I have 1992 oldsmobile eighty eight royale 3.8l 3800 engine. The car has a rough idle when cold sometimes but the car wont stall if i take the car out when cold it will stall when i stop and restart right back up and be fine until i shut it off again. Ive replaced maf,tps,cts,iac also ive replaced ignition control module and coil packs also new plugs and wires. Fuel filter was replaced 3k miles ago and was doing it before that. Im not sure if it started happening before or after my father hit a big curb pulling out of a store. Also the problem doesnt happen all the time only maybe once a day sometimes 8 times a day and sometimes no problem for a week.
When i take the oil cap off it throws the car into the rough idle.
Any idea of what it could be?

thanks

sorry about the double post forgot i posted it here before. also theres no check engine light or stored codes.

We have had other cars act this way when a seal on the pcv valve was bad. Try changing the valve and seals and see if that helps.

Also, look around to make sure you don't have any vacuum hoses disconnected or sucking air.

Pcv i have changed after the problem started happening and it didnt fix it. also i replaced the rubber o ring on it and closed it back up on my engine it sits on the side there are no hozes coming off of it its just a plate with 3 10mm screws and the pcv is underneeth. Also all my vaccum hoses are connected.

In a normally operating engine, when the oil cap is removed, there is not usually a significant difference in the idle. The fact that your idle goes rough when the cap is removed suggests that unmetered or improperly metered air from outside the engine is entering the intake manifold through the oil filler hole and crankcase. And the only path that I can think of for this is through the pcv valve and seat.

If you feel a vacuum when you remove the oil cap and place your hand over the hole, with the engine idling, that is another indicator that the pcv valve is stuck open, improperly sealed, or not the correct calibration for your engine.

Other vacuum leaks might occur where the throttle body attaches to the upper intake manifold, at the upper intake joint with the lower intake manifold, and where the lower intake manifold seals against the heads and ends of the block.

Here is a way to check for those leaks. This must be done outside and away from sparks and flames. Take an UNLIT propane torch and open the valve to bleed a little propane out, (you only need a small flow) Direct the propane around these suspect areas with the car running. If the propane is drawn in through a leak, you will hear a sudden difference in the idle speed of the engine.

Another run-rough cold possibility is that you have a leaking injector that gives you a miss until the plug can dry it out as the engine warms up, but that would not explain the symptoms you describe with the oil cap.

You didn't mention a spring. There should be a spring under the cover that pushes the pcv valve down with the seal around the fat bottom part of the pcv valve. If your spring was missing, maybe that could explain the problem?

it had the spring in it but i wll check again tomarow. Wouldnt a vaccum leak cause the problem all the time? nomatter hot or cold or once one day and not for a week or 8 times in a day and once the next?

thanks

Yeah, if you do the hand on the oil filler hole and feel a vacuum, you will know something ain't right.

Vacuum leaks cause more problems at idle. And if you have a bad seal somewhere causing a vacuum leak, it is possible that heat helps to seal the leak as things expand and tighten up. This might occur, for example on the throttle body seal or manifold gasket seals. Can work the other way too, i.e., things get worse when hot.

Usually rough running is caused by ignition problems. But you have changed most of your ignition components. I assume you mis-typed cts, meaning cps for crankshaft position sensor?

And, did you clean the throttle body and the passages around the IAC when you changed it?

nope i typed it correct cts = coolant tempature sensor its right under the throttle body. I was told that that could cause problems in the transition from cold to hot. Also i tried putting my hand over the oil filler cap and i do not feel a vaccum but when i put my hand over the hole it smooths out the idle again.

uhh i was just looking at your picture mine did not have a gasket between the intake manifold and the metal plate

Well that could certainly cause a vacuum leak.