89 chevy c1500, 5.7l, won't idle in gear, "automatic"

If it stalls like your foot slipped off the clutch on a manual transmission car. (Sorry for the analogy, but it's the best way I know how to describe it.) The lockup converter solenoid has failed. On the 700R4 transmission the ecm prevents converter lockup until the vehicle reaches operating temperature. So that may explain why it doesn't do it when cold. If you want to verify this condition. There is an electrical plug on the drivers side of the transmission. It is square and can have up to four wires coming out of it. Disconnect it and drive the vehicle. If the problem is gone. The lockup solenoid or torque converter clutch is failing.

Don't confuse this plug with the neutral safety switch bolted in behind the linkage with two wire connectors going to it.

I will try that, but it doesn't stall as you described. sometime stalls rounding a corner. If I increase idle (a little too high) by raising coolant temp resistance, it doesn't stall, but that may be affecting other things ? I did try different sensors there. what about MAT sensor (air temp. intake sensor), could that cause stalling ?

The 89 was a throttle body truck and should not be equipped with a MAT sensor. It will have a MAP sensor Manifold air pressure. These can start to degrade over time and can give a false reading. But a bad or dirty idle air control motor will do the same. The TBI idle control motor and passage had a tendency to get dirty and sludgy. The breather tube dumps oil vapor very near the idle control passage.

I replaced the idle control motor with new, also the TPS. I'm running out of ideas. also tried your suggestion about unplugging the lockup solenoid. no different. I tried a different MAP sensor, no change. I did not check fuel pressure, but didn't think it acted like it was that issue.

Did you clean the throttle body idle passage. they can get pretty gunky. Is the tps adjustable? aligned properly? Check the grounds on the thermostat housing. Base timing. This is a pretty good puzzle.

I cleaned and rebuilt the throttle body with no change. ,,,,,TPS not adjustable. I will have timing checked. and check grounds.

does anyone know the effects of putting a fixed resistor where the coolant temp sensor goes. it seems the only way I can get an idle, without stalling.

You mentioned that you changed the ECU. Is the new ecu the correct part number for the application? It sounds like this could be a bad PROM.

The ecm had the same numbers and came from 88 truck. ,,mine is 89, it supposed to be compatible.

Most of those ECM's Had the same or very similar part numbers. There is an access door on the ECM, Its held on with two screws. Under the door is a MEMCAL, or PROM. You should check to see if those numbers match as well. The PROM is the calibration chip that is specific to the type of vehicle the ECM is to be installed in. Things like altitude correction, and base parameters are stored there.

I had fuel pressure checked, it was normal. I'm about ready to get a Ford.

you had fuel presure ckecked but did you ck the volume of fuel coming from pump?

Also you can pop the plug out for idle screw and manually adjust idle there and see if that change anything