Rough idle and dipping revs on lift off....

Hi Jules I'm sure Richard will be along shortly to give you his verdict. In your list of what you have done you have not mentioned checking or changing the MAF sensor? as symptoms could point to it. Electrical issue shorting when hot? Very frustrating having issues like this . Best of luck on finding cause.

We had a lumpy, low idle on the 993 recently and was nothing more than a split vacuum pipe!

Thanks for the quick reply. The maintenance history indicates the MAF sensor was replaced shortly before I got the car, maybe 20 months ago; possibly the previous owners attempts to rectify it! Again, nothing has shown up in the fault log to suggest an issue with same - although as an engineer myself I have a feeling this is an empirical rather than recordable problem! Thanks for reminding me.

Soooo, just back from a big spend to fix all the snags that have been on my mind since I got the car 18 months ago. Since I've owned it I've had a niggling starting problem when the engine is warm - always the same, drive it (easy or hard) park it up for about an hour, restart, fires, stalls straightaway. Then it won't restart (just chugs on turnover) unless you tickle the accelerator, eventually catches (more sputters to life), and will stall again if you don't hit the throttle quick; and there's usually a rich mixture smell around by this stage - I think its a fine line between starting it and flooding it. Once started it is usually OK, but occasionally as you move off it will stall again - maddening! The fault has never lit the check engine light, or registered a fault we can narrow down, although I think the rich mixture issue has tripped the O2 sensor a couple of times. So, the saga of trying to rectify this problem (over the last 18 months) is as follows: - Replace position centre on flywheel - Replace ICV - Clean throttle housing generally - Replace starter - Replace a faulty variocam solenoid (just done - much quids) - 2 intermediate services, inc plugs both times and coils/alternator checked fully operational I really thought he last one was going to nail it, and spent a few hundred doing so. Drove the car out of the indy after the work, down the road for a few miles to the pub, lunch for an hour then, grrrrrr! you've guessed it. Also I've noticed the idle is a bit more lumpy now (dipping low on lift off, almost stalling), but that might be the engine remapping after the variocam. It's definitely a warm engine issue - NEVER happens when its cold. And I think I would add that it has been less frequent since I changed the exhaust (that may be coincidental). I'm really out of ideas - any suggestions? Comments from the floor welcome. J

Lumpy idle and stalling is often a Variocam solenoid issue. You say one has been replaced. Was that because it registered a fault code? If not, why did they replace one - ie how did they know which one to replace? If it isn't producing any fault codes now, then it is incredibly difficult to diagnose. Does your indy use a proper Porsche tester, or another scan tool? Most generic scanners don't show many of the Porsche specific codes. If there are indeed no fault codes, I would suggest reading the values of the sensor inputs (Actual Values on a Porsche tester) and check them against the DME Setpoints. If you want a list of these, drop me an email, and I'll attach it to a reply.

ALso consider air oil separator as I know someone who had very similar issue and after replacing a few items it came back to this. They also had a MAF replaced before they bought car so maybe someone had already spotted the issue and tried to address it. Warm engine, car coming to halt at lights and it would cut out or stationary and idling the rev counter would dip low into the 100's of revs then swing back up a few times then invariably cut out. Diagnostics showed nothing and no errors thrown. Worth a try.

Thanks Richard - Northway Porsche did the work, there's no doubt they have all the gear, and know what they are doing, and I'm satisfied the variocam issue has been correctly addressed. They cleared the fault code buffer down after the service and other work last week, and I'm going to pop down again to see if any codes have thrown again this week, although I suspect not. I've always felt this is something electro mechanical (non sensored) - I haven't considered the oil air separator before - any visual symptoms I could look for? Thanks as always.

Was there any oily residue in the throttle body when it was cleaned? That's a sign that the AOS has failed. Take off the inlet pipe to the TB, hold the butterfly open, and peek inside with a torch. If the housing has a glossy surface, it is probably the AOS.

Nuts - I know the throttle body housing was checked as we discussed it at pick up, and was found clean (didn't realise the significance) - back to the drawing board!

Just back from diagnostics, MAF has a question mark, but the clue to the root of it may well be a wheezing carbon filter cylinder located in the offside wing (so I gather) - the wheezing noise is a characteristic that's been there since I go the car, I didn't realize it may be significant. Another new thing on me - didn't even know it had a carbon filter cylinder before today - I would welcome any comments on its function? Booked in for further investigation work end of next month.

Throttle body was clean on the car I referenced above. I'd be dubious about MAF especially as it is a fresh one - 2 faulty MAFs like that must be a hens teeth issue. AOS ;-)

More info here. http://www.pelicanparts.com/techarticles/Porsche-996-997-Carrera/106-FUEL-Replacing_Your_Fuel_Line_Vent_Valve/106-FUEL-Replacing_Your_Fuel_Line_Vent_Valve.htm

Here is a description of the tank vent system. The solenoid valve in the engine compartment, in the vacuum line to the canister, could be failing. It might be worth investigating that too.

Thanks chaps, enlightening - I'm not convinced its the MAF although I have read stories of more than one failing. To me the carbon cannister/valve issue finally has the feel of being on the right track, because I've always thought it was a fuel related issue as most of the symptoms could always be worked around with a bit of throttle gunning. It will be a while before we find out due to hols commitments, and then its in for a bit of bodywork for a week, so it will be end of July to see if we can finally solve it! I will post again then - and hopefully be cracking open a bottle to celebrate a long niggle finally resolved! Best J

It might also explain the fuel/rich mixture smell. If it is drawing vapour from the carbon canister (valve stuck open) it would give a rich mixture.

Finally resolved - it turned out it was the vent valve at the engine end, not at the tank (carbon canister end) - the valve was not operating correctly, seems it was sticking open when the engine got warm - hence hot start and rough idle issues, as well as petrol smell - finally fixed, hallelujah!

happy days

Hi Julius Thanks for the feedback, I could see this possibly being a future common problem, Was the said valve repaired or replaced and does anybody the exact location of the valve.

The valve is about 6" to the left of the throttle body, above the alternator, and you can follow the small vacuum tube to where it plugs in the side of the TB. The diagram I posted before shows what it looks like. I don't think they are terribly expensive, but Jules can confirm the price.

Thanks Richard as always, Have you known this valve to be an issue in the past?

It is pretty rare, but yes I have heard of one or two incidences. Spoke too soon about the price - £104+VAT. (They used to be £39!)