My 1990 F-150 has about 2000 miles on a rebuilt shortblock. Total miles, about 185,000. Before I had the rebuilt motor installed, and still with the rebuild, the engine will " hunt" for the idle and if you did not drive with your foot on the gas and brake at a light, it would hunt and eventually stall. When parked after driving for a while, you can smell gas. The transmission seems to be shifting hard(maybe because of the idle issues?) And my fuel mileage is down to about 8.5MPG from 16-18MPG. I have changed the EGR Valve, fuel pressure regulator, O2 sensor, engine coolant temp sensor, TPS Sensor, MAP Sensor, ECM, and still can't get the problem to stop. I even put the muffler and catalytic converter back on the truck because one mechanic said there was not enough back pressure for the O2 Sensor to heat up. The truck runs fine when cold, but once it runs for a minute or two, the problems begin. One thing I did notice when I changed the TPS was that it looks like the intake manifold plenum was not cleaned when the rebuilt motor was installed. Will not cleaning this plenum make that much of difference? It has a fair amount of carbon/sludge from blowby in the old engine.