Cold idle problem?

[Guest]

Started my car up the other morning and its not liking ticking over when cold. It starts absolutely fine but once running its as if its going to cut out as the revs drop to almost nothing, then the needle pops back up to normal, then drops again and repeats the process. It drives normally when warm or cold, and once its warmed up it idles perfectly.

If anyone has any idea what could be wrong any surgestions would be massively appreciated.

Cheers

Will

 

[j16jrf]

i too have this problem, i just let the car tick over for a good 5 mins before i drive it in the mornings.

not a cure though so i would like to know the answer too please.

I have replaced the idle control valve and this is still happening so nothing ot do with that then

 

[iainfuller]

I had a similar problem with mine which started last year but finally came to a head earlier this year when the car refused to start one day - although at the time I didn't appreciate that the two were related. I'd already replaced the thermostat so knew it wasn't that, anyway after testing the obvious ignition components my indie diagnosed a faulty engine temperature sensor - I think the part was about £15 - which cured the starting and idling problem.

Given my experience I would suggest you don't try and drive around the problem as it will leave you stranded eventually.

 

[sawood12]

Does this only happen on particularly cold mornings? This is prbably a long shot but is a quick check and you never know....It could be that your throttle cable stop is set so that the idle microswitch is just being pressed and when it is particularly cold the contraction of the cable might be just pulling the throttle off the Idle microswitch. I had similar symptoms when I first got my car and was fixed by adjusting the throttle cable stop position. With the engine off just check the throttle cam by rotating it off the stop and listen for the click of the microswitch. If there is no click either your microswitch needs replacing or your cable needs adjusting. On my car there is a very small amount of slack in the throttle cable so I know that the throttle return spring has pulled the throttle all the way back to the stop, if it is taught then it could be that it doesn't need to contract much to pull it off the switch.

 

[Axl Rose]

Same problem here..
Iwas just changing the antifreeze yesterday and thougt it would be a good idea to replacethe thermostat as well..but..is there some really very special tool to get the snap ring out or has it really been in there for the past 21 years(which wouldn't suprise me)..got a fresh antifreeze but the thermostat remained the same..[&o]

 

[j16jrf]

cold start valve sounds like a good idea.

where is this located and how easy is it to change?

 

[jr2015]

The cold start valve is under the inlet manifold on my `83 model.Its resistance can be checked with an A/Vo meter if the connector is unplugged; it should be 20-55 ohms. You can get an indication of whether its working from the "economy" gauge on the dash.Start up the engine from cold and watch the gauge, the needle will move to the right over the next 30 seconds if the valve is working .
The inlet manifold needs to be removed to change it.
jr.

 

[zcacogp]

ORIGINAL: jr

You can get an indication of whether its working from the "economy" gauge on the dash.

Economy gauge on the dash? What's that then? (I have an oval-dash model ... was this specific to the early-dash ones?)


Oli.

 

[944Turbo]

I think it may have only been there for the first oval dash cars - and the turbos have a boost guage there,
Tony

 

[jr2015]

Its a vacuum gauge in an early dash (mk 1 model, registered Oct`82) described and labelled up as an "econometer", a ridiculous concept in a sports car.However it is useful as a vacuum gauge, and as the cold start valve is actually an auxilliary air valve, the vacuum is zero when the valve is open at the beginning of a cold start, and builds up as the aux air valve closes.While the valve is open the fuel injectors squirt more fuel in to maintain correct mixture ratio.Evidently ithe gauge was discontinued for later models between mine and the introduction of the oval dash.
jr.

 

[hotblack944]

My '86 Lux oval dash has it. Can't say I've ever payed much attention to it...[]

Jim.

 

[AndrewS]

ORIGINAL: will944t

Started my car up the other morning and its not liking ticking over when cold. It starts absolutely fine but once running its as if its going to cut out as the revs drop to almost nothing, then the needle pops back up to normal, then drops again and repeats the process. It drives normally when warm or cold, and once its warmed up it idles perfectly.

If anyone has any idea what could be wrong any surgestions would be massively appreciated.

Cheers

Will

Hi Will,

Idle stabilisation is achieved using an ISV (Idle Stabilisation Valve). This is controlled by the Motronic DME ECU that depends on numerous sensory inputs to enable the correct output to be sent to the valve.

In diagnosing this problem, first check all vacuum lines and intercooler pipe connections (if 944T). Pressure testing the inlet side is worthwhile to eliminate anything here. Assuming all are OK, you then need access to a good multi-meter to diagnose the ISV and ECU.

If your probem is internittent, it may be the cold start sensor or the wiring from it to the DME. There could of course be other causes - but that should give you some ideas to start with.

Regards,
Andrew

 

[JayKayEm]

Had an identical problem, used the forum search facility and stumbled accross this thread. Thought i'd add my experiences for the archive.....

1991 - S2 (2.5/2.7 similar)

When starting up (started fine) from stone cold to mid-temp (approx 70c) engine would hunt like crazy from 600rpm to 1300rpm oscilations. This would last for 20 seconds, until one final almost stall (450 rpm) and then perfectly flat-line at 800.

This was reproduceable every time, to the same pattern, for the same 20 second duration. It could be overridden by the throttle and be rock steady with a foot controlled 1000rpm.

It was like the ISV was trying to stabilise revs, but responding too slowy - sort of an ooops too much, oooops too little fashion. ISV = Idle Stabilisation Valve or Idle Control Valve. A rotary spindle type valve that provides a bypass air route past the throttle butterfly - hence raising engine speed.

With most other early 1990 Bosch systems, you can clamp the ISV inlet or outlet hoses to eliminate the ISV. When clamped, the engine will either stall, or struggle at a low 600rpm. When you release the clamp, there will be an audible 'woosh' of air internal to the ISV and revs will rise to what ever they should be. ISV test complete. However, Porsche have burried the ISV under the inlet manifold and it is a swine to get to.

As with all things, I eliminated the easiest stuff first.

First of all I checked the throttle body switch for the 'click' when the butterfly closes at rest. All OK. Then I detached all vaccum lines from the inlet manifold and inlet tract. No difference. Then I disconnected the Fuel Tank Evap Canister system from the inlet manifold, plugging all ports. No difference. ISV next.....

I have read the inlet manfold on/off discussions for ISV removal. I started to take the inlet manifold off, but the inlet manifold and throttle body is water heated so I stopped halfway as I didn't want to disturb the cooling circuit. I have heard they are pigs to bleed. Now, it may be that I had success with the 'manifold on' method as I have arms not too disimilar to pipe cleaners. It's a do-by-feel job as well. Still, got the ISV out by removing the air flow meter and inlet tract; plus cutting arms to pieces.

I flushed ISV rotary valve with brake cleaner until it was free to move by my little finger. So much black gunk came out it was un true. Had a quick play with a multi-meter; the windings were not open circuit so that was good enough for me. I had to replace some of the quick release hose clips for more traditional jubilee items as I dont have the quick release tool. Ho hum.

Now, totally messing up the elimination process, I also replaced the coolant temperature sensor (the one for the ECU - not the one for the dash!) at the same time. My tools were out, the new part was there, so I just went for it - which means I can't honestly say ISV or CTS, but as both are 15+ years old, a new CTS would not go a miss.

Anyway, problem since has dissapeared. Eliminated. Gone.

I can only attach one photo per post, so the next couple will be from me. Sorry for the poor quality - only my old camera phone gets greasy!

Good luck to anyone else with this common Bosch problem; replicated on 911's, Vauxhalls, BMW's etc........

 

[JayKayEm]

Note my arm!

 

[JayKayEm]

ISV coming out...

 

[JayKayEm]

ISV removed and in vice for cleaning....

 

[xenon]

Mine hunts a bit too on idle - I assumed it was the 'cammy' nature of the engine but perhaps not. I going to try cleaning the ISV on Saturday as per James's post. Saves me having to go shopping with the missus []