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Old 06-09-2021, 07:07 AM   #1
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Engine Misfiring

Hey there, I recently bought a 2000 Chevy Express 3500. Awesome shape, good deal, driving me nuts trying to find a mechanical gremlin. It has an occasional misfire, known when I bought it. Drove it about 200 miles and it started getting worse, check engine light came on misfired quite a lot so I took it to a good mechanic I knew....His computer told him MAP Sensor and Air Flow sensor. He replaced those, and the old ones appeared almost melted. Drove home, did the same thing. Hauled it back to the mechanic who spent 2 weeks looking and testing but didn't find anything else. Did a global reset, it drove fine--for about 100 miles. Battery light came on for alternator and check engine light came on, giving P0175--rich fuel mixture. Took it to another experienced Chevy Mechanic who got a similar van and swapped known good parts to try to track down the culprit. He did discover the fuel line wasn't advancing the fuel and replaced a small vacuum I believe in that (I think, hopefully that makes sense). It seemed when he drove it with the dog house off, it drove fine, but with it on and when it warmed up it would misfire on various cylinders. He heated the computer to see if that was causing the issue, and was convinced that wasn't the case. Eventually he didn't know what else to try and gave up. I tried a 3rd mechanic and had him replace the O2 sensors, as the check engine light still gave the "fuel running rich" P0175. Didn't help. He suggested switching out the Distributor cap and plugs, which he did. Still runs poorly. I'm $2000 deep in repairs and getting to my wits end. Any suggestions?!

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Old 06-09-2021, 07:35 AM   #2
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Check the wiring harnesses that run among the exhaust to the 02 sensors. These are susceptible to heat damage, and these also contain the power circuit for the oxygen sensors. If that power supply wire wears or melts through and grounds out, it’ll overload the PCM. If this is the case, the symptoms will keep getting worse until it finally pops the PCM fuse (#19 iirc). The oxygen sensors most likely now are not getting enough voltage to heat up and work properly, causing your codes.

(Note I’m assuming this is a 5.7L. Small v8 probably has a similar issue. Disregard if you have the 4.3L or 7.4L).
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Old 06-09-2021, 08:30 AM   #3
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Thanks for the idea, it has the 7.4 L.... Any other ideas welcome!! Do you think it's worth getting a new computer?
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Old 06-10-2021, 04:32 AM   #4
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Quote:
Originally Posted by keegan View Post
Thanks for the idea, it has the 7.4 L.... Any other ideas welcome!! Do you think it's worth getting a new computer?
No its not until or unless you can determine the existing PCM is the real issue.
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Old 07-03-2021, 09:30 AM   #5
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I ended up taking a gamble and having the PCM replaced, as the mechanic was able to get one at a reasonable price. It ran perfect--for about 200 miles, and now it's symptoms are back in full force. Random intermittent misfire--sometimes to the point I think it will quit me entirely....
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Old 07-03-2021, 12:38 PM   #6
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That era GM power train electrical is very hard to diagnose, as you’re finding out. You need a good auto-electric shop. It potentially could get $$$ if they spend 10 or 15 hours or more. There is some merit in considering just doing a full wire harness and sensor replacement. It might not cost more in the end. The EFI system on that motor is basically an 80s design so on-board diagnostics is relatively limited in usefulness.
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Old 07-03-2021, 02:00 PM   #7
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Thanks for the input. I'll check into that...What sensor are you referring to replacing?
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Old 07-03-2021, 02:14 PM   #8
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Come to think of it, you’ve already replace the main culprits. I remember reading a TSB where the knock sensor could cause drive ability issues. Basically it starts sending a signal when it’s not knocking, retarding the timing so much it no longer fires properly. You can check this signal with an oscilloscope.
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Old 07-04-2021, 10:36 AM   #9
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Sorry the PCM didn't do the trick. My question is, what happens now that the motor is misfiring? Will it misfire now after sitting and cooling down or will it run well for a short spell before acting up again? Uncle Lance
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Old 07-04-2021, 11:06 AM   #10
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hi uncle--I know, such a bummer! Well, it is baffling. The last mechanic was convinced the issue was heat related....so driving the other day put it to the test as it was mid 90s. I don't think the issue is heat related though, as it drove about 70 miles before misfiring (at least to where I could notice it)...and then just intermittent random misfires. It seems that at about 70mph, going straight, without a hill, it would be fine, but any alterations in that would cause a misfire. I made it to Missoula, it was off for the evening and overnight, and I drove it at about 5am thinking it would be better when it was cool, and it misfired right away. It misfired all the way home....haven't driven it the last couple days but the check engine light never went off, and I suspect it will act the same. I'm not sure why the new computer kept the problem subdued for 200+ miles though, doesn't make sense to me...(which is not saying much
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