Mystery Fuel Problem, driving me crazy

[waynep]

Hello All:

97 3/4t SUB. Problem, very randomly and intermittently, and also most of the time, vehicle has fuel starvation problems, with little or no flow out the water drain.
Replaced the lift pump two times and verified that it is working OK.
Replaced the fuel tank and sender unit.
Verified that proper voltage to lift pump.
When checking fuel flow, powered by the lift pump test connector, and with filter removed, there is almost no flow.
Checking fuel flow from water drain, with filter, and powered by test connector, almost no flow.
When disconnecting lift pump output, and powering, there is some flow but seems low.
Checked all feed lines, and they seem OK and air blows through OK.
Question, what coud I be missing? Could Bio Diesel do something odd in the fuel lines to cause a restriction?
If the return lines were blocked or restricted, could it somehow cause this?
Any issue with the type of fuel filler cap? Is there some type of vacuume balance that is needed to flow fuel properly? Is there a more powerfull lift pump that I should be using?

I am totally mystified, covered with diesel fuel, and just cannot get it to flow fuel properly out the drain. It used to flow fine, and would even run OK with the lift pump un powered, not any more.

Any suggestions appreciated. I have never seen anything like this.

Regards,

Wayne

[More Power]

If you're using commercial grade bio-diesel, this shouldn't be a fuel problem.

Test the lift pump off the vehicle using hoses and a bucket. A pint in 15 seconds is the minimum acceptable flow.

If the lift pump passes the bucket test, work your way back to the tank pickup & fuel pickup sock.

The fuel tank filler cap is rated to 2-psi positive pressure and 1" vacuum. It's supposed to vent when either pressure/vacuum is exceeded. Running/testing without the cap on the tank will let you know if that's the problem.

Jim

[gerard]

I presume you tested the voltage on the lift pump "on charge", not by taking off the connector and thus mesuring? Check your metal fuel line and the little piece of rubber line between tank and lift pump, a little perforation makes the pump drawing air thus deminishing the fuel flow.
Check fuel flow directly after lift pump to determine if the problem is between tank and lift pump or between lift pump and filter.

Good luck!

[waynep]

Hello All:

I get a little bit better flow from at the lift pump than I do at the water drain valve, but not much. But when connected back togather, reduces it further. Pump,tested ok in a bucket.
Checked voltage at the pump with pump on and even wired pump directly from battery, no change.

I replaced the flexable lines, but will check again to make sure that they are not kinked.

IT is so strange. Not much between the pump and fuel sender unit.

Regards, Wayne

[simon]

with enough fuel in the tank,when removing LP ,there should be steady gravity flow from the line. if not,then you have a restriction some where up stream.Maybe pinched or kinked line or hose behind the tank, plugged sock or something wrong with newly installed sender unit in tank. plugged filler cap orifice causes to much vacuum in tank ,restricting flow as well. Check LP inlet side for obstruction sucked in.

[Robyn]

Start at the tank as close as you can get and have a helper apply a tiny bit of air pressure with a rag wrapped around a blow nozzle.

Check the flow as close to the tank as possible. if OK
Move ahead to the next break point and do the same test.
Fuel should flow very freely from the fuel lines.

Any rubber hoses between the tank and the lift pump should be suspect and probably replaced, the same for the lines from the pump to the engine.

The rubber hoses can see degradation of the inner liner even to the point that they will close off almost completely.

Dont leave any stone unturned. Be sure that the steel lines are free and clean too. A chunk of rubber material from a hose could have come loose and is blocking the flow.

Once you have checked the lines completely you should find the cause.

The lift pumps on these rigs will pump plenty of fuel.

With a fresh pickup tube assembly you are good to that point so the issue has to be from the tank connection forward.

An issue with the return will cause issues but not stop the pump from flowing correctly.

A voltage issue is never to be discounted BUTTTTTTT these pumps will still work fine even at 9-10 Volts.

***Some bio diesel can cause issues with hoses so be sure this is checked***

I wont run bio in any of my rigs due to the issues have have seen with the big rigs using it.
I can get enough problems with the regular stuff.
Here the Bio is so expensive that there is no reason to use it.

Keep us posted

Robyn

[daustin]

I'd also try it with the fuel cap removed, you may have a vent issue.
Don

[waynep]

Hello All:

Finally gave up and decided to essentially remove all the fuel lines and check them inch by inch.

What I found was I believe GM parts sabotage, as I can't imagine what else could have happened.

About half way down the inside, of the purchased brand new from GM in Aug half flexable and half rigid fuel line, was a piece of what looked like yellow saran wrap, about 2 x 1 inches, that was partially blocking fuel.
When blowing with low pressure air, everything seemed OK, but when using high pressure
80 psi, out came the little piece of plastic wrap.

Now fuel flows fine with and without the lift pump as posters above said it should.

I just can't believe that this piece of plastic wrap got in there by itself?????

Thanks to all who responded, and I would suggest for any fuel problems to make sure and blow out the lines.

Regards,

Wayne

[sailun]

Had a similar problem on one of my boats. Ran fine at idle up to half throttle, but at cruise speeds would suck the Racor filter bowl dry, and die.

Eventually, after much time and frustration, I removed all lines, and ran a wire thru them, or blew them out. Out came a very well-preserved hornet, stuck at one of the elbows.

There was no screen on the fuel tank vent, and no screen on the pickup tube.

[waynep]

Hello All:

Well, after dropping the fuel tank and blowing out the first supply line from the tank, half rubber and half metal tubing, and brand new from dealer, it ran OK for about a week and then back to its old tricks again.

I have been running pump Bio Diesel here in New England and am now wondering if that could somehow be the problem causing the in tank filter to clog or something. New tank also and sender unit.

I have switched to regular diesel and it now seems to be OK, but for how long?

Opoening the fuel cap does not seem to make a difference. When it stops flowing, nothing seems to work.

Any others have seen problems with pump Bio?

Going totally crazy.

Regards,

Wayne

[donbfishin]

I had a fuel starvation issue after replacing my torque converter. Turns out the trans shop pinched the rubber hose behind the fuel filter (95 K2500) when replacing the trans. Loosened filter assy & straightened out hose, no problems for last year.

[waynep]

Hello All:

Switched back to regular diesel, 1/2 tank diesel and 1/2 tank B-20 from pump and it seems like it is running better and not totally stalling due to lack of fuel.

I am not convinced that it is 100% better, but a lot better than it was. Will have to wait till all the B-20 is consumed.

Really weird. Maybe I will add some diesel treatment to help.

Crossed fingers.

Thanks to all,

Wayne

[93GMCSierra]

I have run 100% bio with no problems if you have cleaned your tank so there could be nothing being pulled up and the sock and filter are clear bio should not be a problem.