I have a 96 K3500 with this great 6.5. The truck is dying on me. I have checked all the connections. Changed the fuel filter and put an additive in the fuel. I'm now ready to start changing parts to find the problem. No codes! I know it's a fuel problem because once it dies if you open the bleed on top of the fuel filter there is no fuel. Work the key a few times and you get fuel after the lift pump works a little. I was ready to throw a PMD on it for $389, but here's my question. Could it be the lift pump? The lift pump works sometimes, but doesn't others. See after it dies sometimes when you hit the key lift pump runs, but sometimes it doesn't. You can crank the oil pressure up, but still won't work. Wait a while and it will work again. What I'm getting at is the lift pump only works part of the time even when you try to start it. I say it's working because I hear it pumping.
I'm ready to start trying to get this thing dependable again. Where should I start? Oil Pressure Sensor? Lift Pump? PMD?

Well you're on the right track by checking connections and fuel filter. I'm no expert on Diesels, though from what it sounds like, could be either the Oil pressure switch or lift pump. A test I've heard on here would be while the truck is running, crack open the fuel pet**** located by the thermostat housing, if the truck stalls when you open it, it's usually lift-pump related. Not sure if the lift pump failing would throw any codes or not. If the PMD is in the factory location on the injector pump, it's a wise choice to buy one that is re-located out of the engine compartment like I think you're wanting, indicated by the $389 price.

From my limited lift pump experience...

Might be the 12 volt connection or wiring to the lift pump.

Also look into the lift pump relay, as well as the supply voltage to it.

Finally the lift pump itself.

If your sure it's happening in start mode and run mode that pretty much rules out the OPS.


Most likely it's a bad connection.

Lift pump is electronic - will vibrate and clack without pumping fuel.

Good lift pump will vibrate hard till pressure head builds up, then will quieten down, considerably.

A '96 is well passed due for a lift pump and OPS

And you should seriously consider remoting the FSD\PMD.

Sounds like the sock in the tank is plugged....

...Where should I start? Oil Pressure Sensor? Lift Pump? PMD?

Start here with Item #1. (http://www.dieselplace.com/forum/showthread.php?t=42248) and post your results here. As stated in the Diagnostic Checklist, perform these tests before you buy any parts!!!

Please also fill out your Signature in the User CP link at the top right of the page, so we all know what we are helping you diagnose each time you post. It is a common courtesy that we all like to extend to each other here...

Regards,

Diagnostic checks
1) Fuel runs out- plenty enough to run engine
2)SEL not on shows no codes
3)a&b 120000
4)no codes
5)na
6)clean
7)just changed
8)cleaned/ batteries charged/ tight
9)no
10)no
11)no
12)yes no difference
13)yes
14)yes black under load lots of it
15)seems ok unsure
16)?
17)3500
18)#1
19)yes cetane boost?
20)ky
21)no
22)none
23)no
24)stock
25)haven't checked

Diagnostic Questions for reference:

1.Lift pump test (http://dieselplace.com/forum/showthread.php?t=39350)- describe results
2.Service Engine Light - on/off/intermittent
3.Model year
3a. Odometer reading
3b. Miles on major engine components if been replaced
4.Do you have any engine codes? Yes or No.
5.If you do have engine codes - what are they?
6.Air Filter condition (visual check)
7.Fuel filter condition (freshly changed or condition unknown)
8.Condition of Battery terminal connections (cleaned and tightened)
8A.Known condition and age of the batteries.
8B.Are batteries of differing age or are they a matched set?
9.Upon cold start, does the radiator hose get hard quickly? Yes/no
10.Upon cold start, do you have excessive white smoke? Yes/no
11.Do you have excessive cranking time before the engine starts?
12.Have you used the block heater, and does this effect engine starting?
13.During hard acceleration, do you have excessive black smoke? Yes/no
14.Do you have any unusual exhaust smoke issues?
15.Turbo check out (http://dieselplace.com/forum/showthread.php?t=39368)- pass/fail
16.Do you have an EGR on the engine? (An F or an S engine code)
17.Indicate if you know if it’s a 1500, 2500, 2500HD, 3500, 3500HD.
18.Indicate fuel that you are using: Bio-diesel, #2 Diesel, #1Diesel, SVO/WVO, other
19.Are you using any fuel additives? If so, please list.
20.Please indicate geographic region you are in: (example: Texas or Canada)
21.Do you have any service history available that might pertain to the problem you are having?
22.Please indicate any modifications to the vehicle that might help us diagnose better.
23.Upon unscrewing the fuel cap (http://dieselplace.com/forum/showthread.php?t=42338), do you have a large vacuum formed in the tank? yes/no
24.Location of PMD/FSD (http://www.dieselplace.com/forum/showthread.php?t=39436)? On the pump/remoted over intake/remoted out of engine bay (please specify specifically)
25.Are ALL glow plugs in proper working order as per this thread (http://www.dieselplace.com/forum/showthread.php?t=39182)?

Answers:

Diagnostic checks
1) Fuel runs out- plenty enough to run engine
2)SEL not on shows no codes
3)a&b 120000
4)no codes
5)na
6)clean
7)just changed
8)cleaned/ batteries charged/ tight
9)no
10)no
11)no
12)yes no difference
13)yes
14)yes black under load lots of it
15)seems ok unsure
16)?
17)3500
18)#1
19)yes cetane boost?
20)ky
21)no
22)none
23)no
24)stock
25)haven't checked

...
14)yes black under load lots of it
...

This tells me that it is probably not the fuel socks being plugged.

But this also tells me that when you DO get fuel, then you are getting adequate amounts (pressure/flow).

Perhaps the OPS is on its way out and IIRC on an OBDII truck, the PCM actually runs the pump through a relay during WTS period. IF that assertion is indeed true (maybe GMCTD or other can confirm), then I would suspect the OPS...

I would probably start there, as it is cheap and easy to replace (with no wonderful lingering diesel fuel smell). The next step would be that Delphi FP905 lift pump...

Regards,

On OBD II OPS runs LP once truck is up and running IIRC....

Fuel sock may still be the cause....I had an exact same problem about a year ago with my Dad's 97 6.5 TD....

EDIT - Here is the thread that has my troubleshooting in it...

http://www.dieselplace.com/forum/showthread.php?t=56208&page=1

Yes - there is conflicting information on the OBD2 lift pump system, but near as I can figger -

OBD2 PCM pulls in the relay to run the lift pump in Wait to Start and Start, where the OPS is supposedly redundant - PCM does monitor LP for applied power.

It appears as tho the OPS does run the LP after start, and if LP power is lost, PCM will kick in the relay to supply power.

Ratcheting the key back and forth to get LP operation would indicate failing lift pump, or failing Ign switch, or failing LP Relay

We were both close...but not quite right....here it is straight from the General's mouth...

"When the key is turned to the ON position the PCM energizes the fuel
pump relay for a 2 second prime. During cranking the PCM will also
ground the fuel pump relay and continue to ground it while the engine is
running. The oil pressure switch is used as a secondary power supply for
the fuel pump in case of fuel pump relay failure. A minimum of 28 KPa
(4 psi) is required to close the oil pressure switch contacts.
When the ignition switch is returned to the RUN position, the oil pressure
of the running engine maintains electrical power to the lift pump (up to and
through 1995 trucks). If engine oil pressure drops below 28 kPa (4 psi),
the engine may run poorly or stall when the lift pump circuit opens. 1996
to present trucks operate the lift pump through the fuel pump relay. The oil
pressure switch is only a redundant power supply."

Easy test -

Crank the engine, measure at pin A lift pump prime - if the relay is pulled in, as the book sez, there will be no voltage on that wire.

+12v there, the relay is off and the OPS is the supply

We were both close...but not quite right....here it is straight from the General's mouth...

"When the key is turned to the ON position the PCM energizes the fuel
pump relay for a 2 second prime. During cranking the PCM will also
ground the fuel pump relay and continue to ground it while the engine is
running. The oil pressure switch is used as a secondary power supply for
the fuel pump in case of fuel pump relay failure. A minimum of 28 KPa
(4 psi) is required to close the oil pressure switch contacts.
When the ignition switch is returned to the RUN position, the oil pressure
of the running engine maintains electrical power to the lift pump (up to and
through 1995 trucks). If engine oil pressure drops below 28 kPa (4 psi),
the engine may run poorly or stall when the lift pump circuit opens. 1996
to present trucks operate the lift pump through the fuel pump relay. The oil
pressure switch is only a redundant power supply."

Since it would appear that there is redundancy in the OBDII lift pump power circuits, then I would suspect a failing lift pump since it seems to operate fine at some point (note "lots of black smoke" at WOT) and not to operate at others.

We were both close...but not quite right....here it is straight from the General's mouth...

"When the key is turned to the ON position the PCM energizes the fuel
pump relay for a 2 second prime. During cranking the PCM will also
ground the fuel pump relay and continue to ground it while the engine is
running. The oil pressure switch is used as a secondary power supply for
the fuel pump in case of fuel pump relay failure. A minimum of 28 KPa
(4 psi) is required to close the oil pressure switch contacts.
When the ignition switch is returned to the RUN position, the oil pressure
of the running engine maintains electrical power to the lift pump (up to and
through 1995 trucks). If engine oil pressure drops below 28 kPa (4 psi),
the engine may run poorly or stall when the lift pump circuit opens. 1996
to present trucks operate the lift pump through the fuel pump relay. The oil
pressure switch is only a redundant power supply."

Is this diagram only applicable to OBDII trucks? If so, what does the fuel pump relay on an OBDI truck do?

Thanks,

No fuel during WTS, Start closes the relay, Run oil pressure closes the OPS.

Is this diagram only applicable to OBDII trucks? If so, what does the fuel pump relay on an OBDI truck do?

Thanks,


Here....you can have a look for yourself....to me, it looks like the relay supplies the LP with voltage during cranking (which is why if I put my truck in 'D' without the truck running I can turn the key to 'start' and the fuel system *should* prime IIRC....

It looks like you can also connect power to the red wire and prime the system that way....must be a pin in the ALDL....


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EDIT - GMCTD is right....I was trying to attach pictures while he was posting....

Dudes, site regs prohibit posting copyrighted data in any form, so I gotta edit it out...............abra cadabra - it's gone!

But, fairly certain the OBD2 change is only to run the LP in Wait to Start, so IP is fully primed at start - my OBD1 starts sooner if LP runs during WTS

Test your A LP Prime drop - engine bay, around fuse center - to disprove that.

'94 OBD1 is in ALDL connector, '95 is there or\and in engine bay

Using a 10 foot (2-wire) extention cord:

Cut off both ends of the extension cord.
On one end, solder one wire to the center post of a SPDT Toggle Switch
.................solder the second wire to an outside post of the Toggle Switch
Place the Toggle Switch on the seat inside the cab of the truck.
Route the wire out the window and under the hood.
Hook one of the underhood wires to terminal A3 of the Fuel Pump Relay (maybe with an alligator clip )
.................Hook remaining underhood wire to Battery Positive.
Flip the switch Off and On until you hear the Fuel Pump running.
Leave the pump running and crank the truck.

If the truck dies over and over, you can be confident it is:
NOT the Oil Pressure switch,
NOT the Fuel Pump Relay,
NOT the PCM and
NOT the wiring
.

The lift pump can run and vibrate and make noises just fine, but if the valves are compromised, it will pump no fuel against any head.

Test for this is put your thumb over the open tube - flow stops = bad\equiv pump

Dudes, site regs prohibit posting copyrighted data in any form, so I gotta edit it out...............abra cadabra - it's gone!



That's bogus!!!

Sorry mods....:p: