Unusual Lift Pump Problem

[86bandit]

After reading many posts concerning lift pump and OPS I have determined that my truck has an unusual failure mode. When running and opening the drain line I have plenty of fuel flow and the truck doesn't die. However, I can't hear the lift pump running when I shut the truck down ( like it used to) and I can't hear it run during glow cycle (also like it used to. What I do know is that it is somewhat harder to start but the fuel mileage remians about the same as always, if that information helps. Any advice or ideas are always welcome.

[Hye]

My '89's lift pump was noisey when the truck was new, but very quiet for years and probably 200K miles.
Replacement pumps, one on the '89 and two or three on the '94, have always been noisey at first and then get quieter. I think that any air in the fuel line or the filter assembly will also increase the pump noise. You didn't give many clues, but I see that you've been under the hood quite a bit, so I'd guess that harder starting is a slow starter motor (worn brushes, worn bearings, missing mounting bolt, bad battery or ground connections)or some open glow plugs. But not the lift pump.

[86bandit]

As a precaution I'll check for fuel flow at the filter during the glow cycle. A quiter pump I can handle but I don't trust dead silent. As far as the starter goes I can't imaine why it might be slow.....it only has 180K on it. :D But thnaks for the reply.

[ol9465er]

86... During Crank FLP powers from cct440 through FPR to cct120 (cct 120 is gray pump wire). During Run FLP powers from cct 440 through OPS to cct120. I see no circuits that give power during GP cycle (I may be wrong, but cannot see in diagrams).

You might be starting to have trouble with OPS not connecting well with lower pressures at first - check for voltages at G on DTC connector - should be 12 volt as soon as Run (will be lower of course at Crank). I had similar trouble and only had 8.5 Volts. Changed OPS and was ok.

As for shut down - with my 94, when firstly started cold and shut down soon thereafter, OPS stays closed for few seconds and FP runs until oil pressure drops (presume because of higher visc of oil). When started hot and shut down soon OPS looses pressure right away and FP doesn't continue for any time (because of lower visc oil presume).

I concur with checking starter - just had to rebuild mine - you need good, fast cranking.

Might also look at return lines for plugging or restrictions. Do search for "plugged return lines" to see if other posts on methods...

Good luck

ol9465er
1994 6.5TD, SOL-D FSD, otherwise stock
279500 KM's
4x4 Ext Cab

[JohnC]

On the '94s the lift pump does not ordinarily run during the glow cycle. It runs either when the starter motor is engaged or there is oil pressure. During the first glow cycle you wouldn't expect either. If it's not running when the starter runs, check the lift pump relay, which is activated by the starter solenoid circuit.

[ 11-11-2004, 07:40 AM: Message edited by: JohnC ]

[86bandit]

I wasn't aware that the 94 had a relay so I guesss I have learned soemthing. WOuld you have any pinouts or troubleshooting tips?

[JohnC]

It's a SPDT relay. The starter solenoid circuit pulls the relay and powers the lift pump directly through the NO contact. When the starter is off power flows through the oil pressure switch and the NC contact to the pump. the purpose is to isolate the starting circuit from the lift pump.

[86bandit]

Thanks I'll check that out this evening.