1990 Jetta Running Rich

[Brit in Canada]

1990 VW Jetta 1.8GL Automatic, 4 Cylinder, 8 Valve, Gas Engine Type PF, 280,000Km (175,000 Miles)

Digifant II ECU Fuel Injected

Recent relevant parts: Distributor Cap and Leads, Plugs, Air Filter, Injector Cleaner added to fuel.

At unpredictable moments the car will run very rich (over fueling) generally after having come to a stop and accelerating away. This can be with a hot or cold engine. The car hesitates will not accelerate and blows out a lot of black exhaust smoke. The performance improves after a few seconds especially if put into neutral and the engine reved. It will missfire and stutter but finally clear. When running normally the performance is good and when checked the spark plugs look as they should be - light grey/brown and clean. When running rich ALL 4 plugs are black and sooty. The tail pipe is also black and sooty.

Using the genuine Volkswagon Service Manual I have checked the Air Flow Sensor (moves freely and resistance varies proportionally to position) and the Intake Air Temperature Sensor. Both are good.

The Coolant Temperature Sensor is good.

The Exhaust Oxygen Sensor has 60,000Km (recommended to change every 100,000Km)

There is no blockage in the air intake system.

Any advice would be very welcome - Thanks, Roger

[stevethefolkie]

If you have a service manual and a multimeter you can check your O2 sensor for correct operation - are you throwing any OBD1 codes (is your CEL coming on?)? -

[Brit in Canada]

Thanks STF I will check the O2 sensor.

Although I have been doing home mechanics for many years I still have a lot to learn about the new electronics. I do not think this model has a plug for code reading and there is nothing on the dash apart from the usual oil, temp, battery etc. There is a row of 5 of what appear to be LED's below those for the above mentioned functions but they have never come on - is this the CEL you mention?

This is my wife's car but I have been driving it to try and figure out the problem. It may not run badly for a couple of days and then twice in one trip. If the ignition is turned off and the engine re-started it seems to temporarily solve the problem.

I'll let you know how the O2 sensor checks out. I had a couple of old one's that I intended fitting to see what happened but for once I must have thrown them away.

[Brit in Canada]

Having got progressively worse over the last few days the Jetta is effectively undrivable now. I replaced the O2 sensor with a new one but it has made no difference.

I am going through all the checks again.

[Brit in Canada]

Problem turned out to be a blocked Catalytic Converter. I hope this helps somebody else.

Thanks

[alg]

hey brit i own the same car with a bit more kms and its doing the exact same thing for the last two weeks seems to be getting worse. I was just about to change the o2 sensor because its been running so rich i thought it must be it, checked everything else. and it seems to be ok. I would have never thought about the catalytic converter how did u find out it was that Thanks a million Al

[JensAroo]

hey, my car is doing the same thing.

how do you check the cat? does it just look clogged? and why does that make the engine run rich?

i removed my coolant temperature sensor plug and it seems to run alot better. and i checked mine with an ohm meter and it works too.


how do you check the o2 sensor without pulling the plug?

[Brit in Canada]

Sorry Guys. I unblocked the catalytic converter by beating out the insides with a steel bar until it was completely clear.

This made the car run a lot better and made me think it was fixed. I went away at that time on vacation and on returning found out that the blocked catalytic was caused by the main problem and the car returned to running rich intermittently.

I have just bought a used ECM from a wrecker and fitted it today. The problem is still there and with the new ECM it will not idle.

I'll keep you updated if I find a solution and please do the same if you have any success.

Roger

[Brit in Canada]

By the way the catalytic looked OK but was difficult to blow air through. I first realized that it had a problem by the flow out of the exhaust pipe being very weak. My catalytic was filled with a ceramic with small holes running through it. I am told some are different but have no experience with any others.

[Brit in Canada]

I finally found the problem - bad grounds which cause an overload in the ECM damaging it or burning it out.

My ECM is a Digifant II 037 906 022AT KEN Made in Germany. The first working replacement I installed was suffix AH which worked but with a slight misfire and no idle. The second is suffix CS which works better but poor idle. I had to speed up the idle by adjusting the butterfly valve.

The car is now drivable with no complaints from my wife but I will try to fit the correct ECM when my friendly wrecker gets one.

I found this link (too late for me) but it explains things very well.

http://forums.vwvortex.com/zerothread?id=3291139