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)