 and G104. But G105 figures in there sometimes. Get yourself on
a first-name basis with these grounds, and help yourself avoid the need for a spendy Pico scope.
(Sometimes it is best to just fix it instead of study it. "Analysis Leads to Paralysis" & all that jazz. :0)

You must be registered for see images attach


****

I actually feel a little silly for asking this now, for normally this is one of my first questions, not my
last? What is a brief history of how this vehicle has run for you? In other words, have you
ever experienced smooth operation from this vehicle where it is now rough? Or did you purchase
this from the PO with this issue part of the negociated price?

If if used to run right at one time, can you remember anything distinct that happened to it right
around the time it went from good to bad?

* Truck was involved in a fender bender. Tech accidentally plunges drill into the wiring harness but
doesn't say anything & walks away?

* #1 son borrows truck over the weekend. Unbeknownst to you, he is a closet hooligan hoonigan,
and put the old soldier through it's paces. And somewhere in all the excitement the high speed
miss made it's presence known shortly afterwards?

* A recent tuneup was performed using parts sourced from Amazon. You bought from a legit
dealer with good scores, but thanks to the practice of 'binning', you actually got new stuff
from a different vendor that 'looks right' but ain't. Hint: The counterfeiters are still trying to
figure out how to get their product even close to meeting the OEM specs? ;0)

I exaggerate slightly in order to make my point, but I have personally troubleshot to each of
the 3 scenarios above. Although not probable, it's *possible* that there's something really
interesting that happened to your faithful traveling companion in the vicinity of the time that
your truck went from good to bad.

****

I know that the above is a bit of a read to slog through, but I'm a firm believer that with
(accurate) Spark, Fuel, & Compression...the engine has to run right. And since all the normal
stuff didn't clear the higher speed/no load misfire, it's time to dig in a little deeper.

(NOTE: Schurkey replied while I was weaving this tale, so no doubt he's all over that low compression
issue. I'm gonna leave in what I already wrote for what it's worth.)

Hope this proves helpful. I'm looking forward to finding out what you uncover on this difficult misfire.

Best of luck --
 

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Alex_m

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Thank you all for all of this help! I have no update at the moment and it may be several days at the least before I get a chance to get back at it, but I have printed all this information to continue reviewing and I will update back when I reach some kind of resolution, or if I gain enough information to report back for additional help. There were a couple questions posed in your responses and I will come back and answer those fairly soon as well. Thank you all!
 
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Road Trip

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Thank you all for all of this help! I have no update at the moment and it may be several days at the least before I get a chance to get back at it, but I have printed all this information to continue reviewing and I will update back when I reach some kind of resolution, or if I gain enough information to report back for additional help. Thank you all!

Perfect! We understand completely that life happens, so continuous 24x7 effort on your end
isn't expected or required. The virtual handshake is that if you ask for some technical assistance,
we do our best to respond, and then once you get a real world fix in place, that you come back
and document it for the benefit of others researching similar issues.

This way we all walk away a little more informed thanks to the shared experience.

Looking forward to your next status update. The troubleshooting theory is interesting, but
the documented real world fix is really where it's at. :0)

Best of luck with the hunt --
 
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Alex_m

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The virtual handshake is that if you ask for some technical assistance,
we do our best to respond, and then once you get a real world fix in place, that you come back
and document it for the benefit of others researching similar issues.

I've got years and years on other forums, so I understand completely. This has been one of the most helpful in a long time; I'll be back.

The question is whether you can restore compression with chemicals; or whether an engine teardown is required. Clearly, I'd try the chemicals first.

Warm the engine, run it at ~2000 rpm. Dump a gallon of water into the throttle body as a "urine stream" so as not to overload the engine with water. If the engine misfires, stop pouring the water until it clears up. This removes the carbon from the chambers.

Engine fully warmed-up, remove spark plugs. Dump 4 oz of GM Top Engine Cleaner (TEC) into each plug hole. Put the plugs back in loosely. Turn crank by hand at least two full revolutions (more is better, but after the first two revolutions you can use the starter motor if you disable the fuel spray and the ignition) to spread the solvent around. Let it soak overnight. Turn crank at least two full revolutions by hand. Tighten plugs, replace plug wires, start engine. It's gonna smoke! When engine is warmed-up, shut it off and repeat the process of dumping TEC in the plug holes, turn the crank, and let it soak overnight.

Probably best to change oil. Gets rid of the moisture from the steam-cleaning and also removes any TEC and carbon that drizzled past the rings.

I do have a minor update. I stopped on my way home from my daughters daycare. I altered the script a bit, picked up some seafoam, berrymans, had half a gallon of water. Some slightly easier to aquire and use items that I can try in the mean time. One can of Vitamin B12 in the fuel tank, partial can of seafoam in the crankcase. I thought I was slick sh** - I unplugged the MAF, popped the intake "muffler" off so I had that perfect little turn up in the intake tube, put the edge of the water cap under the throttle stop which brought it right to 2000rpm, and I started pouring away. I poured a little over half a gallon of water and one can of b12 down. Put it back together, went to pull out, and it won't take throttle.

I assumed that the air rushing into the throttle would be enough to pick the liquids up and carry them into the engine. Judging by the amount of liquid in the air cleaner housing, I was only about 30-40% correct. Hopefully I did not cook the MAF. I unhooked it again, poured out the air cleaner housing, and left the air filter out to dry before continuing home. I did grab a video when I got home of the miss. I will say, either because of the MAF being unplugged or because of the water and berrymans that did make it down the intake, the miss seems to not be happening until around 3500 now. We will know more when everything is dry and I plug the MAF back in. I will put that video at the bottom of this post.

On the assumption that the fuel delivery system is 100%, and that all of the cylinder leakdown is purely
past the piston rings, then let's take a deep dive into the Vortec Ignition system.

1) Let's start where the timing does -- the CKP sensor. I noticed that this was on the punchlist of parts
replaced. But since the Master Reference pulse that sets the timing for everything inside the VCM (including
triggering the fuel injectors and driving the coil via the Ignition Control Module) is so important, I'd ask you
to give the CKP & associated wiring a careful visual inspection.

There's been reports scattered through the forum where a CKP sensor gets too close to the spinning reluctor
underneath it to the point where there's physical rubbing. It's easy to prove/disprove that there's an issue
in this area.

I did check when I replaced the CKP - there were no rub marks, and it was dated 2019 (same as the dizzy I replaced) which was just before my buddy bought it so the PO to my buddy was also chasing this issue.

2) Give the underneath of your distributor cap (and the rotor too) an extra careful visual inspection. For several
reasons, it's pretty common to see carbon tracking between adjacent spark plug terminals. And due to the
revised distributor 'crab cap' used in the Vortec design, it's pretty common to see the center terminal short out to
the path leading to the #3 cylinder's spark plug terminal:

Now why would this cause only a high rpm misfire? The only informed guess would be that either
the higher PPS (pulses per second) or higher KV (needed to fire at higher engine speeds) would
cause the wounded insulation to roll over and turn into a conductor.

Disclaimer: The quickest way to be wrong in public is trying to predict what high voltage will do when
given more than 1 choice to work with, so I won't try to guess at the probability that you are suffering
from the above.

I'm an electrician by day, you're speaking my language.

In order to answer this question quickly & easily, you need to use a scan tool to monitor the proper
synchronization of the CMP & CKP sensors. Most tools refer to this as "Cam Retard". (The factory
specs are 0°, +/- 2°, at 1000-1100 rpm.)

Note: The VCM is supposed to kick a P1345 DTC if this isn't within factory specification, but for
various reasons the test that performs this check may be precluded from running for other reasons.

Have you got a recommendation on a cost effective scanner that has that option? I have a good scanner, but it is Land Rover specific. For other models it is just OBD2.

When all else fails, check the grounds. :0)

Seriously, most folks don't realize that when the key is in either START or RUN, one side of
the injectors has a constant +12v on one side, and the injectors are actually fired by the VCM
sending them individual ground pulses on the order of milliseconds. (!)

I did not know that!

I actually feel a little silly for asking this now, for normally this is one of my first questions, not my
last? What is a brief history of how this vehicle has run for you? In other words, have you
ever experienced smooth operation from this vehicle where it is now rough? Or did you purchase
this from the PO with this issue part of the negociated price?

I purchased this from my buddy knowing about the issue. I got a good price, but I also felt like it would be an easy solve... Hey, you win some you lose some. Verdict is still out on this one. I believe he also purchased it with the issue, but was not really aware until he got it home. He is a nice guy. Test drives stuff like it belongs to someone else.

If if used to run right at one time, can you remember anything distinct that happened to it right
around the time it went from good to bad?

* Truck was involved in a fender bender. Tech accidentally plunges drill into the wiring harness but
doesn't say anything & walks away?

So you'll forgive me for holding this piece of information a bit close to my chest - it seems like a possible smoking gun and I wanted as much diagnostic information as possible without this particular piece of information swaying the input I was getting at first. It is actually a salvage title truck. I do not know for sure the nature of the damage that caused this. There is some body damage to the driver rear door and front of the rear wheel well that easily could have totaled the truck, assuming it was not totaled before this damage occurred. In truth I do not know. I will say, it does not appear to have had any parts replaced due to collision anywhere in the front of the truck. No ill fitted parts, bad body lines, no parts that have been marked up with a paint marker at a scrap yard. Other than the damage to the door and wheel arch it is a pretty sharp rust free example. Rare around here (salt belt).

Ah! One other piece that I only just remembered - when I very first got the truck from my buddy, maybe three days after, it quit on me and would not start back up. I found a blown ECU fuse. I went tracing wires, and I found a spot where the fiberglass sleeve on the egr tube had slid back and the wire loom was broken allowing a couple of wires to contact the egr tube, melt the insulation, and short out. I peeled the harness apart there, repaired the damage, and fixed it so the same would not happen again. Unfortunately instead of completely solving all issues, it simply returned it to the same state of operation that I bought it in. This was also prior to my damaging of and subsequent replacement of the ECM.

* A recent tuneup was performed using parts sourced from Amazon. You bought from a legit
dealer with good scores, but thanks to the practice of 'binning', you actually got new stuff
from a different vendor that 'looks right' but ain't. Hint: The counterfeiters are still trying to
figure out how to get their product even close to meeting the OEM specs? ;0)

Unrelated, but this cost me a 6.0 in my work truck about a year and a half ago. I purchased what were supposed to be genuine AC Delco injectors. They looked the part and I installed them. One of them stuck open after shut off and filled a cylinder with fuel. I went to restart the truck not knowing and it bent a rod. When comparing the new injector to a known genuine, it was in fact very slightly different - a fake. The seller did at least refund the cost of the injectors.... That was a $3000 and multiple day mistake at a time when I did not have a backup truck and I was drowning in work I had to get to. Thankfully another buddy traded me an '89 Dodge W150 for some electrical work and parts that got me by while I was getting my truck back together. Anyway, unrelated but there's my anecdote.

Here is the video. You can watch the tach needed jump around when it is missing. Also very interesting and I just noticed, the speedometer needle also comes up a little in the high RPMs. Odd.

xc_hide_links_from_guests_guests_error_hide_media
 
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Scooterwrench

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I've got years and years on other forums, so I understand completely. This has been one of the most helpful in a long time; I'll be back.



I do have a minor update. I stopped on my way home from my daughters daycare. I altered the script a bit, picked up some seafoam, berrymans, had half a gallon of water. Some slightly easier to aquire and use items that I can try in the mean time. One can of Vitamin B12 in the fuel tank, partial can of seafoam in the crankcase. I thought I was slick sh** - I unplugged the MAF, popped the intake "muffler" off so I had that perfect little turn up in the intake tube, put the edge of the water cap under the throttle stop which brought it right to 2000rpm, and I started pouring away. I poured a little over half a gallon of water and one can of b12 down. Put it back together, went to pull out, and it won't take throttle.

I assumed that the air rushing into the throttle would be enough to pick the liquids up and carry them into the engine. Judging by the amount of liquid in the air cleaner housing, I was only about 30-40% correct. Hopefully I did not cook the MAF. I unhooked it again, poured out the air cleaner housing, and left the air filter out to dry before continuing home. I did grab a video when I got home of the miss. I will say, either because of the MAF being unplugged or because of the water and berrymans that did make it down the intake, the miss seems to not be happening until around 3500 now. We will know more when everything is dry and I plug the MAF back in. I will put that video at the bottom of this post.



I did check when I replaced the CKP - there were no rub marks, and it was dated 2019 (same as the dizzy I replaced) which was just before my buddy bought it so the PO to my buddy was also chasing this issue.



I'm an electrician by day, you're speaking my language.



Have you got a recommendation on a cost effective scanner that has that option? I have a good scanner, but it is Land Rover specific. For other models it is just OBD2.



I did not know that!



I purchased this from my buddy knowing about the issue. I got a good price, but I also felt like it would be an easy solve... Hey, you win some you lose some. Verdict is still out on this one. I believe he also purchased it with the issue, but was not really aware until he got it home. He is a nice guy. Test drives stuff like it belongs to someone else.



So you'll forgive me for holding this piece of information a bit close to my chest - it seems like a possible smoking gun and I wanted as much diagnostic information as possible without this particular piece of information swaying the input I was getting at first. It is actually a salvage title truck. I do not know for sure the nature of the damage that caused this. There is some body damage to the driver rear door and front of the rear wheel well that easily could have totaled the truck, assuming it was not totaled before this damage occurred. In truth I do not know. I will say, it does not appear to have had any parts replaced due to collision anywhere in the front of the truck. No ill fitted parts, bad body lines, no parts that have been marked up with a paint marker at a scrap yard. Other than the damage to the door and wheel arch it is a pretty sharp rust free example. Rare around here (salt belt).

Ah! One other piece that I only just remembered - when I very first got the truck from my buddy, maybe three days after, it quit on me and would not start back up. I found a blown ECU fuse. I went tracing wires, and I found a spot where the fiberglass sleeve on the egr tube had slid back and the wire loom was broken allowing a couple of wires to contact the egr tube, melt the insulation, and short out. I peeled the harness apart there, repaired the damage, and fixed it so the same would not happen again. Unfortunately instead of completely solving all issues, it simply returned it to the same state of operation that I bought it in. This was also prior to my damaging of and subsequent replacement of the ECM.



Unrelated, but this cost me a 6.0 in my work truck about a year and a half ago. I purchased what were supposed to be genuine AC Delco injectors. They looked the part and I installed them. One of them stuck open after shut off and filled a cylinder with fuel. I went to restart the truck not knowing and it bent a rod. When comparing the new injector to a known genuine, it was in fact very slightly different - a fake. The seller did at least refund the cost of the injectors.... That was a $3000 and multiple day mistake at a time when I did not have a backup truck and I was drowning in work I had to get to. Thankfully another buddy traded me an '89 Dodge W150 for some electrical work and parts that got me by while I was getting my truck back together. Anyway, unrelated but there's my anecdote.

Here is the video. You can watch the tach needed jump around when it is missing. Also very interesting and I just noticed, the speedometer needle also comes up a little in the high RPMs. Odd.

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That tach jumping leads me to believe the ICM is bad. O'Reilly's lifetime guaranteed parts are still elcheapo chinese crap. You're better off going to a salvage yard and pulling a used factory ICM.
For example I installed a complete NAPA dist. in my 305 and when I was datalogging I noticed my tach line was fuzzy like there were little oscillations in the tach signal. The truck ran fine but me having a bad case of OCD wasn't gonna have it. I pulled the factory ICM out of my old dist. and installed it in the new dist. and the tach signal cleaned right up. I also gained a mile to the gallon.
 

docstoy

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Read this thread a couple of times. So has a compression ck been done on all cylinders ? Appears you have looked over ignition and fuel system a couple of times. So #3 is the suspect cylinder because of low compression ? From what I've read missfire can happen with no load in neutral ? Because of difference in pressure being different on #3 I would suspect a valve train problem, but you won't know until leakdown test is done . Reason being your just going by tach jumping and it has a miss. Probably a stupid question, does your scanner have missfire data ? If it doesn't, do you have access to an oscilloscope or have a friend that has a scanner with missfire data to confirm that your missfire is electrical or fuel related ? It seems your self employed so I know your more than busy plus trying to fix a truck that's had at least 2 owners that passed it on , also been there done that. Best of luck and let us know and welcome to forum .
 
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