Still battling a severe fuel consumption problem on my 89 740T wagon. Have done pretty much everything that can cause excessive consumption from full tune up, new ECT and fuel pressure regulator, intake tract leak repaired, new O2 sensor, AMM, everything cleaned, etc, etc, Nothing has improved mileage from the 13-15 mpg one bit. Not in the slightest. I'm pretty much stumped but thinking about it yesterday I have a thought. When I replaced my ECT I found it had a black one for an LH-2.4 instead of the blue for LH-2.2. Figuring that has to be the problem I replaced it with the blue one but nothing changed in the slightest. Mileage and performance remained exactly the same. After thinking about it and doing some more research I don't see how changing the ect could not have an effect one way or another. Their values are completely different and having the wrong one should create havoc with the ecu. So I'm thinking maybe there is a problem with either the wiring or the ecu itself. If its getting completely different readings from the ect and it is processsing it as designed I don't see how it could not affect the cars performance. Is my logic sound here? Is there a way of testing the ecu or a good source for testing info? My Haynes manual doesn't say anything about it. The car has 180K and this is my first brick. It was bought a few months ago and I have no knowledge of its service history whatsoever but it seems to be reasonably well cared for. It runs fine. Starts right up, idles good, pulls hard, runs smooth with no hesitation or driveability problem with either ect. If not for this gas mileage issue I would be using and enjoying the car much more.
Dave