WOT is going on?

[Marcus Oxer]

Dear Listers,

I have a new piece of evidence on a long standing problem that I'd like to run past those who know more about such things. I have been struggling with a light-load misfire on my 1990 Audi Coupe 2.3E (engine code NG, CIS-III injection) for nearly two years now, and have spent over £1000 in failed diagnoses and adjustments during that period, all to no avail! The symptoms are hesitancy and jerkiness under light throttle openings, at any speed, making driving smoothly a difficult task. I am told that everything obvious appears to be in spec (ignition timing, mixture), and apparently the sensors are giving sensible voltages to the injection ECU as determined by measurement at the ECU interface. I have had the fuel system pressures checked, the injectors replaced and the airflow meter replaced (albeit with a second hand one). I have also fitted a new oxygen sensor. Both idle and WOT switches seem to be working fine.

Recently, I have discovered something that seems to cure the problem, but only on a temporary basis, and I found it quite by chance while fiddling under the bonnet/hood.  Working under the bonnet, if I manually open the throttle just slightly (say to 1000-1200 rpm), the engine starts to hesitate and buck in an erratic manner, corresponding with the driveability symptoms I experience. If I then, whilst still holding the throttle slightly open, close the WOT switch for a second or so, the engine smooths out, and the jerkiness completely disappears! This effect remains after I subsequently release the throttle and WOT switch, and in fact the "cure" remains in effect until I turn off the ignition - so will last for a whole journey as long as I keep the engine running. This workaround as I should call it only works after the engine has warmed up, and so a typical journey for me consists of a stop after about 2 miles, lift the bonnet, perform the magic, and continue on my way. I could live without having to do this ;-)

I found that this effect is the same whether or not the differential pressure regulator is connected or not, so the influence doesn't seem to be on the fuel injection side. I therefore decided to look at the ignition timing during my procedure, and it looks like this is where I need to focus more. Using a stroboscopic timing lamp, I noticed that before I close the WOT switch as detailed above, the ignition timing is jumping around a lot and as I increase the revs, the timing marks completely disappear, so I presume that the ignition is advancing off scale. However, as soon as I close the WOT, the timing marks magically jump back into view, and are pretty stable (and within spec). Normal steady advance seems to then occur with increasing RPM, and the misfiring has disappeared.

As far as I can see, the WOT sends a +ve signal to both the fuel injection and the ignition control modules. No idea what happens after that, other than fuel enrichment and some timing adjustment presumably results. What I'm curious about is whether anyone can imagine what is wrong with my system such that a temporary +ve signal like this is curing the misfire. Does it point to a poor power supply to the igntion control module perhaps, or just a plain bad ignition control module?

I'm intrigued, but stumped. This feels like it could lead to a breakthrough in fixing my car, but I just don't have the knowledge of where to go with it. I hope someone else does!

Thanks very much

Marcus

1990 Coupe 2.3E
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