TBI Fuel Pressure

[biggsteve97]

Originally Posted by rfmaster

During POST ECM will energize FP relay for about 2 seconds. You may observe 9 to 12 PSI readings during this time, but this reading is unreliable. To verify fuel pump operation with engine off you can apply +12volts to the red pig tail attached to the FP relay socket from a fused jumper wire (use 10 amp inline fuse). Once FP builds pressure you can verify FPR is doing job maintaining pressure between 9 to 12 PSI (higher is better). Quick drop-off in fuel pressure after pump is shutdown is normal, but there is no factory spec.

With regard to your issue with difficult warm re-starts. Make sure that FP is energized during cranking! It is common for FP relay to develop bad contacts over time. The oil pressure switch takes over and powers-up FP after oil pressure builds up above 4 to 6 PSI. When engine is cold oil pressure build up is very quick. Once engine oil is hot oil pressure build-up takes time to exceed 4 to 6 PSI threshold of the oil pressure switch. You can verify if FP relay is functional by temporarily disconnecting oil pressure switch. If you get no start with engine cold you've found root cause - FP relay circuit.

Pull FP relay - take a close look at socket contacts, verify relay contact resistance with DVM or simply replace it with known good unit. On a very rare occasions ECM may blow switching transistor used to apply +12volts to FP relay coil. However this failure mode is rare and happens if harness is damaged and relay power circuit has been grounded.


//RF

I have dual tanks and doesnt matter which tank Im running I can hear the pump run every time I turn the key during the hard start and even with the intermittent no start. And that is with both just turning the key on not cranking the engine as well as when cranking. The injectors are spraying fuel while cranking. Though on the turn of the key before even letting go of the key the engine will fire, imediately die and then just crank. I have watched the injectors when this happens, they spray briefly, pause then slowly start spraying again, with each pulse spray getting courser until it usually starts. Ironically, whether its the hard start condition or the intermittent no start condition, if I shoot a very short burst of starting fluid into just one of the throttle bores, the engine will fire right up. And the injectors are spraying in all situations.
My chip guy burnt a new chip with longer perameters for injector pulse width while cranking and it has helped alot both cold and hot starts. So far since I put the new chip in the no start condition hasnt happend but it will still fire, die then crank for about 2-3 seconds then fire at operation temp. So I am installing a permanent fuel pressure gauge tomorrow so I can see exactly whats happening at the moment when the hard start happens.