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GMC Forum: 2008 Envoy 4.2L PO300, PO352 , PO353
View Poll Results: PO352, PO353
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Old 05-25-2017, 01:37 PM
Swampfox21 Swampfox21 is offline
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Join Date: Jan 2015
Location: Westchester Cty, NY
Posts: 4
Smile 2008 Envoy 4.2L PO300, PO352 , PO353

Started my Envoy after being parked for 4 days in my garage and engine had a severe miss.. 91,000 miles on her. Had been running fine and this is the first engine issue in 3 years.

Pulled down the codes.. PO300, PO352, PO353.. Intermittent miss cyl #2 & #3. So I pulled the plugs and started trouble shooting. Plugs looked good but since I was there A pulled them all and replaced them all with new AC iridium plugs.

Checked for spark from the coils with HEI tester. No spark from #2 & #3.

Have solid 12 volts to the coil wire and a solid ground, Signal line did not show any pulse. (3 wire COP)

Couldn't find a wiring diag. on line so I ended up taking the thing back to the dealer figuring it was either the ECM (new one would need to be programmed)or wiring (Need print).

Problem turned out to be in the wiring harness from the ECM to the coils. The Tech found 6 broken/frayed wires in the harness where the wiring goes down under and behind the ECM. He had to strip back a good amount of the plastic to find the broken wires. He found the broken wires for the coils by following their color codes. Along the way he found 4 additional frayed/broken wires.

The tech did the repairs and sealed and repacked the wires in the plastic shield. Spark had now returned to the cyl #2 & #3. $$ (4) labor Hrs later the Job was done.

Glad to have my Envoy back on the road and running properly.

Steve D.
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