I have a 94 Suburban with the 6.5 turbo diesel and my lift pump is not running.

If I open the drain on my fuel filter housing, I see bubble being sucked up into the clear bowl!
This truck has an aftermarket Yanmar clear bowl fuel filter on the firewall in lieu of the fuel filter on the motor. This was the previous owner's mod.

I tried running a jumper wire to the battery and the pump runs and pumps fuel as evidenced by the fuel draining from my fuel filter drain.

I checked that there is no voltage to the lift pump when the truck is running.

There is no voltage to the purple wire on the lift pump solenoid.

The fuel pump fuse in the under dash fuse block is ok.

I tried opening the fuel filler cap. No change.

Where to go from here?

Can someone explain the wiring and logic for the fuel lift pump? I thought that it runs continuously when the truck is running.

I've owned this truck for about a year and I drain the fuel filter bowl about once a week by cracking the drain valve with the engine running until I got bubbles today. Of course, I'm concerned about damaging the injector pump by running the truck this way.

Thanks in advance.

Roger

 

On a 1994 you will want to do the dual lift pump relay upgrade. There are schematics available if you google dual lift pump relay.

Or you can order one from leroydiesel.com

If you make your own, do not cut and splice any wires as many of the home made schematics show. Make it the way Leroy does, plug and play.

If you replace your lift pump with OEM style - direct fit - Use only AC Delco or GM. None of the after market lift pumps have a good record for dependability or longevity. They can also take out your Oil Pressure Switch / OPS if they draw to much current.

Get a lift pump for a 1993 - they have higher output

Your immediate problem sounds like a bad OPS. The lift pumps on 95 and older are run by the OPS with undersized wiring. You should have some voltage at the lift pump plug when the vehicle is running.

If you have no voltage, either your OPS is completely shot, you have a bad connection or a broken wire.
You can show 12 volts at the lift pump plug and still not have a good lift pump run. Often when the OPS goes bad, it will put out a readable amount of voltage but will not carry the amps to run the lift pump.

If you have any voltage coming from the OPS to the plug with the truck running, I would start by doing the dual lift pump relay upgrade. Whatever voltage the OPS is putting out should be enough to trigger a relay and your lift pump will run as it should.

Investing in the relay upgrade is money better spent than a new OPS. Only use an AC Delco or GM OPS. I have been running for years on a couple of "bad" OPS's that would not run a lift pump but trigger the relays just fine.

Use 14 ga wire or bigger for the mod.

If you order from Leroy, make sure you ask for the prime feature. I do not know if he includes it, unless you ask for it. This runs the lift pump while your glow plugs are on.

Also install a toggle switch for priming and diagnosing the fuel system when you install the dual lift pump relay upgrade. The toggle switch can also be used to run the trucl in the event something else fails. Such as if you have a relay go bad. That happens very rarely - I've never had it happen to me in the over 2 million miles we have on our 6.5's. But it does happen.

The dual lift pump relay upgrade can be done with one relay. But I have not seen that schematic posted.


Replace

 

Thanks for the quick response. I agree that the problem is most likely the OPS. Considering that it's Sunday and I need to go to the airport on Monday, I wired the relay to an ignition switched circuit at the risk of burning up in a horrible accident and the risk of this temp fix becoming permanent. Any chance this could be related to a recent switch to Rotella T6 synthetic in preparation for Winter.

 

I don't see any possibilities there other than better starts, less wear and possibly a tiny better fuel economy