GM 3400 mystery misfire

I recently did an extensive repair on my 2002 Chevy Venture with a 3.4L engine.I replaced head gaskets, intake manifold gaskets, timing chain and sprockets, including the timing gasket. All ignition components, coils plug wires, plugs, etc., are AC Delco original and new. I went into the engine due to the common intake manifold gasket failure and coolant in the oil.

Here are some symptoms of the “soft” misfire that vehicle experiences. I define “soft” as compared to a harder electrical ignition type misfire. I have included some other information that is not symptomatic, but might be pertinent to trouble shooting the issue.

* The vehicle cranks right away and usually runs smooth at first. Sometimes it will miss a bit right after it starts. At other times it runs very well until warmed up.

* After the vehicle warms up, a rhythmic, ‘loping’ idol sometimes occurs. The car never totally dies. The headlights will slightly dim when the loping occurs.

* The vehicle misses intermittently throughout the driving range.

* It seems to run best at sustained speeds/ interstate, etc.

* The vehicle occasionally stutters at take off in traffic.

* no check engine light has come on.

* vehicle has not totally stalled out.

Any ideas/ trouble shooing pointers would be helpful. I have checked all ground wires. They seems to be tight and in good order.

Thanks much

Topic

[BillParticipant]

That’s a great find. Just goes to show everyone that OEM parts are still the best replacement parts.

[DAN TWYMANParticipant]

I am also running an aftermarket Balancer (Dorman) . Seemed alright for awhile, but now getting some misfires. Scan tool shows engine rpm at a different value than the 24x rpm. Are those supposed to match?
https://photos.app.goo.gl/WuA71F6oErFP5wG19 link to screen shot of scan tool data

[Author]

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