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Lift pump

3K views 21 replies 7 participants last post by  teedo 
#1 · (Edited)
Hello all new to the group been looking through tons of your topics and finding tons of help and wanted to thank you all.
I do have one question to ask about lift pumps I need some clarification on how they work I recently purchased a 1994 GMC K2500 with a 6.5TD my first diesel it was non running in my testing I found out that the lift pump was rewired to a hot wire while the key was in the on only position but lost power in the start position if I'm not mistaken there should be a prime in the key on for a few sec then be on in the on position while cranking and after the ops picks up pressure then that system takes over in the on position if this is not correct can you point me in the right direction on how this works?

Thank You
 
#2 ·
Thats it.
 
#3 ·
On your '94 the OEM system has the fuel pump (LP) relay on the firewall energize in the start position, then the OPS takes over after oil pressure has been established. Since the LP was rewired to be hot with the key in the on position it probably was tapped into a 12v source that gets cut off while all energy is diverted to the starter during the crank cycle. No telling which circuit that might be. Your best bet would be to start from scratch and do the LP relay mod in the Stickies section and also install a fresh AC Delco OPS. Yours will be located under the intake manifold forward of the fuel filter canister. ~FH
 
#6 ·
Yours will be located under the intake manifold forward of the fuel filter canister. ~FH
My OPS on my 94 is at the back of the block to the driver's side of the fuel filter.
 
#4 ·
Welcome!!!

In a 94-95, there is never a priming function despite what people say.
It is suppose to but in my case, the LP never turns on when the key is on until the engine start running and OPS takes over. Which means if OPS is bad, then LP is dead also.

Priming function can be added by adding 2 relays into the LP/OPS circuit or buy a Kennedy Diesel LP/OPS harness.
 
#5 ·
Welcome!!!

In a 94-95, there is never a priming function despite what people say.
It is suppose to but in my case, the LP never turns on when the key is on until the engine start running and OPS takes over. Which means if OPS is bad, then LP is dead also.

Priming function can be added by adding 2 relays into the LP/OPS circuit or buy a Kennedy Diesel LP/OPS harness.
this is different from the OPS relay mod? it's only mentioning one relay... what is the two relay mod you are talking about?
my truck only has one added relay, and I'd like to make sure the LP comes on with the key.
got a diagram for the 2 relay you are discussing?
 
#7 ·
Thanks for the advice i will look into the the mod i did some testing on the stock relay plug and i get voltage to the it from the hot wire in and voltage to the to the starter wire while cranking and voltage from the ops wire after about 2 revs of the engine assuming thats when it picks up oil pressure and that sensor completes its circuit which tells me that the ops is working it looks new so I think im good there. So if there is no prime mode the lp should be on while cranking at least and on after the ops kicks in for the on/run position. I trace the wire from the lp and it goes through the firewall where the hood release cable runs and was connected to a orange wire and orange plug that is mounted right there on the firewall to the left and above the brake pedal?
 
#8 ·
I thought someone posted a diagram of the 94-95 system with that relay and it did run during cranking but dropped out and went back to just the oil switch for run.
 
#11 ·
I thought I remembered a OE diagram.
 
#14 ·
I still can't figure this out I took the fss out took out the stopper to make sure fuel was getting through. I then took out the return check valve and still get just a small slow flow I have good flow going into the IP like 5 psi but just a small flow coming out the return? Where is the fuel going thats going in? Anyone know?
 
#15 ·
Fuel flowing into the IP and dribbling out the the return is too subjective a description to properly troubleshoot. It would be much more helpful if you Tee in a fuel pressure gauge on the IP inlet line and get a pressure reading with key ON and LP working. ~FH
 
#16 ·
Ok I put a gauge on the inlet to the IP and get like 5 psi with the LP running I took the FSS out and fuel pours out that hole and the FSS works great the IP is full of fuel I took the top cover off to verify that and seen that it was full so I know that fuel is getting to the IP and filling it up I took out the return check valve and made sure it was in working order while I had that out I had the LP running and very little fuel comes out of the hole where the return check valve goes nothing like what is going into the IP or coming out of the hole where the FSS sits when I had it out
 
#17 ·
That return check valve opens just enough to keep the internal pressure of the IP from going over 5 PSI during operation, so the amount coming out will be negligible at 5 PSI in. At this point I would try a spare known-to-be-working PMD. Where is your present PMD located and do you have a spare? ~FH
 
#18 · (Edited)
I think that is about as much dismantling you can do.

Now, the fuel have to build pressure and things have to be in closed to build that pressure. Button it up for now.

Then you crank till there is fuel at the injector.
Loose the injector input line at the injector and tighten it when fuel start coming out of there. Keep cranking and it should start.

While cranking, make sure you pause in between.
Make sure LP is running all during this process by putting 12V direct source to it.
5 psi should be enough till the IP.

If there is not fuel at injector, then either IP is bad or just the PMD.
 
#19 ·
Well somehow I knew that was gonna come up lol. Right now its still the pump mount and no I do not have a spare. I did the test according to this site http://www.accuratediesel.com/nostart.html and my numbers seem to be fine but I guess that doesn't mean its good. Looks like I'm gonna have to get another one and do the relocation for it. What leads you to suggest a new PMD anyways just out of curiosity
 
#21 · (Edited)
PMD is the weak point in the 94+ 6.5L. They fail OFTEN. The PMD drives the IP solenoid to send properly timed fuel delivery to the injectors.

You need to do as JMJ suggested and loosen the injector line at the #1 injector, crank the engine and see if fuel comes out the line. Crank for no more than 15 seconds at a time with a 1 minute starter cool down between attempts. If you don't get fuel out the injector line it's either a croaked PMD or a failed IP. The PMD is the cheaper of the two to replace first and you should always have a spare anyhow.

Since the PMD you have is still on the IP and not remote mounted a failure would be very likely. But if you did the PMD test from Accurate Diesel correctly and the numbers look good you may be needing a IP. Try the injector line bleed and see what happens. ~FH
 
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