I have replaced a head gasket on my 2001 civic LX recently. I have drain the coolant and put it back on again. Now I have idle issues, the car starts fine with high RPM (because it is cold) about 5 min latter the RPM changes, it revs low and high (about 2000 rpm) when it should be idling normal. I have taken the radiator cap (before i start the engine) and run it a while with the cap off to purge any air pockets, no luck. Another thing I have realized that there is a pressure build up in the upper and lower radiator hoses when it is running with the radiator cap on.
Any ideas on how to fix this problem?