[Shindog32]

I recently had an overheating problem and Im in the process of replacing my EWP and TS. While i was checking under the hood for signs of overheating damage I removed the oil cap to check for water in the oil and noticed that after I put the cap back on ther was a hissing sound while the engine was running? Removing the cap while the engine was running caused the check engine light to come on and I cant get the hissing noise (vacume leek) to go away. This may sound like a stupid question but did I remove it incorrectly or is it normal to vent a little? The gasket looks fine BTW.

[Shindog32]

Anyone have any thoughts on this?

[jwzimm]

Well we have PCV (Positive Crankcase Ventilation) which uses intake vacuum to draw air out of the crankcase and through the motor. Opening the oil cap while running will spoil the seal in the crankcase and I wouldn't be surprised that the CEL came on from that. As for why the cap won't seal again, not sure. It could be that it is designed to vent a little (to provide flow into the crankcase for the system to work) but I don't know that. Maybe just need a new 710 cap (oil cap).

[GMWNashville]

This engine uses very very little engine vacuum (hence the need for a vacuum pump) you possibly have a bad valve cover which has the pcv system integrated in

[Shindog32]

Problem solved! I have 71000 on this car and have never changed the oil myself. Come to find out the retainer ring is not supposed to come off with the cap! It did and when I put the cap back on I did not "snap" the ring on first. Once i did this right no more vacume leak. Feel free to insert rookie remarks and dumb a%# replies at will.