Audi 80 B4 ABT soots when cold?

Question

Hello, my dears,

I've had a problem with my 80s for a while now and I'm slowly losing my clue where to look for the problem.

I have an Audi 80 b4 with the 2.0L 90hp engine (ABT, MonoMotronic), 260,000km. The problem started a year ago:

The engine starts without any problems, regardless of whether it's warm or cold outside. However, it immediately becomes rough when running. Deep black smoke comes out of the exhaust, and it smells strongly of gasoline. It responds well to the gas, but sometimes chokes when accelerating hard. Fuel consumption is also, of course, much too high. It looks as if the engine isn't burning the fuel completely. The problem is more noticeable the colder it is outside.

But once the engine warms up, the problems disappear. The engine runs perfectly, no soot, no (barely any) gasoline smell!

Now to what I have already done:

New coolant temperature sensor
New spark plugs
Different ignition distributor installed (used), due to possibly faulty Hall sensor
Compression test OK
No unusual changes in the oil level
Timing belt replaced + ignition timing checked
Other complete injection unit (used) including intake air temperature sensor, throttle potentiometer, idle speed control.
New “carburettor flange”
Spray pattern of the injector looks good, no dripping
New lambda sensor

None of this solved the problem (it felt like it improved it a little for a while). During one of these failed repair attempts, the motor suddenly seized up – motor gone!

So I looked for a new engine in the classifieds and actually found one. The car had half the mileage on it as mine. And it was still installed, which meant I could test its functionality before buying it. The engine ran great, so I bought it and installed it in mine. I transferred the entire engine and exhaust system from the other car to mine. The injection unit, which I had already replaced once as described above, the transmission, ignition coil, and engine control unit remained from my car. I started the engine, and the problem was solved!

After about five months, it slowly started again. Shaking, sooting, and a gasoline smell until it warmed up. So I installed a different engine control unit (used). And even after that, the symptoms went away for two months. Then, who would have thought it, it started again, and so far, a new ignition coil and new ignition cables haven't fixed it.

I really don't know what to do anymore and hope someone can give me an idea or a tip?!

Thank you in advance!

nobodyathome · Answer

The only thing that hasn't been changed is how I see the cabling. So I would test the cabling thoroughly

scusselbudd · Answer

Defective cold-running regulator that hopelessly overgreases the engine in the warm-running phase. Possibly also measures your Lambda false values

verdraait · Answer

The usual suspects are sick injectors.

ixxiac · Answer

Hello

Check and/or relocate cable set for NTC II to cylinder head. Or, instead of the NTC, you mount a pot that you put in the interior. The ohm values can be measured from the NTC or taken from the technical data otherwise around 2000 ohms are about 20°C, 100 ohms are about 100°C. Operating temperature is around 200-300 ohms

Bosch Messtechnik in Plochingen has a "tester" with 6 meters of cable and pot for D, L, KE Jetronic. The device shows NTC I and II value but also short circuits or mass problems. It was integrated into the Pocketmaster from over 80s.

http://www.google.com/search?q=NTC+II+Motronic+Ohm value

While cold-starting or hot-running, the injection system works primarily after NTC II with correction of NTC I (suction air) and specifically until at least 0.5 volt signal level of the lambda probe is present, as long as the engine remains fat in cold-running mode, a hot-running regulator to Oberland/Mangold has been installed (GAT, HJS, Eberspächer)

The Monojetronic have a "gel" for heating the suction tube directly under the injection head, which is separate with its own NTC temperature sensor and heating circuit control. Many mechatronics confuse NTC II cylinder head with NTC suction pipe preheating. At VAG, the ones in the cylinder head neck are opposite (plastic neck with spring clip) or next to one another (aluminum neck)

Used lambda probes land in old iron. Lambda sensors swallowed (80000km at short distance, 160000 at long distance) and are "traumatized" by the extension. Lambda probes are quite sensitive if they are not detachable with a long ring wrench without beating the "bread". Drop new unpacked jump lambda probes from over 50cm height on workshop floor > is not installed

Simply incorporate a heated Bosch, Temic, NGK or Würth "universal probes" which have been robust planar probes since 2010.

https://triscan.dk/de/why-the-use-of-universellen-lambdasonden-ein-bad-ideeist