Hey guys, on my 86 2m4, i replaced the fuel pump about six thousand miles ago (some time in august?) after the rubber hose went bad. I had the tank down already, car had ninety thousand miles on it, i figured replacing it while down would save me down the road from having to take it apart again. I have noticed though that the pump seems to run much longer after i shut the key off, till it stops, esp if i start the car and shut it down real quick again. how long about do your stock fuel pumps run for in this case? thanks for any input

Matt

Many years ago, the fuel pump on my 2M6 wouldn't shut off - the oil pressure sender had malfunctioned, which caused the oil pressure gauge to peg AND kept the fuel pump on all the time. The solution, of course, was to replace the OP sender.

I gather that the problem was a little rare, but not unheard of.

You can see from the schematic below, Doc John is correct. Since the fuel pump fuse is hot at all times, if the oil pressure switch gets stuck in the ON position, the fuel pump will continue to run even with the ignition off. Change your oil pressure sender/switch.

"Should" stop as soon as the key is off.

quote Originally posted by Lou6t4gto: "Should" stop as soon as the key is off.

With the key off it will stop when the oil pressure drops below 4 psi. Both of mine stay on for a couple of seconds after the engine stops.
Do you hear the fuel pump prime for a couple of seconds when you first turn the key on? ( Just wanting to make sure the FP relay is working)

ok guys sounds good. never would have thought the oil pressure sending unit would be related, thanks.

yes, it does prime when i first turn the key on, everything seems to work fine on it, the car runs fine and everything, i was just concerned with how long the pump continued to run when i shut the key off, sometimes as long as ten seconds. its not everytime I shut it down, like i said definitely runs longer after shut down when the car is started and shut right back off again.

quote Originally posted by mtjd25: definitely runs longer after shut down when the car is started and shut right back off again.

That's because when the oil is cold, it's considerably thicker creating higher oil pressure. It takes that much longer to depressurize back down to 4 psi as 2farnorth said. Ten seconds seems a little long but as long as it's working, don't fix something that ain't broke. In your case, I'd be sure to stick around and wait for the pump to turn off before leaving the car, because as I mentioned earlier, it will run with the key in any position and if the oil pressure switch is sticking, then you could return to a dead battery.

What would cause the fuel pump run time to increase? Mine is running much longer after shutting off than it used to. It used to shut off almost instantly. Now, it runs at least 5 seconds.

GM TSB...

quote Vehicle: All Technical Service Bulletins Fuel Pump Run-On Number 87-6(Gasoline)-63 Date 9/87 Subject: FUEL PUMP RUN-ON 1982-87 VEHICLES EQUIPPED WITH FUEL INJECTION Some vehicles with engines equipped with Throttle-Body-Injection (TBI) or Multi-Port Fuel Injection (MPFI) fuel systems may exhibit a condition where the fuel pump continues to run for 3 to 4 seconds after the ignition is shut off. This condition will most likely occur when a cold engine has been started, run for a very short time (oil still cold), and shut off. This condition occurs because some engine fuel systems are wired so that when the engine oil pressure is above 4 psi, the fuel pump will continue to run via a switch in the oil pressure sensor. Customers whose cars exhibit this condition should be assured that their vehicles are operating properly, and that the fuel control system insures that all fuel pumped to the engine after shut-off is returned to the fuel tank. NO REPAIR SHOULD BE ATTEMPTED. If the fuel pump continues to run for significantly longer than 3 to 4 seconds after the ignition is shut off, refer to Service Manual Vacuum and Wiring Diagrams/Electronic Fuel Injection/Fuel and Idle Speed Control Wiring Circuit for wiring information and to section 8A of the chassis service manual for information on diagnosis and repair/ replacement of the oil pressure sensor switch.

If oil is out of spec, ie you run 20w-50, it runs longer then normal. Especial when it's cold.

In cold weather, pump may run-on 5-10 sec in a cold engine. Depends on oil type and weight. If engine is hot then pump run-on will/should drop 3-4 sec and correct oil.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top of every forum page...)

Ahh now that you explained the oil pressure part of it it makes perfect sense, I had no the pump was in anyway connected to the op sending unit. Learn something new everyday on here, thanks everyone for answering question.

quote Originally posted by mtjd25: ok guys sounds good. never would have thought the oil pressure sending unit would be related, thanks.

The oil pressure switch acts as a backup if the fuel pump relay fails. Many people have had the main relay die and not realize it but they have to crank the engine longer than normal for it to start. That's because without the main relay, the oil pressure switch doesn't bring the fuel pump on until it cranks over enough to build pressure.

If you're hearing the pump prime, though, you know your fuel pump relay is working fine.
FYI, those relays are interchangeable, if one ever fails.

quote Originally posted by theogre: In cold weather, pump may run-on 5-10 sec in a cold engine. Depends on oil type and weight. If engine is hot then pump run-on will/should drop 3-4 sec and correct oil.

Thanks, Ogre. You didn't address my question directly, but you answered it anyway. When I first noticed it and became concerned, I had just cranked it in the driveway and pulled it into the garage.

One of the things that I have seen happen, is people never realize their relay has gone out, and just assume it is supposed to take as long as it does for the eng to start. If your engine seems to take a little longer to fire off than you think it should, disconnect the Oil Pressure switch connector. If the engine does not start at all in this condition, your relay is more than likely bad.

BTW-the a/c relay is identical to the fp relay--is right beside the fp relay and can be used to power the fp in an emergency.