Useful info regarding oil cap. Well I found it useful anyway.

[stupidget]

So, yesterday I was driving back from North Wales to Wolverhampton and something 'odd' happened. Car started off fine but I noticed that the revs were slightly lower than normal, so much so that I actually stalled at some lights. I put this down to piss poor clutch control and carried on with the journey. About 40 minutes later, as I plodded along at 30mpg through some road works on the A55, the CEL comes on even though the car is driving like an absolute dream. As we got closer to home I stopped at a petrol station, bough some stuff and got back in the car, CEL still on. At home I plugged in my faithful £30 ODBII and got 2 HO2S Errors, went straight on the interweb (as you do) and started to panic as there was talk about failed CAT's, replacement sensors etc etc. I cleared the codes and the engine ran like an absolute dog, just on the verge of stalling. As is customary whenever there is ever an issue with my car I pop open the engine cover, look for anything obvious, check coolant (no, I don't know why either), Power Steering fluid (i'd just had new hi and low pipes replaced) and finally the oil cap (I'm obsessed with making sure there is no mayonase). This is when I realised that after my last 'check' I'd not replaced the oil cap correctly!!!!! Screwed the cap on correctly and the engine purred like a kitten within half a second!!!!!!

It's amazing how we always expect the worse when sometimes you just need to take a step back, calm down and check the obvious.

 

[Les Richards]

Seem to recall reading somewhere that on the air-cooled engines there is a noticeable drop in revs if the cap is removed with the engine running, too. Perhaps even more critical on later 911s.

 

[911UK]

good result and useful check to make

 

[Luddite]

My old pseudo vintage sports car has the ubiquitous Rover V8 engine of 80`s vintage and it runs poorly if the oil cap is removed, thus it seems not only Porsche engines suffer this issue.
Yeah it would seem handy if a list of things to check were available relative to the fault codes generated starting with the simplest or most likely first.... Though never having used a code reader, perhaps some already have such a facility...?

 

[gbarron]

The 996 runs a closed-loop scavenge style crankcase ventilation system in conjunction with the air/oil separator. If the oil filler cap is left loose or off, excessive un-metered air enters the intake system via this circuit causing the ECM to try and adjust fuel trim to compensate. This why the car wanted to stall and eventually threw a CEL.

 

[bananaz]

If only all mechanical issues were as inexpensive and simple to fix as this one!!!!

 

[Carrera4Boy'sDad]

Thanks. Exactly the same happens with the BMW N52 engine in my Z4.

 

[Luddite]

From my air cooled days I remember that a possible poor running issue could be tracked to a worn seal on the oil cap, might be worth checking on older water cooled cars these days..?