chamilka

Hello 928 owners,
After my car (1988 928 S4, stick shift) is warmed up (15-20 minutes), if I leave the car idling, then the idle starts to go up and down. There is absolutely no issue while driving it, the issue only happens while idling (stopping at a traffic light for example). The issue will also not happen when the engine is cold.
What could be the possible source for this issue.
Thanks

Speedtoys

Auto trans?

Mrmerlin

When was the intake refresh done
how old is the MAF

have the computers ever been rebuilt

Was the temp2 sensor ever replaced sensor ever been changed

Petza914

Do you notice that the hunting idle occurs when the cooling fans engage due to engine temp rising to a certain level and doesn't happen when the fan isn't running?

puyi

+ have you cleaned up the engine ground (and all ground in general)

ddire333

Had similar on my 88 (non CAT / auto) only once fully warm and only sometimes it would stall when coming to a stop. I had adjusted the idle mix resistor too low for emission test , one turn back and problem went away.

chamilka

Below are answers to your questions:

1. Current mileage : 184,000 miles

2. Transmission: manual transmission

3. When was the intake refresh done?: On 2013 at 161,000 miles following services were done: Major Timing belt service, Clutch service, engine seals, Valve cover gasket, air filter, oil change, ignition service/tune up, cam sprockets etc.

4. How old is the MAF?: There are no records indicating it has been changed. I have service records since 2003, so I would guess it is old.

5. Have the computers ever been rebuilt?: Yes computer was professionally rebuilt on 9/5/19 @ 182,300 miles.

6. Was the temp2 sensor ever replaced censor ever been changed : Not sure about Temp2. On 4/27/07 @ 133,155 miles there were sensors replaced as a part of full timing belt service. It does not say which sensors. Thermostat was replaced on 11/19/18 at 180,077 miles.

7. have you cleaned up the engine ground (and all ground in general) : No. could this cause idling issues after warming up?

8. Do you notice that the hunting idle occurs when the cooling fans engage due to engine temp rising to a certain level and doesn't happen when the fan isn't running? I will check.

Mrmerlin

Has the O 2 sensor been replaced with a factory part?
I would put in a fresh MAF,
and swap in a new temp 2 sensor,
NOTE cut off the temp 2 connector and put on a fresh one if its ratty.

Did you replace the 2 knock sensors and the hall and CPS and TPS and install new throttle shaft bearings

davek9

If O2 was not replaced, I'd start there they are usually shot after 60k miles, or if run extremely rich for any period of time.
Temp II would be next on my list, as said above ^^^

chamilka

Thanks all. The O2 sensor is a real pain to replace.., I would first replace Temp II sensor which is so much easier , and see if that helps .

chamilka

Replaced the Temp II sensor and went for long drive. I did not notice issues during idling this time. I will post back again when I get a change to drive for longer duration. . Thank you all for your advices etc.