P0300 random misfire

[Anusface69]

Hello. I am having issues with a 2005 Tahoe 5.3. The truck starts and runs fine for a few minutes and then out of the blue it will throw p0300 and randomly misfire. The truck will misfire for a few minutes and then run perfectly fine after.
If I watch the misfires I’m live data when this occurs, it shows cylinders 2 and 7 misfiring. When the misfire counter resets, it will show misfires on 3 and 6.
It’s strange that the misfire randomly jumps to other cylinders, and ones on opposing banks.
I’ve smoke tested for vacuum leaks, done ckp variation relearn, the truck has new plugs, wires, coils, injectors. Could it be pcm failure? Has anyone experienced this before? Any help would be greatly appreciated.

 

[iamdub]

Could be faulty crankshaft position sensor or PCM but I'd lean more towards the sensor. Maybe the sensor acts up when it gets warm? Since you have the ability to perform a CKP relearn, try a new sensor. Don't cheap out- get OE!

Have you ever watched the RPM on your scan tool? If the sensor is faulty, it's "misses" might be too short for the tach on the dash to show a response. Your scan tool would probably not have enough resolution as well. But, if you see a blip of low or no RPM, then that would help to confirm. Looked at any of the freeze frames for any of your DTCs?

 

[Myfirstz71]

Hello. I am having issues with a 2005 Tahoe 5.3. The truck starts and runs fine for a few minutes and then out of the blue it will throw p0300 and randomly misfire. The truck will misfire for a few minutes and then run perfectly fine after.
If I watch the misfires I’m live data when this occurs, it shows cylinders 2 and 7 misfiring. When the misfire counter resets, it will show misfires on 3 and 6.
It’s strange that the misfire randomly jumps to other cylinders, and ones on opposing banks.
I’ve smoke tested for vacuum leaks, done ckp variation relearn, the truck has new plugs, wires, coils, injectors. Could it be pcm failure? Has anyone experienced this before? Any help would be greatly appreciated.

I was having a similar issue in cold weather. Truck would start, run about 1 min then cough, sputter and throw codes for a few minutes and then smooth out. It was only cold start, and went away once engine warmed. For me I traced it to the intake manifold gaskets. Always got P0300, but had lean bank codes as well.

I know you said you smoke checked for vacuum leaks, just a thought.

 

[Fless]

What do your short- and long-term fuel trims look like? Might check both at idle and 1500-2000 rpm.

 

[Anusface69]

What do your short- and long-term fuel trims look like? Might check both at idle and 1500-2000 rpm.

I forget what they were off the top of my head but I know they were higher on one bank than the other, I’m assuming cause the manifolds held on by like 2 bolts

 

[Anusface69]

I forget what they were off the top of my head but I know they were higher on one bank than the other, I’m assuming cause the manifolds held on by like 2 bolts

If theyre off what would you suggest,? I’m an Audi mechanic and never seen this before

 

[Fless]

It depends on which way and how far they were off, if they're adding fuel (positive), or taking it away (negative). More than 10% total (LT+ST, same bank) or a distinct imbalance between banks can be an issue. Suggest getting the manifold bolts in place with good gaskets and ARP bolts.

 

[mattbta]

Welcome.

 

[Anusface69]

Welcome.

View attachment 397116

I’m aware my username isn’t all that great. Was just trying to get established here quickly

 

[mattbta]

I’m aware my username isn’t all that great. Was just trying to get established here quickly

You’ve got some good tips already, not sure it is PCM related, just yet. Definitely get those manifold bolts rectified.

Is the CKP new? What brand?

 

[Anusface69]

Today I replaced the CKP sensor. Test drove under the same conditions and had zero misfires. Gonna drive tomorrow to see how it goes, and get the last two readiness monitors ready. I will let you guys know what happens. I really do appreciate all the tips and input, I’m an Audi guy so it was nice to get some info from people who know

 

[Anusface69]

I knew it was too good to be true. Under light load started misfiring only on 6, when I got closer to the shop after about 5 miles misfiring on 6, it started misfiring only on 7. Ltft at idle:
Bank one 12%
Bank two 5%
At 2000 rpm
Bank one 16%
Bank two 14%
Stft at idle: both banks 0
At 2000 rpm
Both banks bouncing from 0 to 7
This misfire seems to happen when I start the truck cold, after driving for 5-10 minutes. Engine warmed up. Yesterday when I let it warm up first no issues. I did smoke test again and made sure I had decent pressure with the smoke so I could just rule out the intake gaskets.

 

[Anusface69]

It’s too bad I can’t post a video of the data stream on here.

 

[Fless]

It’s too bad I can’t post a video of the data stream on here.

You can post it on YouTube or another hosting site and post the link here.

 

[MassHoe04]

I had misfires on an 05 4.0L Jeep. Was actually fuel related.
It kept happening until my fuel pump completed died one day.

I would suggest checking fuel pressure before, during and after start-up and warm-up to see what you have going on.

A gauge can get loaned out from your parts stores with refundable deposit, so checking fuel pressure on your system will cost nothing other than a few minutes of your time.

I would at least want to rue out the fuel pump as a possible cause.

 

[Anusface69]

I have a confirmed fix. Replaced cheap ignition coils with Delphi and the truck is fine. Can I ask questions about the hydra boost here or should I start a new thread?

 

[mattbta]

New thread, please.

 

[Rocket Man]

These trucks don’t like cheap aftermarket parts like coils and sensors. Glad you figured it out.