Finally... progress

The problem with the richness looks to be solved. It was a problem with the lambdas, but not in the obvious way.

After spending a shocking £80 on a new lambda, I did some more testing on the old lambda sensors, and, after heating them up with a blowtorch, the faulty one came back to life. I ended up replacing it anyway, as this and other data suggest that it's on the way out. So, new lambda in place, I ran the engine and attached a multimeter to the lambda output, expecting to see it start to cycle between rich (close to 5v) and lean (closer to 0v), when the engine went into closed-loop mode. Unfortunately, this didn't happen: it stayed rock steady on about 4.8v (i.e., rich).

After much head-scratching, I plugged in the laptop, and noticed that the laptop was reporting 0v coming from the lambda sensors, even though they were outputting close to 5v. A quick scan of youtube (apparently, some people like to video their sessions of plugging a laptop into their car, and in this case I'm glad they do, although in general they should probably get some kind of hobby) yielded a video of someone running the same software on the same engine, which was clearly reading correct (non-zero) lambda sensor values: so the software is clearly capable of displaying correct values. A quick check of the wiring from the lambda sensor to the ECU showed that the wiring was OK, so the evidence pointed, improbably, towards an ECU which functioned fine in all respects except that it couldn't read the lambdas properly. If the ECU thought it was seeing 0v, then it would think that the fuel mix was too lean, and inject extra fuel to try and solve the problem... hence the emissions issues, and the numbers I was seeing from the lambdas.

The theory was promising, but I needed a new ECU to test it (number four, if you're counting - the first two had immobiliser issues, and the third had the lambda issue). This also meant a new immobiliser (they are paired with the ECU) and a matching key (again, the chip in the key is paired with the immobiliser). Fortunately, they are cheap on Ebay (about £50), but unfortunately most of the breakers who sell them are absolute cretins. Example email exchange:

Me: I need the ECU, immobiliser and keys - here is a picture to avoid confusion.
Cretin: Just to confirm, you just need the ecu and the key.

In the end, I found a seller who seemed to understand that I needed three items, not two, so I ordered it. Sadly, he turned out to be useless as well, and I didn't get it until four weeks later. He claimed he was snowed in (in Glasgow), but I'm not convinced the snow lasted for four weeks.

It was worth the wait though: I plugged in the new bits, swapped the chip in the key over and ran the engine. After a short warm-up, the lambdas started cycling nicely between rich and lean readings, suggesting that the engine is no longer running rich.

Assuming there's no snow on Saturday, it looks like I might get to MOT before Christmas after all, although I will probably freeze to death on the way there. I hope they don't object to the new ECU having reduced the milage by 60,000 miles.