1994 k1500 wild misfire

[Schurkey]

The only thing I have not replaced is the Coil but It did check out OK with an volt meter test.

VOLTmeter? Sure you don't mean "Ohmmeter"?

Are these coils noted for creating a miss when they warm up?

ANY (defective) coil can miss when hot.

IF (big IF) the ignition coil passes the ohmmeter tests--primary winding resistance, secondary winding resistance, and both primary and secondary to ground--use a spark-tester calibrated for HEI to verify spark power. The spark tester testing is best done with the coil fully warmed-up, and then misted with water from a squirt-bottle.

There are several brands and styles of spark tester. A decent spark tester will have an actual spark gap NOT A DAMNED LIGHT-BULB.

My favorite style:
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

 

[name]

VOLTmeter? Sure you don't mean "Ohmmeter"?


ANY (defective) coil can miss when hot.

Yes, I mean Ohmmeter.

looks like I got 80K on the fuel filter, I have read somewhere that this may cause a random misfire so I will change that out next.

 

[Road Trip]

Here are the pictures of my current set up. Witch runs good when cold but the miss still comes back when it comes up to running temp.

This is good troubleshooting info. Many times knowing all the ways that an engine will work correctly can help us eliminate some
of the possibilities of what is causing the engine to misbehave at specific times. By the way, I fully believe that an engine can
run well cold but misfire when up to temp. More on this in a minute.

But I do have a question for you. In your first reply (#4) you mentioned that you have a misfire "after installing the Autozone tune up parts".
The following makes a big difference in our troubleshooting. Which of the 2 categories below does your scenario fit into:

A) The engine was running fine prior to the parts swap, but started misfiring after installing the Autozone parts?
-or-
B) The engine was misfiring prior to the parts swap, and after the Autozone parts were installed they didn't improve the misfire?

(Unfortunately, as many in here can attest both scenarios are quite possible these days.)

looks like I got 80K on the fuel filter, I have read somewhere that this may cause a random misfire so I will change that out next.

This is straight up a good preventive maintenance practice -- a win no matter if it fixes your random misfire or not.
I concur with changing that fuel filter asap.

New Dizzy and ICM, New Cap, New Rotor, New Wires, New plugs, New EGR valve, New IAC valve, it is not throwing any codes.

The only thing I have not replaced is the Coil but It did check out OK with an volt meter test. Are these coils noted for creating a miss when they warm up ?

Schurkey's guidance on the coil is spot on. Anything high-voltage (especially vintage high voltage) is pretty much guilty
until proven innocent. Conversely, it's never been easier to buy brand new, out of spec problematic parts that are not as good as the originals.
Check your coil carefully per Schurkey's recommendations, and IF you decide to change the coil do yourself a favor and don't buy the
stuff that's literally the 'too good to be true' price. (Especially if the ebay & Amazon parts also happen to be available on Alibaba. (!)

****

OK, now to further discuss the 'good cold, misfiring hot' thing.

IF the engine was running fine prior to swapping the Autozone parts in, then the highest probability is that
you received one of more parts that was bad out of the box. (Sometimes bad QA by the parts vendor, other times
a part is returned by a previous customer and they unknowingly wounded it, sometimes invisibly.)

But if your engine was running this way before the parts were changed and still no joy, then let's work through
what your engine uses cold, and what gets added to the mix once everything is up to temp.

Since your '94 is a TBI, with the exception of the EGR valve everything you have listed above is involved
and working as advertised when cold. Once the engine coolant temperature approaches normal operating
temps, the EGR valve will start operating under computer control. IF you have the exact right EGR valve,
then adding this into the mix shouldn't adversely affect the driveability. But as with everything else, a lot
of incorrectly-spec'd parts are being sold these days. Be sure to verify that the opening in your new EGR
valve matches your old one. (There are 4 really good photos shared by a forum member that really
clears this up. EGR valves calibrated for smooth OEM-style transient operation.)

****

OK, if you are still reading this then I'm going to assume that you have verified that you have good fuel pressure
(post fuel filter change) a good coil, and have verified that your new EGR valve meets the OEM spec? Then
it's time to talk about Open Loop vs Closed Loop operation.

Open Loop is how the GMT400 engines run A) when the O2 sensor(s) is not up to temp, or B) during
WOT operation. IF the engine operates smoothly cold, and then after 2-3 minutes from a cold start
it's like somebody threw a switch and the rough running starts, then it could be that the O2 sensor
got switched in, and now instead of the fuel injector duty cycle being set by the factory tables, now
the fueling is being 'fine tuned' by the feedback being reported by the O2 sensor(s).

With a healthy O2 sensor going from Open Loop to Closed Loop operation is the equivalent of
going from fairly precise to really precise operation. Obviously, if your O2 sensor is old, tired, lazy, etc.,
this is where a marginal O2 sensor takes a smooth-running engine and messes up the drivability.
Garbage in from the O2 sensor = PCM (ECU) garbage fueling calculations.

Since they live/work in such a harsh environment, O2 sensors are just like tires & spark plugs -- they
are a wear item. NOTE: If your O2 sensors have been recently changed, verify that they are one
of the brands that plays nice with the TBI computers. Some do, many don't.
(And I mention the O2 sensor because it's not on your changed parts list.)

****

And IF your coil, fuel filter, and O2 sensor(s) are all good, then here we have to stop and gird for battle.

Simply put, in order to fix this we need the computer's perspective on what's going on. Schurkey and
others who've been around the block are going to be asking for live data, O2 crossover counts, short and
long term fuel trims, misfire counts by cylinder, etc. This *is* fixable, we just need to be able for the
computer to show us the data, and then we follow that data to guide us to the most probable fix.

One last thing. Even in the pre-computer, pre-feedback loop days, every once in a great while we'd
have to troubleshoot an engine that ran fine cold, but not when fully up to temp. (Not nearly as often
as when the engine ran poorly cold, but fine when up to temp.)

If after ensuring that the spark & fuel systems are 100%, then we'd have no choice but to venture down
the path of a possible mechanical issue. (Think valves not always seating 100% of the time as they rotate
when opened and bad spots mesh, etc.)

But not to worry about that at this time. Please let us know if your problem commenced with the Autozone
parts installation, or was the problem already showing, and the Autozone parts just didn't make a difference?

Apologies for the length, but there's a lot of moving parts in this misfire scenario, so I thought a good
overview might be helpful.

Best of luck. Let us know what you find.

 

[name]

But I do have a question for you. In your first reply (#4) you mentioned that you have a misfire "after installing the Autozone tune up parts".
The following makes a big difference in our troubleshooting. Which of the 2 categories below does your scenario fit into:

The engine was not misfiring before installing the Autozone cap, rotor, wires and plugs , But it was throwing a SES lite and a 32 code before the Autozone parts. It was because of the code 32 and general lack of power that I initiated the tune up. The instalation of the EGR valve took care of the code 32 and SES lite ( Service Engine Soon )

This is straight up a good preventive maintenance practice -- a win no matter if it fixes your random misfire or not.
I concur with changing that fuel filter asap.

The Fuel Filter was in bad shape, I could barley blow threw it and when the new one was installed it really made a big difference in the acceleration and power output of the engine, Truth be told, I am now a firm believer in changing these out more frequently ( 30K like the book suggests ) The Fuel Filter I used was the K&N because it fit the mounting bracket on the truck, The Duralast Fuel Filter appeared to be slightly bigger.

That being said, the Fuel Filter had no effect on the misfire itself.

IF the engine was running fine prior to swapping the Autozone parts in, then the highest probability is that
you received one of more parts that was bad out of the box. (Sometimes bad QA by the parts vendor, other times
a part is returned by a previous customer and they unknowingly wounded it, sometimes invisibly.)

The further I get into this, The more I believe this to be the case. The EGR valve replacement did solve the SES and code 32 issue, and the Fuel Filter replacement did solve the lack of power issue. Had I changed these 2 parts out first I probably would of never changed the cap, rotor and wires as they were not due on the maintaince schedual at 50K miles, ( The Plugs were due at 30K and I figured I might as well do everything at once, especially when I tore a couple of plug boots when replacing the plugs )

Another thing to note is there are some bad reviues online conserning Duralast wire sets, ( FWIW ) and wrapping the coil wire in plastic wire loom and re routing it so it was not laying on the #1 plug wire directly did make the condition better, but did not eliminate it.

But if it is the wires, why would the issue get worse when warm ? Wouldn't bad wires crossfire when cold also ?


Other things to note from the above posts :

1) I know the o2 sensors have not been changed from 80K threw 160K miles . ( and would suspect the original ones are still in the truck )

2) I have not yet verified the coil outside of the ohmmeter test because I have not net sourced a spark tester, But I do know it is not OEM and was changed for whatever reason before the truck hit 80k miles.

 

[Road Trip]

The engine was not misfiring before installing the Autozone cap, rotor, wires and plugs , But it was throwing a SES lite and a 32 code before the Autozone parts. It was because of the code 32 and general lack of power that I initiated the tune up. The instalation of the EGR valve took care of the code 32 and SES lite ( Service Engine Soon )

I appreciate the very complete status update. Glad to hear that the new fuel filter and EGR valve took care of the issues that they did.
And most importantly, you took the time to feed this info back to the forum as another real world data points for others to learn from.

The short answer is that the older our trucks get, the more load they put on the fuel filters, for if you look carefully inside a *lot* of gas tanks
it's like spelunking in the Carlsbad Caverns. (especially ones where the vehicle was sidelined for a long period of time) I have personally troubleshot
cars that were having fueling issues to factory-size fuel filters getting plugged up in as little as 30 minutes of running. And after 3 or 4 quick filter
changes with no improvement a new gas tank solves the problem once and for all.

Given this, I'd recommend that you carry a fresh spare fuel filter in the glove box, and at the first sign of any future loss of power be ready to swap it out. Don't know
the history of your truck, but I am no longer surprised when a 30 year old vehicle needs a fuel filter swap way more often than when it was a 3 year old vehicle. (!)

And bonus for getting the correct new EGR valve on the first try.

The further I get into this, The more I believe this to be the case. The EGR valve replacement did solve the SES and code 32 issue, and the Fuel Filter replacement did solve the lack of power issue. Had I changed these 2 parts out first I probably would of never changed the cap, rotor and wires as they were not due on the maintaince schedual at 50K miles, ( The Plugs were due at 30K and I figured I might as well do everything at once, especially when I tore a couple of plug boots when replacing the plugs )

Another thing to note is there are some bad reviues online conserning Duralast wire sets, ( FWIW ) and wrapping the coil wire in plastic wire loom and re routing it so it was not laying on the #1 plug wire directly did make the condition better, but did not eliminate it.

This may seem a little bit off the wall, but let's assume that your original wires had enough 'headroom'
that no matter how hard or easy it was to fire the plugs they could do the job. Now the new Duralast wires
get installed, and they are barely capable of doing the job when it's easiest to fire your plugs, and are
pushed into failure when your engine is running leaner = higher KV = substandard wires let us down when going
gets hot. (This falls into the general theory that NEW = Never, Ever, Worked. Not always, but it's something to keep
in the back of your troubleshooting mind.) More on the wires in a minute.

But if it is the wires, why would the issue get worse when warm ? Wouldn't bad wires crossfire when cold also ?

As I mentioned above, the KV (kiloVolts) required to jump the gap is higher when the mixture is lean vs
less voltage to jump the gap when the mixture is rich. (Our fuel is carbon based, and carbon is a better conductor
than straight atmosphere.)

Given our discussion of the state of your O2 sensor, I could theorize that when your engine is cold and you are
running purely off of the internal fueling tables (including extra richness to cover cold engine running) then even with
the marginal new wires they can still get the lower KV job done. But when the O2 sensors are warm enough to come
online and put your engine into Closed Loop mode, then let's say that the O2 sensor feedback is out of calibration,
misguiding the PCM (ECU) to run the engine too lean, and now this is just enough to force the new spark wires past their
capability to deliver the sparks as necessary.

Check this out. If you still had your old wires then we might be able to swap them back in & cover up this out of calibration
Closed Loop error for another nn,nnn miles. Or, it's *possible* that fresh O2 sensors make it just easier enough to
allow the new (suspected substandard) spark plug wires to work alright hot or cold.

Now if you're of a scientific bent, we could prove/disprove this theory by looking at fuel trims, misfire data by cylinder, etc.
All you need is a scan tool that delivers accurate real time data.

However, Plan B would be a 2-pronged approach, and since O2 sensors are like tires or air filters (ie: a wear item) then
this is not considered firing the parts cannon:

1) Return those new spark plugs and get a refund. Explain to them that the engine was running smooth on the old wires, but
now runs rough with the Duralast jobs. My recommendation would be to pick a set of wires from your favorite vendor
that are built with these features:

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Note: I have never been disappointed by a set of these wires installed in accordance with the Factory Service Manual. Others will sing the praises
of wire-wound AC Delco wires. I don't get too hung up on the brand, but in all cases I've had much better luck/longevity/smooth running with
wire-wound spark plug wires vs the older/more affordable high (variable) resistance conventional wires. This is one case where installing
better spark plug wires than original is simply money well spent. (I'm pretty frugal, and I cheerfully buy the best wires I can find.)

OK, assuming you found/installed a good set of wire wound spark plug wires, go ahead and also renew your O2 sensor(s).
Even without O2 sensors codes being thrown, folks are reporting better gas mileage AND driveability after retiring the
old O2 soldiers.

The fact that you were savvy enough to clear the Code 32 and also renew the fuel filter are big steps in the right direction.
And if you return the wires that caused the truck to run worse post installation, upgrade to some good wire-wound jobs
plus get new O2 sensor(s) to accurately help control your hot Closed Loop mode operation, then we should be able to get
your engine running smooth in both Hot & Cold engine scenarios.

Let us know what you discover. This will help other GMT400 owners working through similar issues.

PS: Fortunately you are running a conventional (TBI) distributor cap. I know that you have a new cap, but
be sure to check the new cap & rotor for any signs of High Voltage shenanigans while you are swapping the
wires out. So many people have had new/nearly new caps & rotors fail shortly after installation.

Best of luck --