Misfire on 3 and 7 2003 5.3 almost 300k

[Hutch123]

I have a 2003 Tahoe with the 5.3 non flex fuel. It has almost 300k on it. I was getting a random misfire code on my obd. Happens after the truck is warmed up and usually happens at idle. Once it starts it may or may not happen while I'm driving. I took it in to a local reputable shop that specializes in electrical work. I also had 2 other unrelated issues** after several days of messing around with it they informed me the the truck needed and new PCM.
Shop findings:
Fuel injectors: good
Coil packs: good
Wires: good
Plugs: good
Misfire stayed on 3 and 7 after swapping injectors and they also replaced my fuel filter because it was partially clogged.
They wanted a good deal of money to replace and reprogram a PCM. I decided to do it myself. Bought one pre-programmed to my truck online and then installed. Did the key relearn and truck fired right up. Shortly there after the misfire returned. The connectors looked fine to my untrained eyes when installed the knew pcm. The only code I'm currently getting is misfire on cylinder 3 but I'll update with any additional codes if I see them. Let me know what other information I can provide.
**I also had an issue with a parasitic drain, pulsing interior lights, and and a throttle/pedal position sensor code occasionally putting the truck into low power mode, the shop did not seem to think the issues were related and blamed the instrument cluster for the parasitic drain**
Thanks in advanced for any help.

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[Doubeleive]

i've read before something about doing a crank position relearn or something similar for that same misfire problem, may not have been your pcm at all. the same issue came up real recently for some one else on the forum, you can try searching under "misfire" or "random misfire" and it will be towards the end of the thread.

 

[Hutch123]

i've read before something about doing a crank position relearn or something similar for that same misfire problem, may not have been your pcm at all. the same issue came up real recently for some one else on the forum, you can try searching under "misfire" or "random misfire" and it will be towards the end of the thread.

Ok thanks for the suggestion. I looked up the relearn and I'm going to see if my obd has the relearn command option. I'll update later today hopefully.

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[Mechanic421]

Always have to do a crank relearn when you change pcm,timing chain, crank sensor , engine and maybe even the cam sensor.

 

[Hutch123]

Always have to do a crank relearn when you change pcm,timing chain, crank sensor , engine and maybe even the cam sensor.

The misfire happend before the change. You still think that could be the issue?

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[Doubeleive]

The misfire happend before the change. You still think that could be the issue?

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like I said another person reported that they had the same misfire I believe it was 3 & 7 and didn't have to change anything they just did the relearn and they were good to go, I figure it's worth a try if everything else is good.

 

[Mechanic421]

The misfire happend before the change. You still think that could be the issue?

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Regardless when you change the pcm crank relearn as to be done!

 

[Hutch123]

So just an update. My obd cannot do a crank PS relearn. Guess I'm going to track down someone who does hopefully.

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[Hutch123]

Yesterday I cleaned any grounds that I could find. Some were rough looking. That didnt change anything tho. Then I was looking through some different forums and happend to see a guy with a similar issue but not exactly the same. He threw parts at the problem until it stopped. He had essentially done everything I had except I hadn't replaced the MAF sensor. I looked at the MAF data with obd but it was using a different unit of measurement than any data I could find so I couldn't do much with the data. So I thought the hell with it I'll just clean the damn thing and see what happens. Clean the sensor and the connector. While I was at it a changed my air filter and the air box was full of junk. All kinds of leaves and lint looking stuff. Fired the truck and it's still idling a little rough, but no misfire. I drove it around for awhile the truck seems much more responsive and no hesitation. Still it doesnt idle super great but NO MISFIRE!! I can't believe it was something so simple and the shop o used missed it.

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[Hutch123]

Yesterday I cleaned any grounds that I could find. Some were rough looking. That didnt change anything tho. Then I was looking through some different forums and happend to see a guy with a similar issue but not exactly the same. He threw parts at the problem until it stopped. He had essentially done everything I had except I hadn't replaced the MAF sensor. I looked at the MAF data with obd but it was using a different unit of measurement than any data I could find so I couldn't do much with the data. So I thought the hell with it I'll just clean the damn thing and see what happens. Clean the sensor and the connector. While I was at it a changed my air filter and the air box was full of junk. All kinds of leaves and lint looking stuff. Fired the truck and it's still idling a little rough, but no misfire. I drove it around for awhile the truck seems much more responsive and no hesitation. Still it doesnt idle super great but NO MISFIRE!! I can't believe it was something so simple and the shop o used missed it.

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Well I guess I spoke too soon. Went for an out of town drive this morning and it came right back. As bad as ever.

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