I'm pretty sure my car is thirstier than it really ought to be. It's an '89 325i Sport, all standard and runs very well, hot and cold starts no problem and a nice smooth idle.
I've recently renewed the distributor cap (old one was cracked), rotor arm, plugs, air filter, changed the oil and oil filter. Within the past year or so it's had a new timing belt, water pump and thermostat so all should be well engine wise though I must admit I'm yet to get round to checking / setting the tappets and it does tick a little so they probably need doing.
On my last tank of mostly short trips, but always driven very gently it averaged 21mpg (worked out from litres in and miles driven) and on the tank before, which was mostly a long gentle motorway run with very little traffic and light throttle, 29mpg. From reading on here that seems a bit worse than I should be expecting. From previous calculations with mixed driving it rarely seems to get better than 25mpg even in better weather so I feel something is wrong.
Today I checked the blue coolant temp sensor with a multimeter. It read 1400 ohms when cold (wasn't quite cold as used the car some hours earlier) and dropped steadily to 320ohms as the engine warmed up. From checking the Haynes book this seems to be within spec. I note that with the blue sensors plug disconnected the idle isn't as smooth when warming up and it 'chugs' a little and needed some throttle to start so I assume that's doing it's job and getting a signal to the ECU?
Next I checked the wiring as per Brian's advice on previous posts and the plug at the sensor is in good condition under the rubber cover, and the plug and socket under the inlet manifold are very clean with no apparent problems. I can't see where the wiring goes from that plug, looks to join a loom near the starter motor, is there a way to check it further along for continuity, perhaps at the ECU end?
So - where should I be looking next with regard to improving the economy? I was thinking of a chip for the ECU to perk it up a bit / improve the economy but want it running correctly as standard first.
I should add that I asked my MOT man about the mixture last year as I was concerned about the economy (or not!) of this car and he seemed to think the idle mixture was set ok from his gas analyser though I realise the specs can vary widely as to what's acceptable MOT wise and what's correct.
Thanks in advance for any assistance
Cheers
Stu
