I noticed a sound coming from the rear of the car when it was parked with the ignition off. Sounds like the fuel pump, it lasts for a few seconds and then repeats every 20-30 seconds - not regular. If I'm under the hood I can hear the fuel pump relay (drivers side firewall) clicking with the noise at the rear of the car. The temporary cure is to disconnect the battery.

I've just started troubleshooting this and I'm looking for ideas. I searched this forum and found a few hints (oil pressure switch) but nothing makes a lot of sense at this point. Has anyone run across this?

Thanks

 

BTW: Sorry I forgot to mention it - 1988 IROCZ vert with TPI.

 

That would be the first place to check. If not, the relay is getting a non ignition 12 volt supply.

 

Good troubleshooting, it's clear that you did your homework here. I like it.

The fuel pump is powered either by the ECM via the relay or by the oil pressure switch.

The relay is activated by the ECM and ECM only. So it cannot be accidentally tripping and turning on the fuel pump. However, there are other relays in your 1988. Two MAF relays. And one of them is activated by the fuel pump circuit. So you are probably hearing the MAF power relay clicking.

As I mentioned, the fuel pump is also powered by the oil pressure switch. It is a backup mechanism just in case the main relay fails. The oil pressure switch sends power to the fuel pump whenever there is good oil pressure in the engine. So if your $5 relay goes, you are not stranded in the middle of nowhere.

I would suspect that your oil pressure switch is tripping the fuel pump. They tend to go bad (it is a very common failure on our cars). You can reach under the car and disconnect it. It is located on the bottom of the block, right above the oil filter. It has at least two wires: orange and tan/white but it might have a third wire: tan (not sure on your year).

If disconnecting your oil pressure switch fixes the problem, you will just need to replace it eventually (you don't need it badly, remember, it is just a backup circuit). They cost around $35, if my memory serves me right.

Hope this helps.
Lou

 

Your information is much appreciated. I'll start with the oil pressure switch, in the manual it's referred to as the fuel pump switch but I know it ties into the oil system from installing an oil temp gauge in that area (drivers side engine block low).

I may have confused up the fuel pump and MAF relays. They are right next to each other. I should be able to measure any voltage signal at the connector.

The most confusing part was how anything gets power with the ignition key out but if I understand the schematics correctly, the oil switch is not tied to the ignition switch but is hot at all times.

I'll let you know how it turns out.

 

Ah, so you know where exactly the oil pressure switch it. Great.
Yes, it is a switch for the fuel pump and it is actuated by the oil pressure. Sometimes referred to as the FPOP (Fuel Pump Oil Pressure) switch.

You should have 3 relays for the fuel pump and MAF sensor. Plus the cooling fans have one relay each too. So it's not easy to pinpoint the actual clicking relay.

Yup, you understand the diagram right. The FPOP switch gets direct power from the battery, no switches in path. That's why it makes the fuel pump run without the ignition on.

Yes please, definitely let us know. Conclusions in troubleshooting threads are often underrated.

 

Lou:

Well, as you may have guessed, the problem is of the worst kind - intermittant. When I re-connected the battery, the problem wasn't there. Knowing that it (the problem) will return I replaced the oil pressure switch with Standard #PS216. I had ordered the ACDelco referenced by the OEM but it turned out to have the same electrical connector but smaller pipe threads on the end that threads in.

I used the opportunity (the car up) to retighten the tubular K-arm and LCAs to spec.. I take the car to an AutoX event tomorrow.

 

You can figure out which relay is tripping by touching it. You can feel them click.

 

Correct.
Has power direct from the battery; switch only closes to run the pump when engine has oil pressure (or possibly as in your case goes faulty)
The pump relay also has the same hot power source so if the relay is faulty and staying closed will give the same result

 

Great info - thanks. Just waiting for it to happen again.

 

Vetteoz, what year/model is that diagram from?
It does not look like any TPI fuel pump wiring I know (pre-1990 or post-1990). The wire colors are very different plus there is an extra FP fuse. And the C124 is a new one.

 

From a pre '90 Vette when the FSM had color diagrams ; I use it for newbies as the wire colors make it easier to make sense of
The Vettes have a pump fuse in the fuse box as shown between pump relay and the pump
Maybe I should have used a 3rd Gen one ?

 

That's the right diagram, thanks.
I didn't want anybody getting confused with the different colors, connectors and fuses.

 

HA just had same issue
I pulled all the realys off of fire wall (I was in a mall parking lot dressed to the 9's) and fuel pump kept right on humming!!!
My date was like, "We going to da movies or are you gona work on your car again?" I never disconnected my battery so fast... then relised power locks and windows and car alarm dont work that well when battery is disconnected...

1# The fuel pump relay is only closed (flowing power) for a few seconds at start up when working correctly.
2# The "Oil Pressure Sensing" Fuel cut off switch, is closed (flowing power) only if oil pressure is +15lbs or so, when working correctly.

so if either one sticks closed the pump runs 24/7.

#3 Car alarms might have realy to cut power to fuel pump it may fail too.

vibrations,condensation,dew,puddles,rain,floods,melted wires may give intermittent results

 

You don't have to worry so much about the car alarm if the battery is disconnected... have fun trying to steal that :P

 

It's been two weeks since I changed out the oil pressure (fuel pressure) switch and the problem has not occured since. Thanks for all of your help.

 

Thank you for following up. It will help somebody else down the road.

 

The pump relay is run to prime the rails by the ECM for 2 sec when key first turned on , then stops
It is then run again ( as long as the engine is on ) after the ECM see the signal from the dist that the engine is cranking over

The OP switch is a fuel ON switch that makes the pump run anytime engine has > 4 psi oil pressure;
it is a backup to the pump relay should that relay fail ; you can still start and drive your car home
It will not save your engine by cutting off the fuel with low oil pressure because the pump relay is also making the pump operate keeping the engine running

 

Ah yes the oil pressure switch.