8.1 Misfire Trouble Shoot

[shu-dog]

I have a 2001 GMC Sierra 2500HD 4X4 with 8.1 and Allison Tranny.

 

Having serious misfires in overdrive under load. Also backfire through intake.

Most noteable at 50mph going uphill in overdrive or drive with/without tow/haul button.

 

Truck has 115K miles on her.

I replaced spark plugs 20K miles ago

Plug wires and coil packs within last 3K miles when I noticed it getting bad

ALL 4 o2 sensors just done

 

Still doing it.

 

My plan next is to do a Leakdown/compression test.

 

Anything else I should look at?

 

Im assuming leakdown/compression test will tell me a lot.

What numbers should I see?

 

[Doug_Scott]

Back when the 305 engines came with chocolate camshafts one of the symptoms was back firing in the intake under load. You may want to pull valve covers off and take a look at the amount of rocker arm movement while cranking engine over. You do not need to run the engine to see this.

 

Keep in mind the three things required for an engine to run. Fuel, spark and compression. You say it is intermittent, so you can pretty much rule out a compression issue. Low compression caused by a mechanical issue will always be present. You changed the plugs 20k ago, was that done in an attempt to address this issue? Take a good look at them when you pull them for the compression test. If you have a buddy giving you a hand, you can accomplish a good amount of trouble shooting during compression test. You can verify plug condition, you can verify camshaft still has lobes by removing valve covers before compression test is performed and watching rocker arm motion during each cylinders compression test, and you can verify compression across all cylinders.