me and a buddy just put a z6 in a dx coupe with an auto tranny. at idle it runs great and can be revved the **** out of and runs pretty well with only little hesitation. However when under load it seems as if it is missfiring. Also it overheats a little bit. But when under load it will start jerking and run like *****. Spark plugs are good, wires are good, dizzy its self is good. I figured that it was likely that cat could possibly be clogged. any input?

 

the cat is a good place to start. unless you're in the "rust belt" it shouldn't be much trouble to check.

i would also want to know compression test results, loss of coolant, mixing of coolant and oil, white smoke... to put it simply, rule out a leaky head gasket.

is this a junk yard motor? what's the story?

 

Im 30 mins oitside of cola... but not much rust. Headgasket could be a problem but runs fine sitting and revs up well but when driving just goes to hell. When sitting it dosent over heat while driving it does. Checking cat today though.

 

how long are you letting it sit and run (when it doesn't over heat)? do you hold RPM around 3k while doing this? that helps simulate the heat generated while driving.

 

Well it would start acting up and heating up at about half a mile of easy driving. There is no tach so we do not know the rpm range but we held it at we suspected to be around 2k for a good 45 seconds and ran like a top.