Okay I'm usually pretty good about figuring these things out, but my truck has me stumped. 1988 Mitsu SPX, Federal Emissions, manual transmission.

Carb has been rebuilt 3 times now by the same shop. It runs about the same(terribly). Here's what has been replaced: 02 sensor, fuel pump, fuel filter, engine, catalytic convertor, carburetor.

If I probe the TPS I show .6 volts at idle, and 4.6 at full throttle and a smooth transition in between. If I apply vacuum to the EGR valve, the engine dies. The hot air inlet and the choke both work fine.

At full operating temperature the O2 sensor voltage stays at .8 volts - it's definitely running rich. If I unplug the coolant temperature sensor(the 2 prong one) with the engine running I hear a click from a relay, but see no change in the way the engine is running.

The carb shop tried to "fix" my problem by grounding the yellow wire of the MCS to the frame. It passed emissions with this bypass hooked up. It also made the thing totally run like crap.

Having removed that and readjusted the idle, if I touch that wire now to the ground the idle immediately goes to crap, and the o2 sensor voltage starts to fall. So I have concluded that grounding the MCS leans out the mixture, based on what the o2 sensor does. So it almost looks like the computer is either bad(I could be the first one to have that happen reading the other messages....) or there is another sensor somewhere in the feedback system that is keeping the computer from having control.

My 4 cyl dwell meter hooked up to the MCS wires at idle stays at 45 degrees. It says at 45 degees without anything hooked up as well, leading me to believe there is NO signal being sent to the MCS. Can anyone tell me if that means the ECM is commanding the carb to be full-rich? No signal at operating temp either means the computer is brain dead or something is convincing the computer that things need to stay rich(another sensor somewhere that is bad?).

Anyone have any idea as to what the heck might be wrong?

Chris