Fuel pump runs after shut off- help needed!

[53GMC]

Just recently I noticed that after I turn off the engine the fuel pump used to stay on for 3-5 seconds but it started staying on longer.

Here's what I know - check if I'm right... The fuel pump receives its power from the fuel pump relay or the oil pressure switch (they are connected in parallel so that power can be provided from either source). The oil pressure switch provides power to the pump when the oil pressure reaches 4 PSI. The Fuel Pump relay supplies power to the pump when it receives a signal from the ignition switch. They are no other switches or interupts in the circuit



Couldn't I just unplug the oil sending unit and if the fuel pump doesn't run after I shut the truck off, then I will know it's the sending unit that needs to be replaced correct? I replaced the pump only a year ago and do not think it's that. I changed the oil yesterday. Could that have changed oil pressure reading any now?

Thanks for your help!!
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[gmclowsierra]

the fuel pump runs after the key is off is normal. the ecm is what commands the fuel pump on and off.

[jayman68]

If the oil is thicker that could affect the pressure and cause the pump to stay on longer. If you disconnect the wire to the oil switch the engine will start but shut off when you release the key, because the pump is not sensing oil pressure. Depending on the year and setup of the system there is different amounts of computer control.

[53GMC]

changed the oil from 10W-40 to 10W-30 and the pump did shut off when i cut the key off like it used to ....this was the first time i had used 10w-40 and when i made the change it was back to normal...thanks guys!

[jayman68]

Glad it worked out.

[donkeypunch]

Originally Posted by 53GMC

changed the oil from 10W-40 to 10W-30 and the pump did shut off when i cut the key off like it used to ....this was the first time i had used 10w-40 and when i made the change it was back to normal...thanks guys!

What year was the truck you had this issue with? I have an 87 454 tbi that has a fuel pump relay (i think...below drivers exst manifold) that has always been disconnected and once i found the loose wire i hooked it up (ignition off) and pump turns on?....Does not seem to shut off? I should mention the wiring looks hacked and the truck idles like ****.

Any thoughts?