Infuriating problem!

[CarbTuner]

It sounds like we are close to nailing it. I also have something in mind that hasn't yet been mentioned: the air flow meter and its ancillaries...

Although I am not as knowledgeable as all you guys concerning electronics, I do know that the L jetronic system and all the components that contribute to make it work can be hard to pin point when something goes wrong.

Off the top of my head, the following elements take part in the fuel metering process:

The throttle body bypass can get stuck, but I believe that you have checked that already,
The cold start injector could be leaking,
The main injectors might be leaking too,
The water temperature sensor as mentioned above,
The throttle butterfly position sensor,
The butterfly case itself might need a clean and thorough check,
The air flow meter might be internally damaged as well, or gets stuck in a certain position...

There might be one or two other things that I have forgotten though. I hope I am not mistaken for the above none the less.

And of course, all the wiring that links all these elements to the ECU are definitely exposed to corrosion by now, so are well worth checking too.

I first thought of an air leak somewhere, but the symptoms described clearly assess over fueling...

May the hunt continue, but I have a good feeling that you're closing in!

Cheers

[Billwill]

I doubt this is an L jetronic since the ECUs mentioned are all Motronic. 1.3.

However your suggestions are still valid I think....escpecially the AFM

[Fred555]

Test the engine wiring loom that connects the ECU to the various sensors & actuators often something like the screen wash plumbing (or the bonnet mounted screen wash "T" piece with one way valve also cracks slightly & leaks) inside the bonnet leaks into the loom which corrodes the various electrical joints (splices) inside the loom.

A resistance test will only find the very worst examples of this as the load applied by the multi-meter will not be high enough to show up the (now) bad splices - so instead do voltage tests by connecting to the wires at the point just before they go into the connector plug for whatever engine sensor or actuator you are looking at BUT the device MUST remain connected to the loom plug to apply the load to show up the possibly low (or perhaps missing?) voltage caused by the bad splice upstream - then you can compare the two readings by taking apart the ECU Loom multi plug (undo the screw on the side of the plug & slide it apart) and compare the voltage there OR with really long test leads & a helper do voltage drop test (voltage difference reading should be well under 0.5Volt) by probing the wire (with both ends connected & engine running) under applied load between ECU connection end of wire and far end of same wire in engine bay.

Any high volt drop readings remove the loom from the car un-wrap it find all the splices remove them (without shortening the loom length) remake them (ideally [only if you are good at soldering - a badly soldered joint is even worse] solder then use an adhesive lined heat-shrink tube to insulate the join) then re wrap the loom in non adhesive loom tape.

Do you take any static handling (ESD) precautions while working with open ECU's? Some can be quite sensitive & get worse with age.