My 1989 760 Turbo wagon (Canadian spec, 190,000 km) has been getting very poor fuel economy for about 6 months. I thought it was the cold winter but the poor mileage has continued into the warmer weather. It gets ~20mpg for mostly highway driving (same as my 5.3l pickup). I checked the O2 sensor output and found that it was a steady 5.6V at any rpm, even under load. Introducing a vacuum leak at idle would reduce the voltage, but anyway it clearly was shot so I replaced it with a new Bosch unit. Now the voltage is about 7.8V at idle, with little fluctuation. If I disconnect the fuel pressure regulator vacuum line and plug it, the volage increases, but only by 0.5V or so. If I unplug the vacuum line (leaving it loose, so there's a vacuum leak) the computer goes into closed loop and the voltage hunts between about 0.2V and 0.7V rapidly (looks like it's working properly). Reconnect the FPR and it goes back to 7.8V steady. Other notes: air filter is clean, all vacuum lines have been checked, compression is ok (140psi on all 4) and the car has no noticeable driveability problems whatsoever. The temperature sensor is mapping a little higher resistance than Steve Ringlee's info (640 Ohms @ 80C).

Am I correct in deducing that my fuel pressure regulator is also toast? If this is the case, why wouldn't the ECU set a fault code for running rich at idle and set the "check engine" light on? Is this because it still sees the temperature as being only 60C because of the temp sensor?

I've checked for the diagnostic box behind the driver's side shock tower but it is not present on my car. There is a "Jetronic" computer in the passenger side footwell but I cannot see how to check the fault codes on this unit. Any help would be greatly appreciated.

Sorry for the long-winded description,

Martin Kestle