Fuel to LP problem

[JTodd]

After sitting for any amount of time, it appears that the lift pump is not pulling fuel. I can start the truck, so fuel is moving, but when the engine is not running and the LP is, it is noticeably louder than it has been. When it is loud, no fuel comes from the filter drain. when it quiets down, fuel comes from the drain. It is clearly a dry pump/wet pump sound change. The pump always comes on with the key and will stay on after cranking for a couple seconds, so I don't think on OPS. As I read, on the 96 the OPS is not the primary power for the LP anyway.

When the engine is running I have tried to open both the filter drain and the other drain at the t-stat and both at the same time and the engine continues unaffected but fuel does not come out of either one.

The problem is that after sitting, it is very hard to start because of no fuel. Once running, it runs smoothly and will restart easily after a short period. If it sits for 30 minutes or longer it gets very hard to start.

[convert2diesel]

Jeffrey:

Check the fuel line from the tank to the LP. As these get older they do tend to rust out. Just a pin hole leak could break the syphon from the tank and it will take some time for the LP to pick up it's prime after it has been sitting. Also the lines coming out of the top of the tank are susceptable to rust (great place for mud and crap to collect).

Been chasing the same problem on a couple of trucks this winter that belong to the guys who plow my laneway. Have replaced all the fuel lines on these trucks with vitron and cleaned and sealed the pipes coming out of the tanks.

Hope this helps

Bill

[Robyn]

The lift pump should run any time the truck is running.
Any lines from the IP all the way back to the tank can be "suspect"

There is a rubber hose where the metal line connects to the pickup tube at the top of the tank and these do go bad after some time.

Even if the hose is not bad the clamps can relax over time and as the hose hardens the seal between the hose and the hard line can be compromised, allowing air to seep in over the course of from a few hours to a day or more.

To really check this out, disconnect the line at the lift pump and at the tank and pressure test it with 20 PSI air pressure or so.

Any leaks will be obvious at this point.

If you dont find anything here try checking forward of the lift pump to the filter.

The factory filters have a nasty habit of rusting out in the lower well area and may not leak much but will allow air to enter and cause a "back siphoning of the fuel to the tank.

Even the line from the filter to the IP is suspect as is the water drain line.

If these are still factory stuff I would replace all the soft lines, JUST BECAUSE.

All it takes is a tiny weak spot that may not leak under 5 PSI fuel but certainly leak air back in.

For a complete test you can unhook the feed at the IP and then connect air at 20 PSI at the tank fitting and then look for leaks.

This is not dangerous pressure but will certainly expose any issues along the way.

best

Robyn

[waynep]

Hello All;

I have been having almost the exact same problem except mine tends to stall and then not want to restart.
I have replaced the lines from the pump back with new ones, repalced the fuel tank, replaced the tank sender unit, replaced the lift pump at least three times and still it fails.

The only thing that seems to help is a totally full fuel tank for some reason. Once half empty, stalls again and almost impossible to start.

Help also???

Wayne

[JTodd]

Well,
Problem persists. When it is running, it is smooth but I notice that even then the LP is making a tremendous racket. Can these fail but still operate? That is can they still operate mechanically internally, but not actually pump fuel?

[rustyk]

A pump can fail to pump (valve, seal, etc.), and still operate. However, if it's making a racket, it sounds like it's not getting fuel - it would be fairly quiet if it weren't running in air.

[JTodd]

I pulled the outlet fitting on the pump and hit the key (actually had my daughter hit the key). Fuel came out, but not as much as you would think. I also know that no matter how much I ran the LP, no fuel was coming out of the filter drain. I replaced the pump and no fuel flows freely. I have not even been able to crank it yet, I killed the batteries - dead. Now to figure that out.

[waynep]

Hello:

One thing that I noticed but I don't know what it means is if the fuel tank is totally full, it does not seem to happen and fuel flows OK.

At around 1/2 tank, then the problems seem to start. Driving me crazy.

Does this make any difference with yours?

97 3/4 T 4 WD Sub

[JohnC]

If the pump is making a rapid hollow rattling sound it means eiter it's not getting fuel (a leak or obstruction in the suction side) or the check valve(s) have failed and it can't make pressure.

[JTodd]

Hello:

One thing that I noticed but I don't know what it means is if the fuel tank is totally full, it does not seem to happen and fuel flows OK.

At around 1/2 tank, then the problems seem to start. Driving me crazy.

Does this make any difference with yours?

97 3/4 T 4 WD Sub

I noticed that when I parked nose down on an incline, even just a driveway, it would start much better. I think that as the fuel sought its own level it would be closer to the IP than if level. That would apply to a full tank v. partial tank.


JohnC If the pump is making a rapid hollow rattling sound it means eiter it's not getting fuel (a leak or obstruction in the suction side) or the check valve(s) have failed and it can't make pressure.
The pump was very lowed, like rocks in a tin-can type of noise. I noticed that if the tone would change - quieter - it would be pumping fuel when checked at the filter drain and it would fire right up,as in with hardly any (3 seconds) worth of cranking. If it stayed loud, I knew I was in for problems. Also, even when I got it running I could still hear the rocks-in-a-can noise from the LP. Once running, it ran fine even at highway speeds. No apparent hesitation or other issues. It makes me think that once the IP was pulling fuel, it would run, so no blockage.

I am having other issues now, so I have not been able to test that theory.

[JTodd]

Final update.

After replacing the LP, fuel flowed from the filter drain. The LP is also much, much quieter than it was. While trying to nurse it with a failing pump, I killed the starter. Once that was replaced, it fired right up. Interesting that the pump failed, but still made a lot of noise.

Man, what an ordeal.

[DmaxMaverick]

Final update.

After replacing the LP, fuel flowed from the filter drain. The LP is also much, much quieter than it was. While trying to nurse it with a failing pump, I killed the starter. Once that was replaced, it fired right up. Interesting that the pump failed, but still made a lot of noise.

Man, what an ordeal.

Actually, that is the most common way the fuel pumps fail. I can't tell you how many times I've heard of the noisy pump before it finally quit. By the time they start to be more than regularly noisy, they've failed. All the while, folks will chase wild geese with "unrelated" issues (that mysteriously disappear with a new LP), because "the pump is still pumping". Glad you got'er fixed.

[JohnC]

Interesting that the pump failed, but still made a lot of noise.


That's what happens when one of the check valves in the pump won't seal.