Lift Pump diagnostics

[mobilevet]

Okay… I’ve done some preliminary diagnostics and am pretty sure I’ve got either OPS or Lift Pump failure.

Here’s what I did: opened the fuel filter drain while running and the engine stalled - was hard to re-crank, guess because of air in the line, but then finally did crank up and run/idle rough then normal - repeated the test and got same results.

Now, I’ve searched through the archives and found two ways to differentiate between these two:

1.) Do the test again and this time after it stalls, leave the drain open and go back inside the truck and cycle the ignition switch – bad OPS should get some fuel out, bad LP no fuel.

Question: Will this work on a 94? That was part of what was confusing about my search, it was hard to determine which suggestions worked for which year model. Same issue occurs with suggestion #2:

2.) Unplug the fuse for the LP (?) and short it to the battery. The LP should come on and pump fuel out the drain line (assuming valve open). No vibrations and other physical indication of LP activity and no fuel = bad LP. Physical activity but no fuel = problem up stream.

Question: Again, will this work for the 94?

Also, what about this “electronics upgrade” that is suggested (installing a relay so the OPS doesn’t have to carry to much current)??? Is this something that the 94 needs/would benefit from? It seems fairly simple to do.

Lastly (and maybe should have been first), when I did my check the truck seemed to be running adequately. I did the check because it had a history of “miss” at high speeds and wanted to rule out the LP and everything else before blaming the IP. Only thing other thing is that I did get a code18 during one “miss” event – this was followed by a reduction in power… possibly due to failure of the LP??? I don’t know I’m just trying to work through this. Any help is appreciated.

PS - I tried the "crank to build oil pressure then shut off and immediately turn switch back on (but not crank) and listen to hear LP working" test also.... I heard nothing.

[DickWells]

Pull your cover off the power dist box on driver's side under the hood. Look for the little square black relay that runs the LP. Should be in the diagram. I think, but not sure that it's the near left one as you face it from the left fender? Remove the relay. Check numbers of the pins on under side of relay. I think, but not sure (my 6.5 is 2000 miles away) that connecters number 86 and 36 are the right one to jumper with a paper clip. Someone close to the right data please jump in here. Don't be too scared of jumping the wrong ones, I've done it by mistake several times. A good LP should run and pump fuel.
If you don't have Jim's 6.5 books, it's a shame, because all this stuff becomes much easier with them at hand.
Don't remember if the relay for the lift pump is in the manuals or not. You can trace the hot line back to the OPS from the LP and isolate it to the new relay, then put a new wire in from a bat-hot fuse to run the pump and just hook up the relay to be pickled by the OPS. I think the line you're looking for is white, or gray with a black tracer. ???? I think I used a 20 amp relay. Again, it could have been ten. Dunno. You'll want a new OPS, anyway. Might as well bite the bullet. Those little contacts like milliamps a lot better than the big amps.
All this could be the IP, too. My first trip to the Chevy store had me going in for an OPS, then a LP the next day, then an IP the next day. Well worth the $100 deductible.
I havn't been much help, I'm sure, but maybe you'll get headed in the right direction. With a 6.5, always keep a jumper under that cover. Surprising how often you jump out and check the lift pump. Helps a lot to prime the filter when you put in a new one. No long cranking to re-start after a filter change.
Good lucki

[a5150nut]

Okay…
1.) Do the test again and this time after it stalls, leave the drain open and go back inside the truck and cycle the ignition switch – bad OPS should get some fuel out, bad LP no fuel.

Question: Will this work on a 94? That was part of what was confusing about my search, it was hard to determine which suggestions worked for which year model. Same issue occurs with suggestion #2:

2.) Unplug the fuse for the LP (?) and short it to the battery. The LP should come on and pump fuel out the drain line (assuming valve open). No vibrations and other physical indication of LP activity and no fuel = bad LP. Physical activity but no fuel = problem up stream.

Question: Again, will this work for the 94?

Also, what about this “electronics upgrade” that is suggested (installing a relay so the OPS doesn’t have to carry to much current)??? Is this something that the 94 needs/would benefit from? It seems fairly simple to do.

Lastly (and maybe should have been first), when I did my check the truck seemed to be running adequately. I did the check because it had a history of “miss” at high speeds and wanted to rule out the LP and everything else before blaming the IP. Only thing other thing is that I did get a code18 during one “miss” event – this was followed by a reduction in power… possibly due to failure of the LP??? I don’t know I’m just trying to work through this. Any help is appreciated.

PS - I tried the "crank to build oil pressure then shut off and immediately turn switch back on (but not crank) and listen to hear LP working" test also.... I heard nothing.

1. yes
2. yes
question yes
and yes

you can also lay under the truck while running and put your hand on the lift pump and feel if its running.
at the fuel relay under the black cover, just remove the fuse and hold it back to the soket it came out of and touch the hot post next to it.

[mobilevet]

Thanks! I have the Troubleshooting and R&R book and it has been very helpful - as has the forum.

Sounds like I need to just replace both. Anyone with a 94 that could give advice on getting to the OPS? I'm not exactly sure where it is and have read that some are very difficult to reach.

Also, I really need a good general purpose repair book for this truck that shows me stuff like wiring diagrams, locations of sensors, etc I got a Haynes book but it turned out not to be the right one (was the gas burner version) - any suggestions?

[Artworks]

Your OPS should be by the firewall, drivers side, behind the fuel filter housing. You will need special wrench to remove / install. You can loosen or remove bolts holding filter housing in place to give you more working room. The BEST thing to do is to do the relay option and should have no more OPS / Lift pump problems and it makes bleeding filter a breazze too. Check out KENNEDY'S Site for the relay and OPS. " BIN THERE , DONE THAT !!!!"

[mobilevet]

Is there any difference in the parts available at a local store vs. some available online that claim "heavy duty", etc???

[94 at Large]

..... The BEST thing to do is to do the relay option and should have no more OPS / Lift pump problems and it makes bleeding filter a breazze too. Check out KENNEDY'S Site for the relay and OPS. " BIN THERE , DONE THAT !!!!"

x2

Pretty sure an OPS is an OPS is an OPS no matter where you get it. They routinely go out and replacing them is pretty straight forward, with the proper tools, but as Artworks pointed out the Kennedy set-up is the way to go.

[JohnC]

I think the answer to Q 1 should be "no". The 94 (or earlier) will not pump fuel inless the starter is energized or there is oil pressure.

The '95 (only the '95) will pump if the key is at start and the gear selector is in D or R.

The '96 and later will pump for a few seconds after the key is cycled from Off to Run.

On 95 and earlier trucks (ODB 1) there is a red wire in the ALDL connector. Give it 12 volts and the pump should run.

[Kennedy]

Okay… I’ve done some preliminary diagnostics and am pretty sure I’ve got either OPS or Lift Pump failure.

Here’s what I did: opened the fuel filter drain while running and the engine stalled - was hard to re-crank, guess because of air in the line, but then finally did crank up and run/idle rough then normal - repeated the test and got same results.

Now, I’ve searched through the archives and found two ways to differentiate between these two:

1.) Do the test again and this time after it stalls, leave the drain open and go back inside the truck and cycle the ignition switch – bad OPS should get some fuel out, bad LP no fuel.

Question: Will this work on a 94? That was part of what was confusing about my search, it was hard to determine which suggestions worked for which year model. Same issue occurs with suggestion #2:

2.) Unplug the fuse for the LP (?) and short it to the battery. The LP should come on and pump fuel out the drain line (assuming valve open). No vibrations and other physical indication of LP activity and no fuel = bad LP. Physical activity but no fuel = problem up stream.

Question: Again, will this work for the 94?

Also, what about this “electronics upgrade” that is suggested (installing a relay so the OPS doesn’t have to carry to much current)??? Is this something that the 94 needs/would benefit from? It seems fairly simple to do.

Lastly (and maybe should have been first), when I did my check the truck seemed to be running adequately. I did the check because it had a history of “miss” at high speeds and wanted to rule out the LP and everything else before blaming the IP. Only thing other thing is that I did get a code18 during one “miss” event – this was followed by a reduction in power… possibly due to failure of the LP??? I don’t know I’m just trying to work through this. Any help is appreciated.

PS - I tried the "crank to build oil pressure then shut off and immediately turn switch back on (but not crank) and listen to hear LP working" test also.... I heard nothing.

There are 3 suggested courses of action here:

1) Replace the OE OPS aqnd keep a spare because it's going to fail again

2) Replace the OPS and wire in a relay for the LP.

3) Add my complete harness and not worry about OPS issues again.

Of course, you'll still have to verify that the lift pump is operational.

[DickWells]

The numbers to jump together under the fuel pump relay are 30 and 87. Hope no-one was inconvenienced by my listing of the wrong numbers.:o
This little routine is only good to get fuel to the filter, if your lift pump is good, or to prime and bleed the filter housing after a filter change. Saves a lot of cranking.
DW:)