Rough running if oil cap off?

[Dominic123]

I had some modifications done to engine resulting in having to cut the oil fill tube down approx 4 inches; so now I have a plastic piece with the oil fill cap on yet can pull it out; its not air tight and I was told that a Porsche will run rough if the cap isnt air tight and was told that I need to replace the entire fill tube.

Any insight or opinions?

[Loren]

It needs to be air tight or you will get fault codes and poor performance.

The emissions requires in the US require crankcase vapors be burned in the engine - not vented. So you have a vacuum leak that will affect performance.

I suggest replacing the tube or finding a way to make it seal again.

[JFP in PA]

Your oil system is normally running under a vacuum condition of about five inches of water; it has to be air tight.

[logray]

Replace the tube or determine a way to make it air tight.

[Dominic123]

Thanks everyone for the info; so would having a not air tight oil fill tube result in sluggish and/or hesitate take off? I havent received any fault codes, yet sometimes when I take off the sort of sputters until I give it some solid gas; maybe once it gets going then it builds the vacuum?

[logray]

Without a good seal at the oil filler neck, you are asking for trouble and is a recipe for engine damage.

[MiamiC70]

I made that bonehead move the other day checked oil, saw that was a little low but I failed to let settle. Checked again level was good and closed engine cover car was all over the place running rough, misfiring, sluggish pulled over and the problem became apparent ;)

[ldrhoades]

I'll be the contraian here. I've had my oil cap come off at the track (wasn't tightened up after checking oil - supid, I know). Anyway, car ran fine. Turing great laps at Road Atlanta. High G loading, high RPM, all the normal track rigors. The only reason I figured it out was that I was black flaged for smoak. Made one hell of a mess but the car ran fine. That said, I've got a semi-permanent CEL due to other mods on the car (it's strictly a track rat). But it did RUN fine, no noticible difference in performance. That said, when I'm not being stupid, I prefer to have the cap on and TIGHT.