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Old 07-29-2020, 01:46 AM   #1
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Join Date: Jun 2017
Location: San Antonio, Texas
Posts: 123
98 S-10 Random misfire

4.3 V6
170,000 miles. No ticking or noise at any time and it doesn't burn a drop of oil.

It's my nephew's truck and I'm going to see if I can help correct a misfire problem.

What he's done or replaced so far: New MAP sensor, cap, rotor, plugs, wires..... and then injectors.

The truck was running great and this this came on sudden. With a cold start it has a slight miss but gets worse once it reaches operating temp.

When this all started the codes initially indicated a MAP-MAF sensor and random misfire. He replaced the MAP, cleaned the MAF, completed the tune up and the MAP-MAF codes did not return, but it started reporting a misfire on cyl 5. Next he replaced the injectors with a remanufactured set and the misfire moved to cyl 4.

The code almost always reports a miss on cyl 4 but will occasionally report a random misfire instead.

It idles fine except for the misfire.

He's done a feel and visual check of all vacuum lines and did not find any leaks. To me, it just doesn't feel or sound like a vacuum leak but a check with a gauge wouldn't hurt. I also recommended a compression test but that wouldn't explain the misfire moving from 4 to 5.

Oil and water are clean.

Today I suggested he swap out the #4 plug wire even though the wires are new. He hasn't had a chance to do that yet.

While I'm waiting on him to swap the plug wire can anyone suggest what else to look for? I suppose it's possible that one of the reeman injectors is bad. Whatever it is it gets worse once the engine reaches temp.
Repairing and driving old trucks is an analog experience. No computers or sensors. They can be fixed with wrenches and a feeler gauge
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Old 08-02-2020, 03:07 AM   #2
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Join Date: Mar 2003
Location: South Mississippi
Posts: 36,351
Re: 98 S-10 Random misfire

Did he swap to the newer style "spider" injectors?
"If it ain't broke, fix it 'til it is!"
Being stupid ain't illegal.

We're Still Debt Free Except for the House!!!

70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e
96 S-10 "Pop's Truck"
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