Engine Hunting at Idling

Hello 

I own VW412 with D-Jet 1.7L, Automatic.
Suddenly, Engine hunting occurred at idling regardless engine temperature.
The hunting occurred with the both D and N at idling.
Other than the hunting, engine performance is OK.

I cannot find any air, vacuum leakage. Aux air valve has been replaced with new 6 month before. Ignition timing confirmed as OK.

I also checked the ECU by VW1218. Test no.7 and 15 are failed.
Test No.15 is for Throttle switch. I have not checked the switch yet.
Is there any possibility that the switch could be the problem ?
(However, the test result is the same when I did the test last year)
 
Could you please advise me what the possible cause is ?

Thank you in advance for your help !!

Thank you,

JH

Hi 'JH',
we prefer to use our first name here instead of unpersonal abbreviations.
You wasn't told this before, but better late than never.
I remember the type 1 beetle with automatic had a semi-automatic with a clutch operated by vacuum air. A vacuum-air-reservoir was in the right back fender.

Does the 412 have a similar system? I remember a car with rotten reservoir, hunting at idle...

Regards
Norbert

Hi Unknown,

provided that your tester works correctly it would mean that your ECU is not working properly and acceleration contacts in throttle switch also not.

Please also check you MAP sensor 0 280 100 049 for vacuum leaks.

 

Hello Norbert,

Thank you for your advice.
I don't think that 412 has the similar one as you mentioned...

Best Regards,

JUN 

Hello Dr-Djet,

Thank you for your advice.
I will check the vacuum leak from the MAP sensor.
By the way, malfunction of the throttle switch could be the cause of the hunting or unstable idling ?

Thank you,

JUN
 

Hi Jun,

if hunting appears while cruising, it can well be throttle switch. Just remove its connector and try then. If hunting is gone, you have problem identified.

Hello Dr-DJet,

The hunting problem is occurred at idling only.
Normal driving behavior is no problem....

Any other advice would be appreciated.


Best Regards,

JUN
 

Hi Jun,

that is odd as none of the 412 engines has over-run shutoff. That could cause a high rpm hunting on idle.

Which ECU do you have and how much does rpm vary?

Hello Dr-DJet,

P.No. of the ECU is Bosch 0 280 000 037 or VW 022 906 021E.

Idling speed (rpm) has been set 1200 rpm- 1500 rpm. (Otherwise could be stalled)
However, as kind of nature of this car, idling speed is sometime not stable.
This does not mean "Hunting", it means for example, today 1200rpm, tomorrow 1500 rpm or morning 1200rpm, afternoon 1500rpm, something like that.

But now he has the hunting problem.

Thank you,

JUN
 

Hi Jun,

that is correct ECU and it does not have over-run shutoff. Over-run shutoff can cause huge rpm variations if you have large vacuum leaks on manifold.

In your case it sounds like AAV causes that problem. Otherwise you should have stabile rpm in idle and not vary from 1200 to 1500 rpm every day. Of course also a leak on manifold and vacuum hoses can cause such a problem. We use smoke tests to verify in workshops.

Hi Dr-DJet,

Thank you for your advice.
I will disconnect electric wire from the AAV and check if the hunting occur.

I also need to do Smoke test but I don't have its tester.. I will think how to do...

AAV for my car is on the picture.

Best Regards,

JUN

Hi Jun,

yes this is the special VW and Porsche 914 AAV. It is known to be troublesome and tends to stick.

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

Hello

I blocked the AAV by cork according to your advice.
However, the hunting still occurred.

Common symptom when the hunting occurred is the following.
i. Engine start is OK
ii. Until about 1 minute from engine start, engine run is perfect.
iii. After about 1minute, engine speed drop automatically and then the hunting start.
The above is the same with or without AAV connected.

Voltage for ECU is OK. Main relay and fuel pump replay have been replaced with new 2 months ago at Tuev inspection. 

I am going to change air hoses with new one as next attempt.

Why always 1 minute later the hunting happens, that I have no idea.

Do have any other advice ?
 
Thank you,

JUN

Hi,

an engine in cold start runs very rich. Have you measured exhaust gas in idle and adjusted rheostat on ECU?