Fast Idle Speed

When driving home last night I became aware whilst sitting at lights, the normal idle speed of around 600-700 rpm had risen to 1100 rpm. I "blipped" the accelerator and the needle returned to the normal position only to then steadily rise to 1100 once more. This condition has repeated itself this morning so anybody out there know why this should happen. Met Black C4 MY2000

This is usually caused by faulty lambda sensors or an intake air leak. OPC or specialist can check the operation of the O2 (lambda) sensors with the PST tester. The most common source of air leaks is the oil return bellows splitting - this will normally manifest itself as a noticeable oil leak from the nearside front of the engine - the part is only a couple of £s but OPC's like to charge about £1000 to fit it! It can be done in about an hour by a good specialist however. 1 other possible cause is the MAF sensor (air-flow meter) but these typically tend to cause problems further up the rev range. Hope this helps. Ian W

Thanks Ian, I haven't noticed any oil but now you have given me a clue where to look I will investigate further. From your description of the location of the bellows, not an easy place to check? RB

Hi ArBee My 1999 C4 is just having its 3rd bellows fitted (in under 12 months!) as I type this so it seems they fail regularly. My local Porsche specialist in Manchester even tells me they replace them as a service item now. The bellows itself is relatively easy to spot with the car on a ramp but as it is almost on top of the front of the engine it's a difficult item to see with the car on the ground or to get at for replacement. If it has split however, oil will run down the front of the engine and get blown back along the bottom of the engine towards the rear of the car. It will inevitably get on the nearside exhaust manifold and smells dreadful! If you cannot see any oil under the engine and you are not getting any burnt-oil smell, the bellows is probably OK and I would definitely get the engine checked either by a specialist with a proper Porsche tester or by your local OPC. The tester will pick up on any sensor irregularities and identify any idle and/or emissions-related problems you may have. I would also suggest a thorough visual inspaction of your engine. Check for any other oil leaks - where oil gets out, air gets in - as these can cause odd idle problems and also check for any vacuum pipes, induction pipes etc. that may have come adrift. Good luck Ian W PS - Craziest thing to have caused idle problems on my car was the oil filler pipe - it had snapped in half!!

Ian, again thanks. Funny you mention a smell, I have noticed a smell but I wouldn't call it a burnt oil smell, more of, and this is going to sound odd, but a 'hot' smell, the kind of smell say a new part might omit as the factory coating heats up for the first time. Sounds weird I know. However this smell has been evident long before the fast idle started. I will take a look tonight to see if there is evidence of oil. Thanks RB

Dear Ian, No oil, not even from the RMS (Now I have my fingers crossed and I am touching every bit of wood in sight) and the strange thing is when I started the car up to put it away in the garage, the revs stayed at the correct idle speed without increasing at all. I would have thought that should there be an induction fault that this would have changed eventually. I reversed the car into the garage, came to a halt and the revs went to 1100. I raised the revs again whilst stationary and they returned to 700rpm and remained there. Completely baffled. Car goes away for winter soon so I might just live with it and get it checked as you suggest at the next service. Thanks for taking the time to try and help. RB