MOT with a broken engine

The past week or two has been mostly spent trying to understand the various engine issues.

Firstly, after noticing the quantity of smoke produced, and the appearance of some white-yellow mayonnaise-ish looking gunk under the oil filler, I tried a quick compression test, with alarming results.

Expected range for this engine is 142-156 psi (when hot). I couldn't easily get the engine hot without driving it, so I did it cold / warm, and ended up with values between 35 (cold, dry) - 90 (warm, wet [i.e. with a drop of oil in the cylinder]). This was pretty inconclusive, but:

• cylinder 2 looks a bit lower than the others, indicating head gasket issues or liner wear
• all these numbers are very low

Other issues: the car is, or has been running, very rich, hence the smoke.

However, after discussions with Will and others, I've come to the conclusion that there are no symptoms directly attributable to the low compression, so I am inclined to ignore this until I find otherwise, as the fixes are either expensive (head gasket) or very expensive (liner wear = new engine).

We plugged the car into the laptop, and using that and a multimeter, decided that the MAF (airflow sensor) was probably not working, which would explain the richness. Will tried hooking up a pot in place of the MAF and twiddling it by hand to achieve a good idle, and we found that the car would stall when it was twiddled too far, proving that the ECU is indeed using a reading from the MAF. So it was duly replaced with a second-hand MAF off Ebay (new ones are £200, hence the preference for a £20 second-hand one).

However, results with the new MAF are not very clear. The car runs much the same, and again the laptop does not show a change in airflow when I blip the throttle. However:

• wiring between ECU and MAF has been tested and found good
• The signal coming off the MAF is in the right range and responds to throttle blips

Tonight I tried hooking up a pot in place of the MAF again, and seeing what the laptop had to say. It turns out that the laptop still didn't display useful data, even though the car would once again stall if I twiddled it too far - the obvious conclusion being that the software isn't displaying a correct value.

So, the way forward seems to be to assume that the MAF is working OK, that the over-fueling is a thing of the past, and see what happens at MOT on Saturday. The emissions tests will hopefully tell me some useful things. It might even pass.

Longer term, I doubt that the engine is in good shape given the compression numbers, but I guess I can always look for an M3 engine when my bank account is a little healthier - I suspect the engine will probably run for now.