From chevythunder.com..... read and see if anything sounds familiar.... may be a faulty O2 sensor:
When you first turn the ignition key to the "on" position, the ecm turns the fuel pump on for about two seconds to prime the fuel injectors. The ecm then shuts the fuel pump off until it receives pulses from the ignition module. During this two second period, and when the engine fires off, the ecm is reading the cst, iat or mat, and tps to set the iac to get the proper idle speed. If your tpi is equipped with a cold start injector, the cold start switch is activated to enrich the fuel to help start the engine. Even on a relatively cold morning, your tpi car should start within a couple of seconds.
When the engine is first fired off, the ecm allows the fuel pump to run, an oil pressure switch is closed when it sees good oil pressure. In event of a oil pump or oil pressure switch failure, the fuel pump will not run. It must be noted here, that in the event of the fuel pump relay failing, the fuel pump will run only after the oil pressure is sufficient to close the oil pressure switch and activate the fuel pump relay. NOTE: If you have a faulty oil pressure switch, DO NOT bypass it! During the time the engine is warming up, the ecm is reading all the sensors, including the maf or map sensor, depending on your system, all the time this information is being processed to adjust the fuel calibration and the idle speed. During this time, the ecm is watching the O2 sensor, waiting for it to be heated enough to activate it and to start sending a variable voltage to the ecm. The ecm also has a internal timer to tell it when it is time to change from open to close loop. You will notice, as the engine is warming up, the idle speed is slowly dropping off, approaching warm idle speed, depending on what the mat (iat) and the cst is reading.
The only difference between open and closed loop, is that during closed loop the ecm is monitoring the O2 sensor, to trim the air-fuel ratio to 14:7. It is also during closed loop that the internal memory is being updated (reprogrammed) to reflect changes in the operating condition of the engine.
When you decide to accelerate, the ecm reads changes in the maf or map sensor, tps, and other sensors including O2 to provide the fuel injectors in a wider fuel pulse( more open time) At WOT, the ecm will ignore the O2 sensor, and based on the calibration in the prom chip will allow the engine to go full enrichment, based on tps. If the engine is overheating, as detected by the cst, the ecm will also enrich the fuel rate in attempts to cool the engine.
