1991 K2500 fuel pump keeps running

[wamullins]

UPDATE: I changed the oil pressure switch. When I pulled the wiring connector from the switch, it was filled with oil. Everything back together, now when I turn the truck off (after starting the engine) I still hear a hum/buzzing sound from the tank area, but it only last for about 15-20 seconds. I've owned the truck for about a week now and have driven it just about every day and never noticed this before. Once again the buzzing does stop after several seconds, last night the buzzing continued until I unhooked the battery. Thanks to all for your help.

[crabtruck]

Good to hear it's narrowing down.
May want to look for the oil problem to repeat on the other connector.
If the pump runs for 15sec with key out and relay in, try pulling the relay again.
If now it does not happen, the key is likely bad.
15sec is about the amount of time the ECM
tells it to come on before oil pressure is up.
Does radio work when pump runs and key out?
Would make the same case for key bad.
Something is making the wires I colored blue get hot.

[wamullins]

Thanks for the info, will check more tomorrow.

[mudseeker]

on the vortecs, the pump "primes" the system after key off, engine off to prepare for the next ignition cycle. not sure if your engine has this feature though. normal prime is 1-5 seconds. faulty regulator maybe?

[wamullins]

Originally Posted by crabtruck

Good to hear it's narrowing down.
May want to look for the oil problem to repeat on the other connector.
If the pump runs for 15sec with key out and relay in, try pulling the relay again.
If now it does not happen, the key is likely bad.
15sec is about the amount of time the ECM
tells it to come on before oil pressure is up.
Does radio work when pump runs and key out?
Would make the same case for key bad.
Something is making the wires I colored blue get hot.

Pulled relay, fuel pump will run with relay out
Radio does not work while pump is running (switch off)

[wamullins]

UPDATE Pulled relay and fuel pump fuse, when I turn switch on, I hear buzzing from tank, but engine will not start, put in FP fuse (left relay unplugged) and engine will start. Anything else in tank that could make this noise.

[crabtruck]

Originally Posted by wamullins

UPDATE Pulled relay and fuel pump fuse, when I turn switch on, I hear buzzing from tank, but engine will not start, put in FP fuse (left relay unplugged) and engine will start. Anything else in tank that could make this noise.

I can make that noise if I take a hit of helium, kinda pinch my nose,
and then ....nevermind.

You still have too many variables in play at once if I follow correctly.
Basically when you turn the key, the ECM should enable the relay,
which runs the pump while you crank. This is for a short (15 sec) time.
The ECM then disables the relay and the now-turning-engine has
developed some oil pressure (over 5 psi) which is enough to activate
oil "sensor". While oil pressure remains up, the pump receives power
through that sensor. In other words power for pump comes from
first relay then trades to oil sensor. The sensor is in many cases
two devices in one: pump switch and pressure sender for gauge.

It then follows that if relay (and fuse !!) is removed and engine is not
turning to develop oil pressure, then pump should have no source of power.
But yours magically does.

You need to find the incorrectly occurring source of power. I figure
if you were in love with your voltmeter, you would have already found it.
So we'll go another way. We'll take on faith that new sensor is good.
Radio not working implies (not proves) key is not problem.
You need to disconnect items until key-on does not cause pump to run.

First lets be sure that the pump is running.
When you disconnect the relay you are left with a relay plug in your hand.
Of the 5 contacts, one is ground (blk/wht), one connects to pump (gray).
You want to monitor the voltage there across wires. Not quick read.
An analog meter may actually be easier for you to read, digital works.
May even be good with a test light. Looking to see if voltage is as high
as battery or some noticable lesser value as things change.

Wrong wires will either connect to nothing (red)(so called prime connector)
or be always hot with key(org) (to fuse). Third wrong wire will be
slightly smaller than the two you want and is ECM control of relay(green/wht).

With the above setup, and all connected except relay, if you turn key
and the pump is what you are hearing then voltage should show up.
See if voltage is completely gone when noise stops or is low voltage there.

If you see battery voltage there, you have a "short"
If you see lesser voltages, you have a "leak".

See if any changes if you do the same as above but with oil sensor unplugged.
If not, then follow the grey wire from the relay to sensor manually,
and see if it goes one or two other places besides oil sensor.
One is to ECM, second would be to fuel module.If mod, unplug and retest.
If no mod, and especially if tests above were showing a "lesser" voltage,
suspect oil impregnated wires. Harness may have internal "leak".
Best check on that after visual wold be disconnecting batt, relay out,
oil sensor unplugged, eng harness to ECM unplugged, then ohm the wire
contacts at sensor and do ohm test of relay wires that used above.

Basically all of the above amounts to keep unplugging until no voltage and
if that does not get it, start looking for shorts, whether external damage
caused or internally cross-talking from oil damage.
This last is what I think based on description.

Just noticed I called you waRmullins, sorry bad eyes.

[wamullins]

No prob. on the name......
As far as the prob with the truck, it is no longer doing it.
But I will keep all of this info handy, in case it starts again
I know it is hard to diagnose and fix problems over the internet, but it is a great place to learn some things. Thanks to all for your help and the info...

[crabtruck]

It's working ???
Now what am I supposed to do with all these crayons I just bought ???
On second thought, don't answer that.

Glad to hear it's good.