02 tahoe misfire - ran out of ideas

[hdrox88]

My cousin just recently acquired a 2002 Tahoe 5.3 with about 240k miles on it. SES light was on and was showing P0300 random/multiple misfire. I cleared the code and started the engine to see if/when it came back on. After a few minutes at idle the SES came on flashing. I shut it down and checked the code. This time it was P0304, cylinder 4 misfire. The spark plugs looked like they were replaced recently and all looked the same. We swapped the cylinder 4 spark plug, coil pack, coil-plug lead wire, and injector with the other know firing cylinders. Clearing the code and restarting after each swap showed the P0304 code each time. We decided to do a compression check next. Cylinder 4 was 175 psi while cylinder 2 was 180 psi and cylinder 1 was 175 psi. Wondering if it was a possible intake leak, I sprayed some carb cleaner around the #4 intake tract but the idle didn't change at all. I might mention that the cats have been removed by the previous owner. I figured that was the cause for the P0300 code, but when it came to just a single cylinder it didn't make sense anymore.

I did some searching around here and found people solving their issues with injectors and coil packs, but that doesn't seem to be the problem here. At this point I am out of ideas and any help would be appreciated.

 

[livingez_123]

as long as your within 10% of the other cylinders you should be good. I would double check if you had 2 cylinders that are side by side and the exact same psi.
did you change out the coil pack and plug and wire at the same time or did you do it one at a time?
make sure your injector plug has all the pins properly seated, maybe one could have pushed out and not making contact inside the plug.

 

[hdrox88]

I tested the problem cylinder psi and got 175, the cylinder just in front was 180 and then I checked a cylinder on the opposite bank and got 176. The parts were swapped one at a time, checking the code each time. I have a OBD2 scanner. I'll have to double check the pins in the connector, but when I probed with a test light to check for injector signal pulse they seemed to be in proper position.

 

[electro]

Measure resistance coil packs?

Re gap plugs to .060 or something like that I can't remember the exact spec.

Also measure resistance of plug wires. Could be cracked.

If you can't find exact ohm numbers. Just compare amongst all other coils or wires.

 

[hdrox88]

I compared the resistance on the coil packs and the plug wires and there was minimal difference. I figured that swapping the parts I mentioned from the non firing cylinder with a firing cylinder would tell me what parts were bad, but the misfire remains on cylinder 4.

 

[electro]

How you doing? Check the injectors. Pull the electrical connectors off the injectors and measure resistance of the injector. I can't remember the spec. But you get the Idea.


Sent from my iPhone using Tapatalk

 

[hdrox88]

The truck has been sitting for the past few months and I decided to try to give it another go. Ok, still have the same problem with this thing. I swapped plug, wire, coil pack and injector again to another known firing cylinder and still seems to be no 4 that is missing. Pulling the connector off the no.4 injector while engine is idling makes no difference to the idle. I used a test light to make sure that the injector was receiving a signal. Now my only other idea is to check for a signal going to the coil pack. My question is, how do I do that with out shocking myself. There is 4 different pins and I am not sure how to go about it. Any help again is appreciated.

Edit: I did a little more research and figured out how to test the signal to the coil pack. Everything seems to check out there as well. It now seems that it is a mechanical problem as I feel that I have ruled out all electrical possibilities. With a compression test that seems to check out just fine, what could be going wrong here?

 

[Duramax05blk]

Are Those the only codes? Could still be electrical, where is cylinder four getting its data to determine when to fire. That's where I would go next.


Sent from my iPhone using Tapatalk. Excuse my grammar the iPhone makes me iStupid!

 

[FreshLikeOprah]

I've ran into this a few times. Clogged cats will cause it.

 

[hdrox88]

It is currently throwing P0300 random/multiple misfire. I just got it back from a trusted repair shop for bad intake seals determined with a smoke test. It got better, but not 100%. They also did a top engine clean to remove carbon build-up but it made no difference. It still has a #4 misfire at idle only. They checked compression and leakdown, results were good. They checked waveform from the crank sensor, results were good. Tested signal duration to the coil, as well as injector pulse width compared to other cylinders and results were inconclusive. They feel that they have ruled out any possibility of an electrical issue and think it is internal, specifically in the valvetrain considering the mileage on it. They are quoting me for engine replacement.

As for the comment about clogged cats, this truck has no cats.

Is it possible that no cats could cause just one cylinder to misfire only at idle and not multiples? Driving the truck around produces no misfire and it has good power. The tech was using a scanner with misfire counter and it only happens at idle on cylinder #4. It is so slight that you almost can't tell that it is misfiring at all.