I know the ECM primes the pump and then starts. If that doesn't work the oil pressure switch will prime it after some cranking. Once the engine is running does the computer then keep things running or does it depned on oil pressure? If I lost oil pressure completely, would the fuel pump shut off as a safety mechanism?
When does ECM power fuel pump?
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Low Budget
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Well I'm not the best to answer this, but someone has to do more than read your post.
Ok. As I understand it: ECM powers the pump through the fuel pump relay. It does a brief direct test (bypassing the relay) each time the ignition is switched on. After startup, the pump gets power through the relay, and it continues to run as required by the regulator. If the relay fails, the oil pressure switch (if you still have one installed) takes over. When cranking to start the engine, the switch detects oil pressure and signals the ECM of it's existance. The ECM will then in turn power the fuel pump without the relay. (This is a possible explanation for engines that require long periods of cranking before starting. The pump relay may be bad.)
As for will the engine shut down upon loss of oil pressure, I don't think so. Hopefully someone "WHO KNOWS" WTF
they're talking about will jump in here and correct any mistakes I may have made. I'm giving my brain a workout here, so no guarantee's!
Hope this helps.
Jeff
Ok. As I understand it: ECM powers the pump through the fuel pump relay. It does a brief direct test (bypassing the relay) each time the ignition is switched on. After startup, the pump gets power through the relay, and it continues to run as required by the regulator. If the relay fails, the oil pressure switch (if you still have one installed) takes over. When cranking to start the engine, the switch detects oil pressure and signals the ECM of it's existance. The ECM will then in turn power the fuel pump without the relay. (This is a possible explanation for engines that require long periods of cranking before starting. The pump relay may be bad.)
As for will the engine shut down upon loss of oil pressure, I don't think so. Hopefully someone "WHO KNOWS" WTF
they're talking about will jump in here and correct any mistakes I may have made. I'm giving my brain a workout here, so no guarantee's!Hope this helps.
Jeff
Umm, not quite 
. If the key has been off for at least 5 seconds (while the ecm resets itself internally, doing stuff like parking the iac), then at key on the ecm will run the fuel pump (energize the fuel pump relay) for about 2 seconds. It will also turn on the pump whenever it is receiving crank sensor pulses, so if it starts getting those within 2 seconds of key on, the relay stays on. Otherwise, the relay goes off until the engine is crank and it starts getting crank sensor pulses, at which time it goes back on. If the key is off for less than 5 seconds, the ecm doesn't get it's internal reset finished and the pump won't run when you turn the key back on. Completely separate from this, and unknown to the ecm, there are contacts in the oil pressure switch in parallel with the fuel pump relay that also power the pump whenever oil pressure is over the switch setpoint of about 4-6 psi. That way, if the fuel pump relay is bad but you crank long enough to build oil pressure, the pump will come on and you can drive home. These also share the current when the relay is energized and the engine is running (with oil pressure), which makes the relay last longer.
. If the key has been off for at least 5 seconds (while the ecm resets itself internally, doing stuff like parking the iac), then at key on the ecm will run the fuel pump (energize the fuel pump relay) for about 2 seconds. It will also turn on the pump whenever it is receiving crank sensor pulses, so if it starts getting those within 2 seconds of key on, the relay stays on. Otherwise, the relay goes off until the engine is crank and it starts getting crank sensor pulses, at which time it goes back on. If the key is off for less than 5 seconds, the ecm doesn't get it's internal reset finished and the pump won't run when you turn the key back on. Completely separate from this, and unknown to the ecm, there are contacts in the oil pressure switch in parallel with the fuel pump relay that also power the pump whenever oil pressure is over the switch setpoint of about 4-6 psi. That way, if the fuel pump relay is bad but you crank long enough to build oil pressure, the pump will come on and you can drive home. These also share the current when the relay is energized and the engine is running (with oil pressure), which makes the relay last longer.RKHiPerformance
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I recently noticed an increase in how long my wife cranks her Monte before it starts. Is it possible that the fuel pump relay has failed and now the pump does not come on until the engine builds oil pressure?
RK, if that's the Monte with the GN drivetrain/wiring harness, watch the oil pressure idiot light while cranking. If the engine fires right as it goes out then there's a good chance the relay is bad. There's lots of ways to check the relay (voltmeter or test light on the test connector behind the alternator should show 12V for two seconds after key on, etc). If it's the original Chevy engine, I know nothing about their wiring .
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1 RARE T
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What if?
What if the car will start and run but then sometimes the fuel gauge goes past full and the fuel pressure goes away,car dies?
New f/p relay did'nt fix it B.T.W.
Happens to me sometimes and before trying to figure it out the car will run again,fuel gauge goes back to reading properly.
What if the car will start and run but then sometimes the fuel gauge goes past full and the fuel pressure goes away,car dies?
New f/p relay did'nt fix it B.T.W.
Happens to me sometimes and before trying to figure it out the car will run again,fuel gauge goes back to reading properly.
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Help?!!!
John Larkin
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Sounds like you are losing the ground connection at the fuel pump intermittently. They do break off at the top of the sender. Perhaps hook an ohmmeter to the ground wire at the connector and shake the wire. If you lose continuity, you have found the problem.
Low Budget
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But would that explain the overfull fuel gauge readings? Would'nt the fuel gauge default to empty upon loss of it's ground?
John Larkin
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GM fuel level sending units use odd resistance parameters (66-230 ohms?) or something like that. I can't remember off the top of my head but it's not 0-90 like the aftermarket universal units. Regardless, I've seen sending units go bad and most fuel gauges peg to overfull so I'm guessing highest resistance = full tank. Just an edumacated guess.
Also keep in mind this isn't a powered circuit because the needle still shows fuel level even with the key off, right? So it's a ground based system. I'm going to say you have a broken wire or bad connection, one of the two. Start at the back and trace the wire forward; that's all you can do. Good luck.
Also keep in mind this isn't a powered circuit because the needle still shows fuel level even with the key off, right? So it's a ground based system. I'm going to say you have a broken wire or bad connection, one of the two. Start at the back and trace the wire forward; that's all you can do. Good luck.Similar threads
