Hello all, hope I do this right and please correct me if not. I have a 99 GMC with a 5.7 V8 that I have been restoring for about a year now. The truck has developed a miss on a long pull and I’m at a loss as to why. I have changed the plugs, wires, cap, button, and now the distributor. The truck runs and idles great but it’s this little quick miss that is driving me crazy to find! I’ve even gone as far as changing the EGR valve thinking it maybe an issue. The miss happens so swift that it doesn’t flinch the RPMs and no lights are being thrown. I have replaced two of the O2 sensors because there was a light when I first got it but hasn’t popped on since. I have noticed it more pronounced when the truck warms up but not when it’s cold (engine temp not ambient temp). It has been in a few shops and their mechanics would call and say they have found it but I have had to call back and say not yet we haven’t. Has this happened to anyone else? The truck has 91,000 original miles and it did sit for 12 years. The motor has been freshened up (due to sitting and all the gaskets dry rotting), radiator and water pump been replaced, alternator replaced, head gaskets tested fine and intake gaskets tested fine. Please help if you can and thank you in advance!

 

For clarification - what do you mean by long pull? Just extended driving, when towing/hauling, or other?

Just because the CEL isn’t on doesn’t mean there’s no codes - try checking for them anyway. It’s possible what feels like a miss is actually a transmission issue.

 

My apologies-it seems to be on long grade hills (over a mile) and I have had it happen on down hill grade as well. The best way to describe it is I can feel it slightly while engine is cold but more pronounced when warm.

 

No need to apologize - to me a long pull means towing which is why I asked.

Are you also noticing an abnormal lack of power during the uphill drive?

Generally speaking the ignition coil and ICM are suspect when misfire occurs under load, but with the added factor of long grades and changes in altitude, I’d rule out a faulty MAP sensor first.

The sensor can be tested manually by back probing the signal circuit (center wire on connector) and applying vacuum with a hand pump at key on, engine off. A scope would be the easiest way to identify the problem, but a digital meter with a min/max recording function or an analog meter would also work. Much like a TPS, you’re looking for a steady change in voltage - a sudden change indicates the sensor is faulty.

If the MAP sensor tests ok, the next step would be monitoring the ICM control and secondary ignition circuits using a scope.