79k20350 said:
when you turn the key to run it should start up until it builds pressure... If it doenst stop the pump isnt holding pressure...
That's true for some of the low pressure aftermarket pumps that are used to feed a carb. They shut off when the float bowl is full and the needle shuts off fuel flow. But TBI pumps run continuously, IF the engine is running. TBI and MPFI systems use a regulator at the engine to control pressure, so there is no need to switch the pump on and off all the time to control pressure. The sequence the ECM uses is:
Key switched on, but engine not running or being cranked - pump will run for 2 seconds then shut off. The pump will start back up when the key goes to the start position. It will also start again if the key is swithced off and then back on, but will only run for 2 seconds again if there is no attempt to start the engine.
Any time the ECM is getting a reference signal from the distributor (meaning that the engine is running), then the ECM is commanding the fuel pump relay to be on.
When the key is switched off, the ECM commands the fuel pump relay to turn off. The exception to this is some 1-tons which have an additional fuel module that keeps the pump running for an additional 20 seconds, then lets the pump turn off.
As a fail safe, there is also an oil pressure switch that bypasses the fuel pump relay and sends power directly to the fuel pump when the oil pressure exceeds 2 to 4 PSI. This is so that an engine with a failed fuel pump relay can still run. It takes more cranking, to build up oil pressure first, but at least you don't have to walk home.
