'94 Lift Pump?

[fastcat800]

Truck ran great when I parked it after a 250 mile drive. Started it the next day and it would idle fine but would buck when I tried to drive it and after a block it almost quit and droped to an idle. Thought maybe I had a gelling problem (got down to -10 overnight) so I poured in some Power Service fuel suplement and let it idle for a couple of hours. It was a little better but still not driveable. I was going to install a new fuel filter but could not hear the pump when I turned the key to run. I put in a lft pump I removed when I first bought the truck and the problem turned out to be the OPS (it looked almost like new). I still don't hear the pump when I turn the key to run., but I was doing it by myself and had some background noise. I believe on the '94's the lift pump gets power when the switch is in the on position and remains powered until the OPS sees pressure. If not, how long does it see power when turned to the run position? If there is no initial power to the LP is there a fuse and where is it? What other things could cause no power to the LP? I am assuming the pump I put in works and will check for power to the pump when one of you really sharp guys tells me how long I have to check for power at the connector when you turn the key on. If it is only a short time I will have to wait till tonight for an assistant.

[willisk]

Pump does't get any voltage until you turn the key to the crank position.

[DickWells]

Please look at the next thread. It might not help, but hey, you never know.
G luck.
Dick W.:)

[fastcat800]

I did look there first and did a search too. Could not find when power was to the pump as all posts said there should be power in the run position not the crank position as willist pointed out. Going to check power now (that was my main ?) and change the filter. Didn't want to pull the filter if the pump wasn't working. I'll let you know if that fixes it.

[fastcat800]

Changed the filter and it runs better than ever. Took quite a bit of cranking to get the filter housing full and after a half dozen times cranking until the glow plug lite went out it fired a little. After that I could hear the pump continue to run when cranking had finished (must have been getting the run signal through the OPS) and that's when fuel started to come out he hose I attached to the bleeder valve on the top of the filter housing. Closed the bleader valve, let the glow plugs heat and it strated in less than 2 seconds and never missed a beat. I think it was a combination of a bad filter and a weak pump. It seems to have a lot more power now. The filter did not look dirty but it was 3 years old and probably 40K miles on it. Sure am glad this happened at home. Change those filters and check the LP once and a while by opening the water bleed valve.

[JohnC]

On MY '94 the pump should run only while the starter is engaged and while the engine has oil pressure.

[Robyn]

The lift pump has two (2) circuits.
1 is activated through the start (Cranking circuit) and runs the lift pump when starting the engine.
The second engages after oil pressure is up above a couple pounds and when running the pump runs constantly.

To reprime afrer a filter change, loosen the air bleed nut on the filter.
Remove the plastic cover on the junction block (Over the AC unit on the upper RH side of the engine bay)
There is a relay in there as well as a little container with a fuse in it.
These are marked.
Bend the clip and slide the fuse out of its holder. Now touch the center of the fuses plastic part (colored part) to the hot terminal in close proximity and the pump will run)
Dont need to crank the thing as this just fills the IP with air and leads to confusion, irritation and general attitude degradation :eek:

Once the fuel coming from the filter is air free, plug the fuse back up in the holder, tighten the bleed nut and your good to go.

If the OPS is bad the pump will not run when the engine is.
The fuse is also to be checked too as well as the relay.

best

Robyn