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Author Topic: Fuel Pump Run-on  (Read 14118 times)

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Fierofool

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Fuel Pump Run-on
« on: October 29, 2020, 01:52:09 pm »
Some months ago I replaced both AC and Fuel Pump relays just as a matter of maintenance.  It was also precautionary as I had occasionally had issues with the fuel pump continuing to run for 20-30 seconds after the ignition was turned off.  The new relays didn't totally eliminate the problem because the pump would occasionally continue to run, but for a much shorter period of time.

Within the last week, I have had to resort to pulling the fuel pump fuse to stop the pump.  With the ignition off, immediately reinstalling it would start the pump again, but if hours lapsed, installing the fuse didn't start the pump while the ignition was off.

Today, I decided to check the relay to see if it was stuck.  My test was to swap AC and Fuel Pump relays.  After starting the car and letting it run for about a minute, I turned it off and the fuel pump continued to run.  I did this test several times with the same result. 

I then unplugged the oil pressure sending unit, restarted the car and let it run for about a minute.  When I turned off the ignition, the fuel pump stopped running, too.  The oil pressure sending unit, as we know, is a backup power supply for the fuel pump in the event the relay fails.  Once sufficient oil pressure is built, the sending unit closes the fuel pump circuit to supply power. 

So, if you are ever having issues with the fuel pump that just keeps on pumping, try disconnecting your oil pressure sending unit. 
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scottb

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Re: Fuel Pump Run-on
« Reply #1 on: October 29, 2020, 02:51:25 pm »
Sending unit shorted out by water?
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Fierofool

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Re: Fuel Pump Run-on
« Reply #2 on: October 29, 2020, 03:32:55 pm »
I installed it horizontally to avoid that problem.  It has never worked correctly.  It keeps the pressure gauge pegged at about 120 lbs. 
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GTRS Fiero

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Re: Fuel Pump Run-on
« Reply #3 on: October 29, 2020, 05:42:52 pm »
Don't hurt yourself, but if you replace the sender, does that correct the issue?
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Fierofool

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Re: Fuel Pump Run-on
« Reply #4 on: October 29, 2020, 07:39:32 pm »
I don't know.  The first one would blow the fuse for the gauges and the instrument lights as soon as I turned on the ignition.  I replaced ith sender with another and it has pegged ever since I installed it.  I just left it at that because it was no longer blowing the fuse.  It's an 88 style sending unit. 
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GTRS Fiero

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Re: Fuel Pump Run-on
« Reply #5 on: October 29, 2020, 07:43:18 pm »
Do you have a high-volume oil pump?

Need to test the output from the OPS.  The current one is obviously bad on the one circuit.
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Fierofool

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Re: Fuel Pump Run-on
« Reply #6 on: October 29, 2020, 09:31:21 pm »
I have the standard 3.4 oil pump which is considered a high pressure pump for the 2.8.  The gauge pegs with ignition on, engine not running.

Our gauges will peg if they don't have a ground through the pressure sending unit, so it's that effect.  It pegs anytime the ignition is on whether the engine is running or not and whether the harness is plugged in or not. 

I need to confirm that I have the conversion wiring correct.  Just never done it. 
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GTRS Fiero

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Re: Fuel Pump Run-on
« Reply #7 on: October 29, 2020, 09:34:13 pm »
My oil gauge only reads to 80 PSI.
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Fierofool

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Re: Fuel Pump Run-on
« Reply #8 on: October 30, 2020, 08:19:39 am »
I'm just throwing an extreme number out there.  It pegs all the way to the right bottom of the gauge.
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There are three kinds of men:

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2.    The few who learn by observation.
3.    The rest of them have to pee on the electric fence and find out for themselves.    Will Rogers

Raydar

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Re: Fuel Pump Run-on
« Reply #9 on: October 30, 2020, 09:06:53 pm »
My oil sender did that. And then the brand new replacement did it, occasionally.
I went behind my fuse box and rewired the fuel pump circuit to "switched 12V".
It's counter intuitive to what some guys are doing. (The guys with the 3800 SC swaps that are running the killer fuel pumps are actually running separate heavy duty 12V feeds to their fuel pumps.) But it has caused me exactly zero issues.
Cows Patoot has actually made a major wiring improvement to his Formula project, that eliminates a lot of the mickey-mouse wiring that causes so many issues with the Fiero. I plan to try that, when I get the opportunity. Having the battery in the front will help, tremendously, towards that goal.
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GTRS Fiero

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Re: Fuel Pump Run-on
« Reply #10 on: October 30, 2020, 09:17:28 pm »
Make you can start a new thread on getting rid of mickey mouse.
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Raydar

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Re: Fuel Pump Run-on
« Reply #11 on: October 31, 2020, 06:48:06 am »
Quote from: GTRS Fiero on October 30, 2020, 09:17:28 pm
Make you can start a new thread on getting rid of mickey mouse.

I'll try to find a link to his posting on FB. But that doesn't address the crappy oil pressure switches that like to stick in the "on" position.
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Fierofool

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Re: Fuel Pump Run-on
« Reply #12 on: October 31, 2020, 09:27:51 am »
After the first one from Rock Auto was blowing fuses, I decided not to go through the relative cost of shipping it back for credit and I just purchased an AC Delco unit from a local parts store.  I don't remember which store, but it was probably AutoZone.  That's the one I have now.  It has never given a correct reading, but it resolved the blown fuse issue. 
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There are three kinds of men:

1.    The ones that learn by reading.
2.    The few who learn by observation.
3.    The rest of them have to pee on the electric fence and find out for themselves.    Will Rogers

TopNotch

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Re: Fuel Pump Run-on
« Reply #13 on: October 31, 2020, 11:26:27 am »
I have a spare 88-type oil pressure sender somewhere, but finding it after all I have done to my garage recently will be a challenge.
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