What would cause fuel pump to prime with key off?

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David Claflin
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What would cause fuel pump to prime with key off?

Post by David Claflin » Sun Nov 17, 2019 9:37 pm

Noticed when I got out of the car yesterday the fuel pump cycled like it does when the key is turned to run. Tried a different ECU but no difference, pump will randomly cycle when I'm standing by the car with my keys in my pocket.
1990 LX/A9L/stock cobra shortblock, special cam :P , ported Holley SMII intake, ported explorer heads, accufab longtubes/ 4.88, Z-spec trand
1985 LTD LX, A9P, ported AFR165, ported Holley SMII intake, 1 3/4 longtubes, 4.10, 4R70W trans, mach 1 brakes.
1986 coupe, DB-1, former FHP car, stock shortblock, ported TW heads, Ported vic jr intake, Massflo 4 hole TB, 1 3/4 longtubes, 3" exhaust, T5, 4.56

jsa
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Re: What would cause fuel pump to prime with key off?

Post by jsa » Mon Nov 18, 2019 6:15 am

Faulty fuel pump relay
Cheers

John

95 Escort RS Cosworth - GHAJ0 / ANTI on a COSY box code
Moates QH & BE
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cgrey8
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Re: What would cause fuel pump to prime with key off?

Post by cgrey8 » Mon Nov 18, 2019 1:21 pm

Agreed. The most likely cause is a bad relay. But lets explore other possibilities that aren't as likely...

With a stock wiring configuration and no mechanical/physical problems, the EEC has to turn the fuel pump relay ON for the relay to power the pump. This is a safety feature. There's also usually an inertia cut out switch that can lock out the fuel pump in the event of a hard collision. But it's not likely to be related to any of this.

However if you've modified the wiring harness to accidentally/intentionally power the EEC when the ignition is OFF, then it's possible the EEC could be trying to power the relay and thus be turning the fuel pump ON.

Assuming you haven't made any radical modifications of the wiring, the most likely fix is going to be to just replace the fuel pump relay. However if the problem persists, then you'll need to do more diagnostics. I would do the following...

Check whether the pump relay is being triggered or not. The easiest way to do that is to wire a 12v light to the same leads that go to the relay's coil. If you catch the fuel pump kicking on and off, pay attention to whether the light is also flickering with the pump.

If the light is NOT turning on with the pump, then it's either another bad relay that's failing in the same unlikely way or you've got something else powering the pump. That would suggest wire damage somewhere.

If the light IS illuminating, then the problem is not with the relay or any downstream wiring to the pump. It's upstream between the relay and the EEC. Either you got something backfeeding the relay intermittently (i.e. something you've spliced in, wire damage) or possibly something backfeeding the EEC with power causing it to intermittently power up when the ignition is OFF. If you unwittingly tapped into the power wire to the EEC to power something else, then it's possible this could happen due to leakage current or possibly capacitance discharge. I don't know what could possibly do that in your case, but I have seen instances where DC motors produce voltage as they are winding down and thus cause things to get powered or continue to be powered when they weren't expected. I remember having a small 12v relay for something tied into a wire that was also powering my blower motor. I remember being able to hear that if the blower motor was running wide-open, the click of the relay OFF was delayed as compared to when the blower wasn't spinning at KOEO. It's not likely inductive voltage like this would have enough current to power the EEC but I don't know that for sure. I'm just throwing it out there as weird things I've seen over the years.
...Always Somethin'

89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, 1.6RRs, FMS Explorer (GT40p) headers, Slot Style MAF, aftermarket T5 'Z-Spec', 8.8" rear w/3.27s, Powertrax Locker, Innovate LC-1, GUFB, Moates QuarterHorse tuned using BE&EA

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EDS50
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Re: What would cause fuel pump to prime with key off?

Post by EDS50 » Mon Nov 18, 2019 6:46 pm

Faulty ignition switch/harness connector?
1992 LX - 25.1c Chassis, Vortech Blown Dart 332, Lentech Trans, TRZ Backhalf, A9L, Moates QH/SL v1.9, BE, EA, TunerView

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Re: What would cause fuel pump to prime with key off?

Post by Chucko » Wed Nov 20, 2019 11:54 pm

I've had that happen with the original FPR in my '89. I gave it a good rap with a wrench and it stopped. Hasn't happened again.

A new relay is cheap insurance if you're nervous.
'89 LX 5.0, Crane 2040, Pro-M 75mm Bullet, 65 mm TB, ported stock intake & E7TE heads, 24 lb injs., JBA shortys, cat X-pipe, A3M1 (GUFB strategy) w/Moates Quarterhorse, BE & EA, close ratio T5Z, 3.55 rear

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Re: What would cause fuel pump to prime with key off?

Post by Chucko » Wed Nov 20, 2019 11:55 pm

EDS50 wrote:
Mon Nov 18, 2019 6:46 pm
Faulty ignition switch/harness connector?
I can't tell you how many times I've had to reassemble the pieces of my ignition switch... definitely not one of Ford's better ideas.

Usually it fails open, though, not shorted.
'89 LX 5.0, Crane 2040, Pro-M 75mm Bullet, 65 mm TB, ported stock intake & E7TE heads, 24 lb injs., JBA shortys, cat X-pipe, A3M1 (GUFB strategy) w/Moates Quarterhorse, BE & EA, close ratio T5Z, 3.55 rear

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Re: What would cause fuel pump to prime with key off?

Post by David Claflin » Thu Dec 05, 2019 9:30 pm

Just as a follow up, I replaced the fuel pump relay and it solved that issue. Then a couple days later it died on me, turned the key and no start, but could hear injectors clicking, so swapped in different EEC relay and it now runs fine.
No mods to wiring harness, while I had car apart before I replaced both switched on column, the key one, and the bigger one under the column sleeve.
1990 LX/A9L/stock cobra shortblock, special cam :P , ported Holley SMII intake, ported explorer heads, accufab longtubes/ 4.88, Z-spec trand
1985 LTD LX, A9P, ported AFR165, ported Holley SMII intake, 1 3/4 longtubes, 4.10, 4R70W trans, mach 1 brakes.
1986 coupe, DB-1, former FHP car, stock shortblock, ported TW heads, Ported vic jr intake, Massflo 4 hole TB, 1 3/4 longtubes, 3" exhaust, T5, 4.56

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Re: What would cause fuel pump to prime with key off?

Post by jsa » Thu Dec 05, 2019 10:09 pm

:thumbup:
Cheers

John

95 Escort RS Cosworth - GHAJ0 / ANTI on a COSY box code
Moates QH & BE
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Re: What would cause fuel pump to prime with key off?

Post by EDS50 » Thu Dec 05, 2019 11:04 pm

Glad you found it and it was simple fix.
1992 LX - 25.1c Chassis, Vortech Blown Dart 332, Lentech Trans, TRZ Backhalf, A9L, Moates QH/SL v1.9, BE, EA, TunerView

David Claflin
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Re: What would cause fuel pump to prime with key off?

Post by David Claflin » Sat Dec 07, 2019 8:39 am

EDS50 wrote:
Thu Dec 05, 2019 11:04 pm
Glad you found it and it was simple fix.
Thanks, me too. Thinking I might need to carry a small tool bag with the two relays, and spare distributor. These things are getting old, and even the parts store replacement parts aren't always going to work for as many years as the factory stuff did.
1990 LX/A9L/stock cobra shortblock, special cam :P , ported Holley SMII intake, ported explorer heads, accufab longtubes/ 4.88, Z-spec trand
1985 LTD LX, A9P, ported AFR165, ported Holley SMII intake, 1 3/4 longtubes, 4.10, 4R70W trans, mach 1 brakes.
1986 coupe, DB-1, former FHP car, stock shortblock, ported TW heads, Ported vic jr intake, Massflo 4 hole TB, 1 3/4 longtubes, 3" exhaust, T5, 4.56

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Re: What would cause fuel pump to prime with key off?

Post by EDS50 » Sat Dec 07, 2019 1:14 pm

I have a spare distributor, coil and f3 chip in the hatch along with tools "just in case"...lol...seriously though... :mrgreen:
1992 LX - 25.1c Chassis, Vortech Blown Dart 332, Lentech Trans, TRZ Backhalf, A9L, Moates QH/SL v1.9, BE, EA, TunerView

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