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Old 06-09-2021, 07:07 AM   #1
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Join Date: Jun 2021
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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).

2000 E450 dually V10 wagon
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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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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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