my car recently developed a real sporadic idle (but it never dies, just sounds like its about to then kicks back up. almost sounds cammed) and it seems to be getting kind of shotty gas mileage, is this the sign of a fault o2 sensor? it also sounds as if i have a bad exhaust manifold gasket, this couldnt be that big of an issue could it? what else could be the culprit. the car has recently had a full tuneup, head work, and all new gaskets. the motor is an L98

 

Is your SES light coming on?

Did these symptoms start after the work, or did they exist before it?

 

could be an injector stuck closed, therefor a dead cylinder, this would make the O2 sensors read lean and then richen up the mix on the rest of the cylinders. Therefor running rough and getting bad gas mileage. Bad ignition to 1 or more cylinders would also cause this.

 

no SES light, the car started doing this after all the work was done. the work was done because i had a bad intake manifold gasket so i was burning coolant and i was told this could f-up the o2 sensor?