stalling

[docdic]

Hi,
I have a May 1993 registered 911 Carrera 2 Cabriolet with Tiptronic transmission.
It has been off the road for five years and has just been brought back into use (hooray!) after a major service at 79,600 miles.
For the first couple of hundred miles it performed (apparently) flawlessly. However, after warming up, it now stalls at traffic lights, etc., and at very low running speeds, although the performance still appears to be normal at anything above slow running.
I am now a long way from the people who serviced it and will have problems getting back to them for a couple of weeks or so. I am told that there is "an adjustment on the throttle" - which I can't see and, in any case, since it is all computer-controlled, I wonder whether it is really more than just that.
Can you, please, tell me what is likely to be the problem and is it something which I can sort out easily or does it need full electronic diagnostics?
With many thanks...

 

[Dave Wilkinson]

Richard,

Developing an untrustworthy idle after a few hundred miles smacks of oil contamination of the Idle Stability Valve. This is most commonly caused by overfilling with oil - an oh so common problem even with those who should know better. Have you checked the oil level under the right circumstances?? What does the oil level gauge read??

If it IS an ISV oil contamination it's not such a big job to fix it. It means removing it from the intake and cleaning carefully with carb cleaner. If the oil level is the cause you need to get some drained as a matter of some urgency.

You are correct in assuming it's all computer controlled. There are 2 microswitches on the throttle mechanism - one for wide open throttle (WOT) and one for throttle closed. If the throttle closed switch is out of adjustment the DME will not switch to idle mode correctly and the engine will stall. There is no other adjustment for idle speed.

It's likely that a Bosch "hammer" might be able to tell more but you'll need to find a decent specialist who will have one.

Regards

Dave

 

[docdic]

Dave,

Thank you very much for your very speedy reply. You are right, of course - I had, inadvertently, overfilled with oil - believing that it was low when it showed red on the gauge and at the lowest notch on the dipstick. I ran the car for about a hundred miles before realising my appalling error and had the excess drained out a couple of days ago. It is now half-way between the two notches on the dipstick.
I certainly won't be making that mistake again...
How and where do I find the ISV, what does it look like and does it have to be dismantled to clean it?
Regards,
Richard

 

[docdic]

Dave,

I found the ISV by "google images-ing" it. I have done the deed - the valve was completely stuck and popped out as soon as the carb cleaner went in. I have given it a good clean out and the engine runs smoothly now at low speed and at rest.

It seems likely to me that, if all that gunge got as far as the valve, then it must be coating up areas before there - should I be thinking of cleaning out the rest of the intake system and, if not, would it be a good idea, anyway, to have another go at the ISV in a little while?

I do hope that I haven't destroyed the catalytic converter... How likely is that?

Regards,

Richard

 

[Dave Wilkinson]

Richard,

I don't think there's any dfanger to the cat - that's on the exhaust side so pretty much everything gets burnt before it gets there. The oil system naturally vents to the induction anyway - it just vents a bit more when it's overfilled. The vented mist is unlikely to gum up much else in the induction system - it's just that the ISV is a little vulnerable. If the car idles OK now I'd be tempted to leave well alone for a while and see how it goes.

Regards

Dave

 

[docdic]

Dave,

I am very grateful to you for your help with the problem and reassurance re the cat.

Although the slow-running did appear to be fine yesterday, it is a bit unpredictable today, having stalled once and ticked over rather slowly on other occasions without stalling. Perhaps there is still oil in the pipe to the ISV and I should give it another clean in a day or two...

Should the tick-over rate be totally constant? It seems to audibly speed up and slow down in a regular cyclical fashion with the revs and oil pressure oscillating slightly on the dials (though nothing like my first ever car - a 1938 Vauxhall used to do when "hunting", donkeys years ago).

Regards,

Richard

 

[Dave Wilkinson]

Richard,

In an ideal situation, yes, the idle should be pretty stable. The DME, which controls such things uses sensor inputs from a variety of places in order to understand the state of the engine and the environment and it may be that you have a problem elsewhere too. It's still possible that the iSV is less than perfect too and I think your idea to cleanb it again in a day or two might reveal more.

It's important to identify the conditions under which the idle is varying. First start from cold -or after a run?? When started from cold the DME specifically ignores the input from the O2 sensor (it is only designed to work when hot) but 30 secs to a minute after start up the input will be recognised and the DME will adjust it's mixture settings accordingly. If you have a faulty O2 sensor you would notice the uneven idle as soon as the DME switches to "O2 aware" mode.

If the uneven idle is only an issue when the engine is hot there's a chance you may have a problem with the cylinder head temperature sensor. If the DME thinks the engine is cold when it's not the fuelling will be wrong.

Lots of possibilities I'm afraid - and lots of potential solutions!!

Regards

Dave

 

[Alexander_Drobik]

Just a thought - may be worth disconnecting the battery for a few minutes, reconnect and then go for a 15-20 minutes run. This will reset the dme and adapt a little to your driving style. Sometimes, the 'system' gets itself into the wrong adaption. If you have done this or it makes no difference I would visit your dealer/specialist and have them do a check/Bosch hammer test etc.

 

[smallspeed]

Im no 964 expert so might be talking out the back of my neck, but its worth checking the idle switch on the throttle linkage; Ive seen this a lot in merc's where it can often be atributed to the 50p secondary throttle return spring that pulls the linkage down onto the switch taking up any slack in the cable, etc.. If this is broken/missing on a merc then the linkage won't depress the switch, so the ECU doesn't trigger into its idle state

A visual inspection should reveal that all the components are present and correct, and when its hunting might be worth just pushing the linkage onto its full closed stop to ensure the switch is being depressed. If this stops the hunting, then a quick adjustment of swithches/replacement of any missing springs may fix it

ben

 

[docdic]

Dave, Alexander, Ben -

Thank you all very much - I am extremely grateful to you for your advices.

In fact, I did a long and quite hard airport run yesterday, since when it has been faultless - the tick-over is smooth and regular and there is no oscillating of the rev counter or oil pressure needles now.

I shall do nothing more for the moment but shall keep in mind what you have suggested and investigate it along those lines if it should play up again.

Once more, thank you very much indeed.

Richard