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Old 09-23-2024, 05:17 PM   #1
Tom
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Join Date: Dec 1999
Location: Mesa AZ
Posts: 5,737
What would you check next? Missfire on my 98

I'm having a problem with a cylinder 7 missfire. It only happens when I've been accelerating mildly for a long time, like an uphill freeway ramp to 80mph. Eventually #7 starts missfiring, which gets worse the longer you accelerate. Let off the gas, missfire is gone.

-I started by swapping the plug and wire from another cylinder.
-Tested compression.
-Then I changed the cap+rotor
-Then I replaced the fuel injector.
-Finally I plugged in an OBD2 and watched fuel trims while spraying around the intake to cylinder head surface area with carb cleaner.

I've eliminated everything that's cylinder 7 specific. Driving around with the OBD plugged in, I can see its only cylinder 7 having any miss fires. I'm at a loss of what to check
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-78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle.
-98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes.
-02 Suburban 4x4: leveled front
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