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.