I finally got around to replacing the knock sensor in the MR2 last Sunday morning.
The purpose of the knock sensor is to detect 'knock' in other words engine detonation and that allows the engine management to run the car with ignition advanced pretty much as far as it can go. If the sensor is faulty on the 4A-GZE engine, the 'Check' light comes on, you get error code 52 and the car goes into a safe mode with reduced performance so as not to damage the engine.
The original sensor was destroyed about 8 years ago by a careless mechanic who promptly went out of business before I could get it replaced. Just as well for him because a replacement ended up costing £150. I had tried several options to get around buying a new one including trying a wide-band one and one out of a Toyota Supra. The problem is that on cars of this vintage, the knock sensor was very narrow band and tuned closely to the knock frequency which is very specific to the engine and a function of stroke, bore, capacity, grandmother's birthday and so on. In other words using anything other than the correct sensor is not going to work. The galling part is that the actual active component is a tuned piezo transducer that you could buy for about £3, so the rest of the £150 is for a steel boss, a connector and Mr Toyota's works do.
The other thing is that the sensor is in a 27mm boss housing and has to be screwed into the block at the rear, a particularly hard to reach spot. Thankfully with the starter removed there is just enough room to get at it with a 27mm socket and that gave me a good opportunity to thoroughly clean the starter before putting it back in the car.
All seems well now with the new sensor, no error codes and the car seems more responsive as it should. The idle seems more stable now as well, which is a bonus although I still intend to change the coolant out to remove air bubbles which are a known cause of wobbly idling.
I also got around to fitting the shorter drivebelt for the alternator and now the air conditioning is pretty much disabled. If I can get it de-gassed soon I can take the compressor out to lighten the car and maybe as something I can sell for a few quid. This will also make fixing the oil pressure sender much easier when I get around to that because that means taking the alternator and bracket right out.
There's not a lot of room to work round the engine in one of these cars.