Engine Hunting at Idling

Hello Dr-DJet,

According to your suggestion,  I checked AAV and found the followings.

When I was checking it, I found insufficient electrical contact of the wiring harness.
This harness lead to the Grand(Earth) for the fuel pump (and also for the ECU?)
When I touched the faulty area of the harness, Engine speed changed and finally stalled by pushing it repeatedly.
When I closer looked the faulty area, it was easily recognized that the harness was almost cut and disconnect from the connector. I fixed the faulty harness. 

Next, I killed the AAV by disconnect positive cable. However, the hunting happened.
This means the above the faulty harness is not the cause of the hunting.

Symptom of the hunting is the following.
1. Engine start at cold condition. Starting is OK.
2. About 1 minute since Engine start (or until Engine speed is reduced automatically) Engine speed at idling is stable, no problem.
3. After 1 minute since Engine start (at the same time when Engine speed get down), the hunting start.
4. While driving, after Engine oil temperature is above 80deg C,  the hunting is almost nothing.

I measured resistance value of the head temp sensor. The senor has been installed with Extension nut  (May be the nut make slower warming the sensor up)
I didn't remove it and measured resistance value by multi meter.
The result by Engine temperature the following.
 30deg C : 969 Ohm
 35deg C : 360 Ohm
 40deg C : 250 Ohm
I think this is out of spec.

If the head sensor is broken, lean combustion is occur and this lean make the hunting ? 

It would be highly appreciated if I could have your advice again.

Thank you,

JUN 

Hi Jun,
you can't 'kill' the AAV by disconnecting the cable.
Doing this means the AAV stays open and gives the extra air needed dururing warmup.

You have to seal the air hoses to disable the AAV.

Without having looked for the air temp sensors spec. I would say the values are uncritical.

Disable the AAV and look what happens...

Regards
Norbert

Hello Norbert,

Oh, Ok. 

Do you mean that I should disconnect the air hose to the intake air distributor and seal the hole of the distributor in order to not extra air sucked ?

If so I will do so and come back !

Thank you for your advice !!

JUN 

Just seal the AAV - by a plug, cork.

Don't touch the other lines.

Regards
Norbert

I recently had very similar symptoms with an air-cooled engine.

The suspicion was also based on the AAV, but when checked everything was functional and power supply o.k.

The cause finally turned out to be a relay for the power supply of the AAV that dropped out at times. If the relay intermittend failed, the electrical heater failed and the additional air caused the idle speed to increase. Since the relay has been changed no further error occurred .

Regards, Guido

Hello Guido,

Thank you for your information!
I will check the relay as well !!

Best Regards,

JUN