Unstable acceleration at specific car speed

Hello

I own VW412 with D-Jet, Automatic.
Since a couple days before, the car has problem which is unstable acceleration at specific car speed (0-40kph). The unstable means that Engine looks "Breathing". Car moves but not smooth.
In addition, this symptom can be observed at wide open throttle. If I keep pressing the gas pedal fully, after 3,4 seconds acceleration become normal and Car speed increase continuously.

When car running on the uphill, this symptom more frequently happen. This problem happen at all engine temperature band but when it is hot (over 70gedC) the frequency of the symptom become lower. Strange thing is this happen only at the specific car speed which is below 40Kph. This symptom also may happen when car start moving from stationary condition.

The following was done for solution but it didn't work.
-Replace spark plugs
-Check and adjust Ignition timing
-Replace distributor cap and rotor
-Replace ECU

As mentioned the above this problem happen at NOT all speed. Happen at only specific speed.

I am planning to replace the injectors. However, if the injectors are the root cause, I guess the problem happen at all car speed band. (not at specific car speed)
I also suspect "Vacuum leak somewhere" or "Automatic Transmission".    


I would appreciate it if you could give me any tips for solution.

 
Best Regards,

JUN     
 

Hi Jun,
you should control the operation of the TPS (throttle-position-switch).
This unit provides the engine with extra fuel when accelerating.

For a first test just switch ignition on. You then should hear the injectors click when opening the throttle by hand.

Don't forget to switch ignition off after the test, the ignition-coil could get damaged.

Regards
Norbert

Hello Norbert,

Thank you for your advice.
I will  do it.

Again thank you,

Jun
 

Hello Norbert,

As you kindly suggested I checked and replaced the Throttle Position Switch (TPS) with the adjustment found on the internet.
It was found that some oil was inside the TPS as shown on the attached pics. 
I guess this is from blow-by gas. Can you please advice me again is the oil in the TPS is normal ?

BTW, after the replacement, the problem was disappeared but after 30 mints from engine start the problem come back. However, the level of the symptom have been much improved.
In order to eliminate this problem completely, do I better to adjust the TPS setting again with more wider idling position ? 

Other suspicious parts are injectors, Trigger contact in the ignition distributor.


Best Regards,
JUN
 

Hi Jun,

there should be no oil or other liquids inside TPS. Please clean carefully and dry with a paper. Especially contacts of continuity switch. Then drive a round without TPS being connected and verify whether your problems are gone. Then reinstall and adjust TPS according to jetronic.org/en/d-jetronic/maintenance#Leerlaufkontakt

Hello Dr. D-Jet,

Thank you for your advice !
I will do it and come back.

Best Regards,

JUN