I am battling a P0300 random misfire code. The truck is a 89 k2500 with a LQ4 and a NV4500. Engine and harness were from an 02 3500. The harness was modified per LT1swap.com and I have added a relay/fuse center for the engine.

Starts and idles fine. Take it out on the road and it will pull hard, and randomly cut out like it’s loosing a few cylinders, and then come back in hard. It’s very random. It also randomly backfires through the exhaust.

I’ve had this issue since it’s been finished and really haven’t put many miles on it.

Problems/parts I’ve found and corrected.

Intake manifold bolt loose. I replaced the gasket with a good new one. (Green I think was the color).

Coil grounds had a high resistance, 7ohms. removed the star washer I used between the ring terminal and cyl head. All connections are crimped and soldered. Close to 0 now.

Good set of used coils.

New plugs and Delco wires

New Carter fuel pump died, replaced it with a Walbro and have good psi.

All of the coil packs were arching, it was obvious at night. I never really solved it, but last time I checked it was not doing it. I believe this the problem. Wires all have diaelectric grease on them.

I bought new (junk I know) Duralast coils and did not see a change. They were arching just like the good used ones and the originals I replaced.

I also have aP0134 code. I know that needs to be addressed but I doubt it’s causing the hard misfire.

Checked compression and all were good.

Vacuum was 17.5” at the intake. Thought I had a table scaling problem because the PCM showed 10.5-11”. Some helpful members pointed out with the PSIA - PSIG difference and cleared that up for me.

I checked resistance between the connector at the coil, and the pin at the PCM. It ranged between .5-1 ohms. I believe it’s a connection or something loose.

should I be using any kind of grease at the harness to pcm connection?


Sorry for the flood of Information. I don’t want anyone to think I haven’t tried or searched. I may be leaving things out.

Any help or advice would be greatly appreciated.

 

the ignition spark (electric) will find the path of least resistance. If you are seeing arching at some place, that means there is higher resistance everywhere past that point. Is it arching at just a few coils? I have seen issues with bad sparkplug wires and spark plugs new from the package.

 

Im sure your tired of replying to my posts, you helped me out on the first one I posted in the diagnostic section a month or so ago. I thought I had it figured out with the Coil ground resistance, the arching stopped then too.

All new Delco plugs. Maybe I’ll try to a new set and see what happens. The problem is very intermittent, and if I bury the pedal it usually clears up. Maybe ones got a small crack I can’t see. I pulled them recently to check the gap and I didn’t notice any cracks.

Your very right, it’s going to find the path of least resistance. It was arching from the body to the bracket on the valve cover. That’s why I assumed the insulation was bad in the coils, but I’m very doubtful 3 sets would of failed the exact same way.

 

Glad to help if I can. This forum is a great place for info. Yeah, I'm sure you are right ... failed 3 times ...I doubt it.
I have also seen graphite spray used around ignition components cause trouble.

 

I’ve been digging through every P0300 thread I could find on here. I have seen a handful of members have the same symptoms I have, and it ended up being the 02 sensors. I bought Bosch sensors from the start because the originals were in terrible cosmetic condition. Maybe that was a mistake?

I’ll take the truck out and watch the 02 data, I can watch the Output voltage and the fuel trim. Maybe that will be telling me something too.

Also, the timing was cutting way back during the misfires. Is that and indication of anything?

I know it’s probably pointless, but I am going to pull the plugs and check resistance through them. I’d like to avoid throwing more parts at it.

 

I fought a missfire on an 03 Silverado 1500HD with an LQ4 years ago. Symptoms sound different than yours other than the random missfire on multiple cylinders, but thought I'd tell you what the fix was on mine that you can check. I spent lots of $ on chasing it down & replaced a lot of parts, took it to a shop that had no clue, took it to a second one & he put like $2500 in parts an d labor into it (but I only paid for a new fuel pump) & he threw in the towel, 3rd shop had it for a week & found the issue because he was going to look at the target wheel on the crank for a chipped tooth. Found that there was rust under the sensor seat & was making the gap between the sensor & target wheel bigger & wasn't reading it correctly. Scraped off the rust with a small screwdriver & ran fine after that.

 

Thanks for telling me your story! That’s definitely something I will check. Crank sensor right? What were your symptoms?

 

Checking the crank sensor is a good thing to check, but it wouldn't make the arching at the coils.

 

Yep crank sensor. Mine would miss bad when it was cold, once it got hot it would run fine. Not just water temp hot, past that, like something in the engine was heat saturated. I finally came to the conclusion (after the culprit was found), that the target wheel was getting hot & expanding a smidge & closing the gap.

 

Agree, arcing at the coils is a different issue.

 

Well I was able to check out the plug resistance on the plugs today. 5-6.8k ohms, after searching I really didn’t come up with any solid numbers on what it should be. The only non I saw said that over 5k ohms is too much, but I’m not taking that as gospel. I didn’t check the wires, because I’ve had 2 new sets on it.

I’m still digging, just curious if those plug resistance numbers are a concern?

Edit:
Took it for a short drive. Still missing terribly, I watched the ignition advance while driving, when it was stumbling hard it would go down to 4 degrees at times.

On a 3 min drive the misfire count was,
#1 1492
#2 41
#3 364
#4 5
#5 32
#6 2
#7 230
#8 23

The regarding of the timing is baffling me. Should that mean something?

 

The ohm readings you have for the plugs sound good for resister spark plugs. Since you are able to see the misfire count/cylinders, swap #1 and #6 plug wires and go drive the vehicle again

 

Tried that and the #1 count still was climbing like crazy. I guess now I need to swap coils and injectors too.


Something else that just came to mind, I bought cheap new eBay or Amazon injectors, and I mean they were cheap. Like $40 for a set. Should I try to buy a good set of used OEMs?

I tried bench cleaning mine but a few seemed to be hung up. So that’s why I changed them.

Revving it in neutral it was backfiring through the intake today. That’s a sign of a lean condition right?

 

Lean condition could do that. I wouldn't trust cheap Chinese injectors. I picked up some used GM injectors and sent them out to have them cleaned and flow checked.

 

Well I’m stumped.

I pulled injectors off a 01 5.3 stashed for a future project. “Bench tested” them using carb cleaner and a battery. Verified I had some that worked. Changed out #1 and #3 injectors because they had the highest count. No changes.

Swapped 1 and 2 coil, no changes. Swapped 1 and 2 plugs, no changed. Had good voltage at the injectors. The oil still looks new. The plugs looked like it was rich.

I did flop o2s and the p0134 hasn’t came back.

I don’t have the tools to do a leak down test, but that’s all I think I have left? The next step is to go internal I guess. I had the valve covers off of it and everything looked good so I didn’t tear it down.

Edit:
one wire I have not checked, the coil reference low wire. I am guessing it would land at the PCM?
Edit 2:
low reference wires all rang out good with low resistance.

something else I haven’t mentioned, the idle speed is very low, even when cold is stays between 550-
600.

Edit 3:
the low idle speed and rich conditions have me thinking a vacuum leak. I dug through my receipt pile and saw that I used Mahle Intake gaskets. Well I’m the VW world Mahle always seemed to be good stuff. After searching on this site everyone seems to not think so. I guess I’ll pick up a fel pro set and try that out.

 

Well positive news.

I got a Fel Pro intake gasket set and installed it. Took it out for a drive, it was clearly stumbling and missing with a flashing SES light and the misfire counter climbing. So I turned around and on the way home I noticed it seemed to be rubbing a little better. Figured I’d try it again and the second attempt it ran normal. No SES light, stumbling or hesitating. Took it out on the 55mph road and tried everything from lugging it to winding it out and it ran good.

I don’t want to say it’s fixed, but it does seem to be running like it should. I guess I’ll figure out tomorrow morning on the 10 min commute to work.

 

Good news . I still wonder what's up with the coils arching.???

 

Well it’s not fixed. #1 cylinder missed the entire drive this morning. I think it’s odd I’m still getting a random misfire code rather than a single cylinder code. The next step I would think is to pull the cylinder heads.

The arching cleared up, I have checked for it a few times and haven’t seen it happen. I think then I fixed the coil ground high resistance, that solved the arching coils.

 

I missed where you had a p0134 code. Sounds like the O2 sensor isn't working. I think that is sensor1 bank 1. That would correlate to the odd bank of cylinders having a misfire. Use your scanner to check out the O2 sensors. Check if both fwd sensors are changing voltage with the engine running.

 

I’m going to sound dumb, but will the 02 sensor cause this severe of a misfire?