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.)

EDIT: This has happened to me so often that I now take each part out of the box and look for 'witness marks'
showing evidence of previous installation? If I see these telltale marks, then I ask for a different one...especially
if the parts store is a long distance from home base.

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 vacuum opening in your
new EGR valve matches your old one. (There are 4 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 wires, coil, fuel filter, O2 sensor(s), etc., are all good and the driveability issue persists,
then here we have to stop and gird for battle.

Simply put, in order to fix from this point on 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 in the direction
of the highest probability 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 rough 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 additional 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 previously 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 points 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 a fine spray of carbon
molecules is a better conductor than straight, dry 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 to the correct destination without detours.

You must be registered for see images attach

(credit: Nice article where DENSO lays out all the variables that affect the voltage it takes to jump a spark plug gap. (DENSO tech support article)

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
error during Closed Loop operation 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:

You must be registered for see images attach

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 conventional wires relying on high resistance vs inductance to control RFI.

This is one case where installing better spark plug wires than original is simply money well spent. (I'm pretty frugal, and yet I cheerfully buy the
best wires I can find.) Note: Both designs work fine brand new, but each time the high resistance wires get disturbed I find their
resistance changes. Meanwhile, the wire-wound spark plug wires work the same as Day 1 for much longer periods of time, even if you have a
busy engine bay where they are being touched/moved all the time. YMMV, but I've found that once I stop & put in a set
of wire-wound spark plug wires they go in, stay in, and don't draw any negative attention to themselves. (Unless of course one or more
of the wires gets incinerated on the exhaust, but that goes back to proper routing.)

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 --

 

[name]

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:

When I am reading the fine print at the bottom of the NKG ad you posted, it talks about having " 1/2 the resistance of traditional carbon core wires "

Are the " wire wound " wires the same as " Low Ohm " wires ? or are the Low Ohm wires something else and the wire wound wires an even better upgrade from them ? I ask because the wires I had on there before from Summit Racing are marked as " 8mm Low Ohm " ( when I did not have this miss ) witch would mean you are on the right path and I will likely return to the Summit Racing wires that I had before witch I believe are made by Packard wire

How much would one expect to pay for "wire wound" wires and can you get them in a kit where you can cut your own length ?

 

[Road Trip]

When I am reading the fine print at the bottom of the NKG ad you posted, it talks about having " 1/2 the resistance of traditional carbon core wires "

Are the " wire wound " wires the same as " Low Ohm " wires ? or are the Low Ohm wires something else and the wire wound wires an even better upgrade from them ? I ask because the wires I had on there before from Summit Racing are marked as " 8mm Low Ohm " ( when I did not have this miss ) witch would mean you are on the right path and I will likely return to the Summit Racing wires that I had before witch I believe are made by Packard wire

How much would one expect to pay for "wire wound" wires and can you get them in a kit where you can cut your own length ?

Hello name,

The differences in the resistance values of various new spark plug wires isn't something to be overly
concerned about since we're using such high voltages to drive them. For me, the primary
attraction of the wire-wound spark plug wires is that they remain the same 1,2...5+ years
later, while the old design 'carbon core' resistance cables seem to change their resistance
with every pull, touch, or rerouting. In other words, the carbon core wires keep gaining
resistance as time passes, and eventually the resulting ever higher voltages needed to
keep firing the plugs causes the pulses to find new/easier paths to ground. (Like through
the side of the coil, arcing inside the cap, etc.

Wire-wound wires are just more robust, yet you still get to have good radio reception.
(And you also aren't subjecting your engine bay's ECU to excess RFI.)

And if the Summit Racing wires that you previously had good luck with are wire-wound, then it
seems to me that returning there would be a safe bet.

And to answer your last question, Summit was offering several kits that looked like this:

You must be registered for see images attach

Cut to exact length, dressed like the FSM illustrations, replace the missing wire looms,
this would be a marine offshore racing quality kind of installation.

If you get to implement this, make sure and take a photo of your finished work so
that others can get an idea of what they should be striving for.

 

[Schurkey]

Summit has "house brand" wire sets that are helical-wound, cut-to-fit, and available in multiple colors and spark-plug-boot angles. Advertised as "Made in USA". My plug-wire of choice for the last decade or so.

Red, 90 degree plug boots example:
www.summitracing.com/parts/sum-860825r

 

[name]

Summit has "house brand" wire sets that are helical-wound, cut-to-fit, and available in multiple colors and spark-plug-boot angles. Advertised as "Made in USA". My plug-wire of choice for the last decade or so.

Red, 90 degree plug boots example:
www.summitracing.com/parts/sum-860825r

Schurkey :

SUM-868858 is the Summit wires I used last time, Looking at the speck sheets they appear to be made of the same material as the ones you posted. Looks like the only thing different between the two is the color and the cut to lenght option.

Summit Racing SUM-868858 Summit Racing™ 8mm Ignition Wires | Summit Racing

Free Shipping - Summit Racing™ 8mm Ignition Wires with qualifying orders of $109. Shop Spark Plug Wire Sets at Summit Racing.

www.summitracing.com

* For those following this thread, the wires I referenced from summit are pre-made and worked out well for me, my only complaint with them is that they seem to be a couple of inches longer than needed ( probably used for multiple applications )

I happen to live a couple of hrs. from a Summit store and warehouse , I will try to get over there this weekend and pick them up. I will get back to you guys when I have something to report...

 

[Schurkey]

Plug wires are goofy.

"Custom Fit" plug wire sets are always Too Damn Long, so they "fit" as many vehicles as possible.

"Universal Fit" plug wire sets get cut to the exact length needed/desired so they're an exactly-perfect fit for the vehicle in question.