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Quick details: car is an 85, originally L69 but swapped to a 350 with regular carb prior to my ownership.
Ran into a fuel starvation issue yesterday that left me stranded and needing a tow. Mechanical fuel pump wasn't pumping fuel into the carb, verified by the lack of fuel in the online filter between the pump and carb. After getting it towed home, I decided to experiment with triggering the fuel pump relay to see if the in tank pump could get the car running. Frankly, wasn't even sure if the pump was even there or if the delay was functional! Triggered everything with my Power Pro (awesome tool) and the little in tank pump sprang to life and filled up the filter in a couple seconds. Car then fired right up.
Did lots of reading here on TGO and found the purpose of the electric pump is to prevent vapor lock. According to the circuit diagram in the FSM, the delay powers the pump while cranking, then the pump receives its power thru the oil pressure switch. Here is a pic of the relevant page:
The wiring is so hacked up in my car that I know the lines to the oil pressure switch are missing. The switch is also gone, I removed it after it cracked and drained out all the way home - fortunately not far away from home! Wanting the pump to help prime the float bowls so I don't need to crank as long for the car to start, I slightly modified the wiring to activate the fuel pump whenever the ignition switch is in Run.
Will there be any issues with that relay providing power to the pump all the time?
I have a new mechanical pump on order and will be swapping that out too, cheap insurance against another stranding.
The pump is only supposed to run when there is oil pressure so that in the event of a roll over/accident the pump doesn't continue to push fuel while the engine isn't running.
You can do what ever you want though
If it were my car I would re-wire the relay to use the oil pressure switch on the control side with the starter as an override so it works similar to factory
The pump is only supposed to run when there is oil pressure so that in the event of a roll over/accident the pump doesn't continue to push fuel while the engine isn't running.
You can do what ever you want though
If it were my car I would re-wire the relay to use the oil pressure switch on the control side with the starter as an override so it works similar to factory
I agree 100%, not trying to be alarmist here, but, the fuel pump running whenever the key is on is a recipe for disaster.
What happens to a carburetor when there is fuel pressure on it but no air flow through the venturis?
In theory, the float bowl would fill until the float totally closed the needle valve, and then no more fuel would flow through it since none would be being drawn out of the float bowl.
In practice, the slightest leak in the needle valve would allow fuel to overfill the bowl and spill over into the intake manifold, resulting in a "liquid locked" engine.
The oil pressure switch portion of the circuit is actually a back-up method to powering the fuel pump in the event the fuel pump relay fails. It's actually not there to prevent the fuel pump from running. This is a common misunderstanding and misreading of the circuit diagram that was posted.
If you look closely at the diagram and basics of a relay, you will see the fuel pump has a constant ground and the 12+ (hot side) comes either from the fuel pump relay (when activated by the ECM) or through the oil pressure switch being closed when there is oil pressure. Look at the diagram to see the oil pressure switch is "spliced" into the circuit for an additional path and not isolated.
The ECM controls the switch side of the fuel pump relay. If the engine were to stall, the ECM would not see an RPM and thus would not keep the fuel pump relay energized. Similarly, this is also why you briefly hear the fuel pump prime when you turn the key to On but not Start for the first 1-2 seconds. The ECM is briefly energizing the fuel pump relay to prime the fuel system. Because the ECM doesn't see an active RPM signal at that time, it then powers off the fuel pump relay until the ignition switch is placed in Start and the ECM begins to see an active RPM. Or if oil pressure builds, power will then go through the oil pressure switch and onto the fuel pump and activate the fuel pump.
The oil pressure switch portion of the circuit is actually a back-up method to powering the fuel pump in the event the fuel pump relay fails. It's actually not there to prevent the fuel pump from running. This is a common misunderstanding and misreading of the circuit diagram that was posted.
If you look closely at the diagram and basics of a relay, you will see the fuel pump has a constant ground and the 12+ (hot side) comes either from the fuel pump relay (when activated by the ECM) or through the oil pressure switch being closed when there is oil pressure. Look at the diagram to see the oil pressure switch is "spliced" into the circuit for an additional path and not isolated.
The ECM controls the switch side of the fuel pump relay. If the engine were to stall, the ECM would not see an RPM and thus would not keep the fuel pump relay energized. Similarly, this is also why you briefly hear the fuel pump prime when you turn the key to On but not Start for the first 1-2 seconds. The ECM is briefly energizing the fuel pump relay to prime the fuel system. Because the ECM doesn't see an active RPM signal at that time, it then powers off the fuel pump relay until the ignition switch is placed in Start and the ECM begins to see an active RPM. Or if oil pressure builds, power will then go through the oil pressure switch and onto the fuel pump and activate the fuel pump.
You should really look at the diagram again, it is actually the path for the power for the fuel pump, unlike the later cars where it's as you described.
If you look the relay has a ground 100% of the time and is only a common connection point for the CCC, it is not commanding ground for the relay. The relay is activated off the start post on the starter, so the relay only has power while the key is in crank, as soon as the key goes back to run the relay loses power. GM was counting on the oil pressure switch to have gotten enough pressure by the time the engine started to keep the switch closed and keep the electric pump running. You need to look at page 30-3 to see the complete path for the starter, I have verified this in my '86 manual.
You should really look at the diagram again, it is actually the path for the power for the fuel pump, unlike the later cars where it's as you described.
If you look the relay has a ground 100% of the time and is only a common connection point for the CCC, it is not commanding ground for the relay. The relay is activated off the start post on the starter, so the relay only has power while the key is in crank, as soon as the key goes back to run the relay loses power. GM was counting on the oil pressure switch to have gotten enough pressure by the time the engine started to keep the switch closed and keep the electric pump running. You need to look at page 30-3 to see the complete path for the starter, I have verified this in my '86 manual.
How you describe the circuit is exactly how I understand it as well for the early CCC cars. The pic I included above is from the 1985 factory service manual. Likely what JT is describing is the method used on later cars where the computer is controlling the fuel pump.
I want the fuel pump to come in when the key is first turned to ON so the pump can prime the system and fill the float bowls so it doesn't work the starter so hard each time I give it a cold start. In the near future, I will be converting the car over to TPI and using a 16197427 ECM to handle the system duties, including the fuel pump.
How you describe the circuit is exactly how I understand it as well for the early CCC cars. The pic I included above is from the 1985 factory service manual. Likely what JT is describing is the method used on later cars where the computer is controlling the fuel pump.
I want the fuel pump to come in when the key is first turned to ON so the pump can prime the system and fill the float bowls so it doesn't work the starter so hard each time I give it a cold start. In the near future, I will be converting the car over to TPI and using a 16197427 ECM to handle the system duties, including the fuel pump.
Using a 12 volt adjustable timer circuit like this one to control your fuel pump relay would enable you to mimic the later car's 3 second prime .
However, the open question here is why do your float bowls need refilling on each start anyway? If the bowls are going empty at each shutdown the problem isn't at the fuel pump(s), It's at the carb itself ....
You should really look at the diagram again, it is actually the path for the power for the fuel pump, unlike the later cars where it's as you described.
If you look the relay has a ground 100% of the time and is only a common connection point for the CCC, it is not commanding ground for the relay. The relay is activated off the start post on the starter, so the relay only has power while the key is in crank, as soon as the key goes back to run the relay loses power. GM was counting on the oil pressure switch to have gotten enough pressure by the time the engine started to keep the switch closed and keep the electric pump running. You need to look at page 30-3 to see the complete path for the starter, I have verified this in my '86 manual.
Scooter, you're correct and I'm completely wrong. I failed to look at that part of the diagram closer and compare it to the later ThirdGens to make me see there was a difference. After I looked at the diagram again, I realized it was different on the relay power side. As you note, that fuel pump relay is energized from the starter circuit and has a full time ground. So once the engine has started and has oil pressure, the oil pressure switch is what keeps the fuel pump powered.
Using a 12 volt adjustable timer circuit like this one to control your fuel pump relay would enable you to mimic the later car's 3 second prime .
However, the open question here is why do your float bowls need refilling on each start anyway? If the bowls are going empty at each shutdown the problem isn't at the fuel pump(s), It's at the carb itself ....
Doesn't the electric fuel pump on the early cars just feed a mechanical pump on the engine? Wasn't it to prevent vapor lock when everything got hot?
Using a 12 volt adjustable timer circuit like this one to control your fuel pump relay would enable you to mimic the later car's 3 second prime .
However, the open question here is why do your float bowls need refilling on each start anyway? If the bowls are going empty at each shutdown the problem isn't at the fuel pump(s), It's at the carb itself ....
You
I should have clarified - the extended cranking is after it has sat for a couple days. Carb that came with the car is a new Holley Street Demon. I had the same issue with my 78 Vette with its original Quadrajet. Converted it to a simple TBI setup and thrilled with the instant start.
Thanks for the link on the timer! Might be just what I need.
Originally Posted by scooter
Doesn't the electric fuel pump on the early cars just feed a mechanical pump on the engine? Wasn't it to prevent vapor lock when everything got hot?
Correct, it feeds the manual pump. I think around 2psi?
I agree 100%, not trying to be alarmist here, but, the fuel pump running whenever the key is on is a recipe for disaster. 
In practice, the slightest leak in the needle valve would allow fuel to overfill the bowl and spill over into the intake manifold, resulting in a "liquid locked" engine. 
However, the open question here is why do your float bowls need refilling on each start anyway? If the bowls are going empty at each shutdown the problem isn't at the fuel pump(s), It's at the carb itself .... 