Idle speed "pulsing"
 

I need some help with my 1988 928S4Auto. Since a layup, the idle speed has been erratic. There seems to be 2 symptoms.

1) When I pull up at lights or junctions the idle will hold 900 for a while then falter and die. It will restart immediately.

2) Sometimes it will pulse between 1200 and 1800 at a standstill, like someone is pressing the throttle every second.

Any pointers would be gratefully received.

Thanks

Colin

This problem (on all cars) is typically a mixture problem. If you have access, go to a shop, and have them check the CO output of the exhaust. This will give you an indication of the current mixture setting.

The hunting problem is ususally a symptom of the mixture being too rich, and the car oscillating, trying to adjust...

-Wayne

I would agree... But you may even have two troubles...

Idle hunting above idle (1200 --> 1800) strong chance of mixture problem.

Idle hunting below idle (200 --> 1000) stong chance of a fuel accumilator failiure or on electronic injection cars, the failiure of the idle stabilizer.

Just my two cents.

Good point - if you can, try to monitor the fuel pressure in the system while the car is running. It may be the fuel accumulator, or a check valve in the fuel pump...

-Wayne

Hello

Sounds like far to rich . First to check is the EFI-NTC on the waterneck.

Grüsse

Thanks for your suggestions. I am out of the country at the moment and am waiting for a decent day back home to try out these tips.

Colin

but out of interest, what is the EFI-NTC ? and where is the waterneck?

As written above, you need to check all the maintenance parts; air filter, spark plugs and wires, distributor caps and rotors,
O2 sensor and coolant temp sensor. Hopefully testing these parts will find the cause.

If not then it's likely there is a vacuum or air leak somewhere or the air flow sensor is beginning to fail.

We have found that many of the S4 modela have vacuum leaks at the intake manifold gasket (there is a bulletin for this) and when the manifold is removed many of the vacuum hoses are cracked or hard and leaking. The last three we have done have had warped manifolds too......

Firstly concentrate on the basics and when you can verify that everything is working correctly start looking at air leaks, air flow sensor, fuel pressure regulator and other stuff.

Hello

EFI = Electronic Fuel Injection
NTC = Negative Temperature Coiffiecient

This descripes the important temperaturesensor for the EFI Fuel calculation. That part sits on the waterneck. Thats the part where the thermostate sits in the mid front from the V Block. Its a bit beside on the driverside.

At least the US V8 crowd say waterneck to that part

There is also a seperate Temperature Sender for the instrument.

If you visit some pages you will find pictures and descriptions.

Top sites:

http://sis125.berkeley.edu/928
http://members.rennlist.com/v1uhoh/index.htm
http://www.nichols.nu/tips.htm
http://addicted.to/928/

Not so top:

http://www.rennlist.org/techarticles/928/928enginefaq.htm
http://www.kondratyev.com/porsche/


Grüsse