Not sure, im not an expert on the 420a engine so i dont know what else it could be. Someone else on here might be able to give some better input. Try Slow420a, he's got alot of knowledge in these cars

 

Ok thanks.

 

Ok i talked with my mechanic and he suggested it might just be a leak in the gasket and not a crack in the manifold itself.

 

Check your throttle body gasket. Its basically a vac leek, my guess is the throttle body, that seems to be common. If its not that make a boost leek check device(pvc cap with a air compressor fitting taped into the top) and see if you can hear a vac leek. If you cant find a vac leek it may be your iac motor... which is also a common problem.

 

That was it, the idle air control motor. I took it off and cleaned it and the hole where it goes, and then put on a new O ring since the original one looked pretty worn and it fixed it.

 

Awesome!

 

good to hear, damn iac will keep every one crazy and ahve mechanics make lies.