Surging idle 88 3.2 carrera

[28121912]

Ok saga still ongoing and have done further investigation.  Car exhibits the following

- starts from cold ok and idles at fast idle , gradually adjusting idle speed down to 950rpm 

- once engine has some warmth the idle speed then drops almost to stall then recovers and does this about three or four times before the idle stabilizes, sometimes the car stalls.  The car does this every 45s or so.

- on the road the car drives and pulls well but exhibits the surging idle issue if you sit at lights etc for a period of time. Car sometimes stalls when you dip the clutch 

reluctant to start hot , when it does it runs rough initially then clears if you rev it with evidence of black smoke from exhaust until it clears itself

have done following so far

- co levels are around 5% 

- exhaust smells like its running rich

- plugs are black when removed

- changed ICV ..... no effect

- checked spec of CHT / AFM all seems ok to Bentley manual.

- cleaned AFM very little dirt or deposits visible, flap moves freely

- bridged the CHT loom connector , no effect , idle continues its cycles of falling rising every 45s or so.

 

I am fairly clear it’s running rich maybe choking itself which system tries to cope with trying to stop it stalling.

Any further ideas advice now I have done further investigation

 

 

 

[NigelJ]

It does sound very much like a CHT sensor issue even though it seems to check out. I think I would try get hold of another one and do a comparison. Perhaps someone here has a spare you could borrow?

[SP72]

Sounds like the mixture is off?

Might be worth an hour of a specialists time to check mixture with a gas analyser?

Stalling is one of the symptoms of incorrect mixture.  

Have you checked for vacuum leaks?

[28121912]

I by passed the CHT by bridging the loom connector and it made no difference. I have a new CHT but reluctant to just swop bits without being sure of root cause.

Checked for vac leaks and replaced any suspect hoses .... no effect.  CO meter shows it runs rich not lean so unlikely to be vac leak

[28121912]

Ok just swapped CHT with new sensor no effect .

[sopor]

check the throttle position switch with an ohmmeter?

[NigelJ]

There was a thread a while ago about a similar problem. In the fuel supply circuit there is a pressure regulator and a damper. In this case the diaphram in the damper had failed and was allowing fuel to be sucked into the vaccuum system and so into the motor. Try disconnecting the vacuum line to the damper and see what happens.

[28121912]

1 hour ago, NigelJ said:

There was a thread a while ago about a similar problem. In the fuel supply circuit there is a pressure regulator and a damper. In this case the diaphram in the damper had failed and was allowing fuel to be sucked into the vaccuum system and so into the motor. Try disconnecting the vacuum line to the damper and see what happens.

Thanks , this would explain mixture being rich and hot start issue as fuel vaporises at lower temp if system not retaining pressure.  Another one for me to investigate

[28121912]

Ureka....... so disconnected vac pipe to fuel regulator and blanked both off started car warmed up and ran perfectly ........ checked regulator and fuel coming out of vac connection so looks like the diaphragm has broken.

So new fuel regulator to be ordered tomorrow and then see if that rectified the mixture running rich and reluctance to start when hot.

Nigel J big big thanks for the advice.   

I did think early on in this saga that the regulator may not be retaining pressure hence reluctance to start hot due to fuel vaporisation but did not fathom the diaphragm allowing the motor to draw additional fuel through vac pipe.

[NigelJ]

Isn't it great when a plan works! Very pleased that you have found the issue. This site is a great resource.

[28121912]

Yes just need to source a new regulator and fit to prove the root cause .

 

[28121912]

Fitted known good regulator and everything running fine now.

thanks for all the inputs , got there in the end