Hunting on idle?

[Growler]

Guys

My 944 Turbo has developed an intermittant fault. Whereas it used to idle very smoothly at approximately 950rpm, it now has an intermittant idle problem. The engine hunts from approx 850 up to 1050 and cycles up and down. If I hold some revs it runs smooth again but as soon as you let it settle back to idle it hunts. Drive the car for a while and it cures itself, next time you stop it may hunt again?

The boost gauge and oil pressure gauge also register some movement in synchronisation with the idle speed.

Has anyone experience a similar problem in the past?

 

[Fen]

My cab used to be bad. I cleaned the idle control valve with carb cleaner (actually i think my mate did it now I think of it). Anyway it's now been fine for 40,000 miles since.

 

[Growler]

Thanks Fen

I did put a petrol treatment (STP) in the lat tank, but it hasn't fixed it so I will hit the carb cleaner next. Is the idle control valve easy to get to?

 

[Fen]

It's just under the inlet manifold between ports 1 & 2 on an S2. On my Turbo it's on the garage floor [8|] so maybe one of the other boostie boys can tell you where it is more conventionally situated...

 

[944Turbo]

Also worth checking the vacuum pipe is connected under the throttle body these seem to come off regularly and the air leak there may give strange idle symptoms.
Tony

 

[richthomas]

Last year mine did a similar thing, sometimes it would hunt so much it would actually die. It turned out to be the throttle position sensor in the end, so it might be worh having yours checked. You can do it yourself, and the info is out there to do it (Haynes, factory manuals etc) Apparently they can fail to read a proper resistence when closed, so tell the DME the wrong signal. Worth a check if the Idle valve doesn't give you any joy.

 

[Mikie_gb]

My beast was doing a similar trick but to add to the excitement it was running on, on release of the throttle, at speed. ie lift off the throttle and it was taking 3 to 4 seconds for the speed to stop dropping. Hair raising in traffic. On doing a search for likely culprits, three possibilities arose:

1. TPS
2.AFM
3. Idle compensation valve.

The TPS got a clean bill of health, when checked with a volt meter (I have the test proceedure somewhere) and the adjustment checked. I then moved onto the AFM and found this site:

AFM Mod and followed the proceedure, working on the principle that if the AFM was duff, a new one was on the cards (£280+ vat) so I didn't have much to lose.

Any how, it worked, and the proceedure was fairly simple, as long as you do nothing else to the AFM. Mucking about with the rest of the settings really is beyond the realms of hack fingured modifiers.

In due course the Idle Valve will be cleaned up and if necessary replaced, but right now the two problems have been greatly reduced.