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 --