Baffled

[RCH]

I have a 1991 K15004WD Sierra, 5.7L, 4L60 trans. There is a distinct detination under load when cold which goes away after reaching operating temp. When in park or neutral there is no detination. The truck has 225,000 miles. I replaced MAP sensor, Map sensor solenoid, O2 sensor, oil pressure switch, EGR valve, Spark plugs, Dist. cap and rotor, replaced throttle body seals with fuel pressure regulator seals. I checked ignition timing , it was good. No exaust leaks. no vacuum leaks around intake manifold. I am the original owner and have maintained it very well. Trans. has 25,000 miles on it(serviced regularly). I get no SES light(no error codes). It was stalling when shifting from drive to reverse or reverse to drive, but the throttle body seal kit fixed that. I figured most of those items were old and should be replaced anyway. Any ideas would be greatly appreciated. I also heard that GM fuel pumps don't die all at once, but they can bind and fail intermittently before giving up the ghost, could it be that it might be running very lean cold and not quite as lean at operating temp.?

[90GMCZ71]

Have you checked the ECT sensor? When the engine is cold, the ECM delivers more fuel. If the sensor is bad, it could be reporting a temp much warmer than the actual engine temp.

 

Use an ohm meter are record the resistance of the ECT sensor when cold and when warm. If the values are very close to each other, the sensor is bad.

 

//90GMCZ71

[RCH]

Thanks, never thought of that because the engine temp. gage works fine. The engine temp. gage runs around 195 deg. Do the ECM and the engine temp. gage get their readings from the same sensor?. I do clean my engine so it might have affected the connection and or the sensor.