Oil pressure switch/lift pump question

[Thor]

I have a 94 6.5 turbo, every 30,000 miles I seem to go thru a oil pressure switch or a frame rail lift pump. It starts out when the trucks starts to miss and chug while shifting. My question is insyead of replace both I want to replace what broke. If I were to take a test light and check power at the lift pump, would I have power with 1.key on, 2.cranking, and 3. running? , or all the time. I have been told the pump gets its power thru the oil pressure switch. Is there a way to bypass the switch? because my gauge seems to work fine. Lastly I assume the inj pump has enough suction to keep the truck idling, so can you feel the lift pump vibrating with your hand, or what is the best way to check that. Thanks

[DmaxMaverick]

It is VERY important you DO NOT bypass the OPS. The OPS is what prevents the pump from running if the engine quits (like in a collision), except for start up. You really don't want to crash, and have fuel pumping all over the place.

There are two fixes to increase both OPS and pump life. One is to install a relay into the circuit between the OPS and pump. Simple and cheap. The second is a controller offered by www.kennedydiesel.com (http://www.kennedydiesel.com) . More simple, but not so cheap. Either way, JK has descriptions of both on his website, www.kennedydiesel.com (http://www.kennedydiesel.com) .

[GMC Hauler]

There are two fixes to increase both OPS and pump life. One is to install a relay into the circuit between the OPS and pump. Simple and cheap. The second is a controller offered by www.kennedydiesel.com (http://www.kennedydiesel.com) . More simple, but not so cheap. Either way, JK has descriptions of both on his website, www.kennedydiesel.com (http://www.kennedydiesel.com) .
Does the use of the relay or controller actually increase the lift pump life or increase the reliability of the system??

I have replaced 2 lift pumps in 2 years, both obtained from a certain supplier, but both of those pumps have failed. I am back, operating on the origional truck lift pump. Both purchased pumps acted up in the same manner: They turn on, but make the pulsing noise part of the time. Fuel pressure is lower and/or erratic. I turn the Suburban off, connect the red wire connector in the harness to the battery, and the pump still does the same thing. So, by doing this, I elliminate the OPS. Each time, after I replace the pump, the new pump works fine.

I will get Kennedy's new type of lift pump eventually. I am tired of getting fuel on myself, and messing with the thing. I'm just cheap, trying to get the last ounce of life out of the origional lift pump.

[DmaxMaverick]

Using a relay should help with pump life. Problem with the OPS is poor continuity, resulting in low current. When they fail, the contacts are usually burned up. Clean, full voltage to the pump should, in theory, help. Using a relay will also lessen the load on the OPS, extending its life.

[rjwest]

MOST problems with 94 - 95 lift pumps are due to OP's
switch controling Volatege/CURRENT for the Lift pump.
( statement based on review of past posts )

96 and newer have a relay to control starting Current for
the lift pump there by inceasing the life of both components.
( again based on review of past posts )

There is one over looked component in the fuel supply sustem. That is the fileter inside the the fuel tank.
In rare incidents, I would expect that dirt and the resultant pluged filter would cause the erratic operation of the lift pump. IE change of noise when operating: loud when starved for fuel, quit when adequate fuel is supplied from tank,

This is only a thought based on a problem on a 93 gas engine, that took me 6 months to resolve..
loss of power after a few hours of driving,
as fuel got warmer, the lift pump would not pull fuel through the plugged filter,. cleaning the dirt from the fuel tank and replacing the filter fixed a very nasty , hard to find problem.

[GMC Hauler]

MOST problems with 94 - 95 lift pumps are due to OP's
switch controling Volatege/CURRENT for the Lift pump.
( statement based on review of past posts )

96 and newer have a relay to control starting Current for
the lift pump there by inceasing the life of both components.
( again based on review of past posts )

There is one over looked component in the fuel supply sustem. That is the fileter inside the the fuel tank.
In rare incidents, I would expect that dirt and the resultant pluged filter would cause the erratic operation of the lift pump. IE change of noise when operating: loud when starved for fuel, quit when adequate fuel is supplied from tank,

This is only a thought based on a problem on a 93 gas engine, that took me 6 months to resolve..
loss of power after a few hours of driving,
as fuel got warmer, the lift pump would not pull fuel through the plugged filter,. cleaning the dirt from the fuel tank and replacing the filter fixed a very nasty , hard to find problem.

I thought so on the first time, but I dropped the tank and it was absolutely clean. I just don't know....

[rjwest]

GMC hauler: Well you know the tank is clean.....
Have you checked the ground for the LP....

I thought my original LP was going bad at 50Kmiles,
Got a 15PSI replacement from Auto Zone,
Now 130K miles no problems,

At ~60k I installed the removed OEM LP along with
another filter down by the tank, The old LP is still running (Limited duty cycle )...

Do you have a dash mounted Fuel pressure gage ?
Probably 1 of the best mod I have made , next to
the additional fuel filter/water seperator. and remote PMD.
The Add on water seperator always traps some water(rusty type), OEM never had any water....

[stingthieves]

Do you have a dash mounted Fuel pressure gage ?
Probably 1 of the best mod I have made

Up by the IP or back at the pump?

[rjwest]

I unhooked the fuel feed line at the Injection Pump.

Squeeze clamp, lift up fuel line.

Added a "T" at end of fuel line and another 4-6 '"
of fuel line to the fitting on the pump,

From the "T" I ran 10" of fuel feed hose,

Than 'ACE Hardware fiting' to go to 1/8 copper tubing
up to dash mechanical gage.

I don't worry about diesel in dash, and 1/8 line does not flow much

[GMC Hauler]

GMC hauler: Well you know the tank is clean.....
Have you checked the ground for the LP....

I thought my original LP was going bad at 50Kmiles,
Got a 15PSI replacement from Auto Zone,
Now 130K miles no problems,

At ~60k I installed the removed OEM LP along with
another filter down by the tank, The old LP is still running (Limited duty cycle )...

Do you have a dash mounted Fuel pressure gage ?
Probably 1 of the best mod I have made , next to
the additional fuel filter/water seperator. and remote PMD.
The Add on water seperator always traps some water(rusty type), OEM never had any water....
I do have a gage in the dash... A mechanical. It is one of the best things I put in there.. makes finding the problems easier, and i don't change fuel filters so often now.

Do you have a part number for the 15 psi replace ment for autozone? That is another option I might want to explore.

Does the 15 PSI lift pump actually run at 15 psi???

[DmaxMaverick]

Before you go experimenting with pumps....

A higher PSI pump may not be just that. The motors/solenoids used to drive the pumps are essentially rated the same between similar models. In order for them to get higher pressure ratings, volume is usually sacrificed. The same pump, otherwise, with a small dia. plunger will provide a higher static pressure, but significantly less volume. The GPH/GPM @ PSI rating is the number to focus on. Except for the high performance pumps ($$), the volume ratings fall off very quickly as the PSI increases.

If you are serious about pumps, check out JK's pumps. A little more money, but should last the life of an engine, and they won't dump trash into the fuel system if the do fail. They can also be serviced.

[Bobbie Martin]

After another OPS failure I installed Kennedy's Lift Pump Control Harness. I can say it appears to be of high quality and all the plugs are the correct ones. The only problem is you must retain the stock OPS sending unit for the oil press gauge. It comes with a tee, but installing it is a real bear. Other than that, its was pretty easy. It works great. Hopefully OPS problems are over now.

I also installed a low fuel pressure light. It uses an adjustable sending unit that I installed in the drain line. It will alert the driver (through the no longer needed "Service Throttle Soon" light) when the fuel pressure drops. Tells me what I need to know with no fuel lines in the cab.

[tommac95]

The LPs have internal power (driver) transistors energizing their solenoid coils. This is 'hard duty' for transistors. The OPS runs the LP directly pre-'96; this means when OPS contact resistance increases (which it will as it arcs off/on/off/on over time, especially in old engine , hot , idling) voltage to the LP drops ... and the transistors really stress out.

That's the scene electrically.

Mechanically , the only filter pre-LP is the pickup nylon 'sock', which also leaks and wears. A little dirt beneath the check-valves in the LP, and pressure drops below regulated level, etc, etc.

So a relay [with an integrated diode across the coil, to shunt "back EMF"] between the OPS and the LP {like the solenoid between IgSw and starter} WILL preserve the OPS points (no need replace, not switching significant load) , and preserve the LP by virtue of providing ample power, relieving stress. J.Kennedy's arrangement is BETTER, solid state switching/relay......but John reports that even his setup doesn't guarantee indefinite life of LP [not sure what issues remain , quite possibly LP quality-control , or lack thereof] .The remaining problems may only exist with the rigs still using the OEM LP.

Other things equal , a filter between the tank and the LP , will preserve the check-valves in the LP. The LP CAN suck fuel from a filter hsg on it's input.