My '90 765T, 140K miles, was giving me progressively worse gas mileage, sarting maybe a couple of months ago. Then it started surging at idle a few weeks back. The surge rapidly got worse and after about 50 miles was swinging from about 2000rpm to a stall. Barely made it home. I checked the usual suspects and replaced stuff that could have been misbehaving: throttle body cleaned and adjusted; new inlet manifold gasket; new vacuum hoses, scrubbed out the IAC and checked it; new coolant temp sensor; new O2 sensor; cleaned and bench-checked the fuel pressure regulator; cleaned electrical connections to all sensors; checked the AMM; cleaned and checked the injectors and fitted new seals; ran the OBD through all its functions (which I'm hoping removes the ECU as a culprit); replaced spark plugs and checked for spark; and checked for vacuum leaks throughout the entire inlet system by inspection and with a stethoscope. I found a couple of problems which I thought I'd pass on since there hasn't been much discussion about them. First, the old O2 sensor checked OK electrically but its vents were about 50% clogged. It would probably have been fine with just a cleaning. Second, there was a union near the forward fuel pump in the return pipe from the fuel pressure regulator that had rusted badly and was partly clogging the return pipe. This probably caused higher than normal fuel pressure and may have contributed to my problem. (I cut out a lot of the return pipe and replaced it with a length of 3/8" copper pipe coupled with 1/2" hose and hose clamps. This may be a stupid idea but I guess only time will tell ...) Unfortunately, I still have the same problem. It's a little better, but still running rich and still surging on idle. Any thoughts, suggestions or criticisms would be greatly appreciated. Pete (from Poughquag)
