I have a 1990 f150 302 5.0.It runs ok for about 2 or 3 minutes
then starts to loape (rpms up down,up down ect.) I removed the pcv
valve from the valve cover and then removed the pcv valve from its hose.Now it runs excellent.I bought a new pcv valve and put it in and it runs awful again.So its not the pcv valve.Can any one tell me what the problem could be.It seems like the vacuum from the hose that the pcv valve is in is too high.Well Im thoughly confused.Any help would be greatly appreciated.thanks mike

 

its probably your IAC contol valve, when you disconnect the pcv valve, it allows the engine to breath in air and probably idles faster.The IAC is the only way the engine gets air while idling. there's only 2 screws holding it onto the intake throttle body. There's a small sliding valve in there that can get carboned up. It must slide freely. Are you getting a check engine lite?

 

There is also a pcv filter located inside the air filter housing that could be causing the problem . Make sure all the pcv passages are clear of sludge etc .

 

I just put(last saturday) a brand new 54.00 iac valve and that wasnt it.Any other suggestions.At least I've found something obviously wrong,which is half the battle.thank you very much mike.

 

Thank you I'll check that tonight after work.thanks Mike