2000 LW2 3.0 L, Low manifold pressure
 

This engine is very difficult to start and won't run at less than 2800 RPM (just stalls). No codes are indicated. Manifold pressure reads 2.5 PSI from DigiMoto OBDII software. (I thought it should be about 20 PSI.) I removed the EGR valve and installed a blocking gasket under it to eliminate that potential source of leak - no change. The car has been sitting a lot and has only 65K miles. I don't know more of the vehicle history as it is new to us. I can't see where there could be any other huge intake leak. Any ideas?

Look in close proximity to where the top radiator hose exits the engine - you should find the Engine Coolant Temperature Sensor (ECTS) (2-wire sensor). Replace the sensor, and the associated connector (about 40 bucks for OEM (GM) parts). Make sure the replacement sensor has a metal tip and not a resin tip on it ....

Thanks for the reply. I was wondering though, what a temperature sensor would have to do with manifold pressure. The engine is cold when these problems exist. There is no such thing as even getting it into gear until I fix the problem. If the sensor were in the intake manifold rather than the coolant system, I guess a ruptured end on it could leak air, but to measure temperature of the engine, it is in the coolant system. Am I missing something here?

Ayup - the ECTS adjusts the air/fuel mixture ratio of the engine depending on the coolant temperature -- dammed hard to get a cold engine started with whonky fuel mixture ratio, and harder still to get it to idle