Fuel Filter Mystery

[Max Mini]

Over time, my '91 7.3 IDI short bus has needed fuel filters more and more often. I'm currently on a long road trip, and so far i've needed a new fuel filter every 1000 miles or so. Tanks have been dropped and checked for gunk/algae/rust. Rear tank is totally clean and front tank has some rust but nothing significant enough to cause this issue. There is no in line filter, just the one under the hood.

It's been suggested the issue is a faulty "fuel filter" light, but if that were true, replacing the filter would have no effect. Before the light starts coming on, I begin to lose steam on hills, and my fuel economy slides down from my average of 11.5 mpg to less than 10. When a new filter is installed, this problem is corrected - for a short time. Eventually, it starts happening again, and a while after that I start getting the fuel filter light on startup.

There is a separate "water in fuel" light which isn't coming on. When I prime it, no water comes out, only a healthy stream of fuel.

Any ideas? Please talk to me like I'm 4 years old because that's how long I've had this bus, and that's as far back as my experience with diesel engines (or any kind of engines) goes.

Thanks in advance!

 

[Austin86250]

the only thing that would make since is the tanks being clogged up for such little fuel filter life, you could always replace the tanks for pretty cheap and simple the rear one costed about 100$
unless the fuel is getting contaminated via outside sources like a really sketchy gas station or a farm tank

 

[Max Mini]

the only thing that would make since is the tanks being clogged up for such little fuel filter life, you could always replace the tanks for pretty cheap and simple the rear one costed about 100$
unless the fuel is getting contaminated via outside sources like a really sketchy gas station or a farm tank

The rear tank is completely clean, the front tank has very minor rust. I'm not consistently fueling at any one place, but it's almost exclusively national chain truck stops.

 

[Austin86250]

do you have pictures?

 

[rhkcommander]

I'm spitballing here since mine doesn't have that sensor, I'm guessing it's just a pressure sensor and your lift pump is getting tired. Also check the contacts, dirty wires etc can cause sensors to read wrong.

Just a guess.

 

[Max Mini]

I'm spitballing here since mine doesn't have that sensor, I'm guessing it's just a pressure sensor and your lift pump is getting tired. Also check the contacts, dirty wires etc can cause sensors to read wrong.

Just a guess.

That makes sense except that the performance improves dramatically with a new fuel filter. Would that happen if it were the lift pump?

 

[rhkcommander]

Mechanical lift pumps are very durable, but nothing is forever. Filters decrease PSI, moreover as they get dirty. A tired lift can go from borderline to less-than easily in that situation.

Have you cut open a filter to see if there is crap in it? If not then it's definitely not a filter issue, and just working on the filter is a sign of another symptom. That would have been step one if you are still having issues or have access to one of the first "bad filters".

Any rubber hoses between the tanks and filter? Its rare but I've heard of guys who used rubber to the steel lines, the rubber looked fine but internally delaminated and pinched shut. Removing the filter could give it a chance to reopen. Just speculating at this point.

You can get a pressure gauge on the filter house for cheap, get some readings at idle.

 

[rhkcommander]

I have a mesh pre-filter on my facet lift pump with clear bowl. The rear tank has rust and algae probably, or just algae. Going to drop it and swap to a bigger tank but it only takes a couple hundred miles TOPS before the fuel flow is cut too much and it starts surging before stalling. I've had to pull it out on the side of the road and give it a rough cleaning.

Maybe the front tank is worse than you think?

Again, cut up a filter and see if it's actually dirty. It would have to be filthy


Couple other oddball ideas:
Have you changed gas caps? I've had ones before that were too sealed, the system would create a vacuum cutting flow at the lift pump. Popping the cap would cause a hiss, performance restored and would start declining again. Drilled a small hole through the caps fixed it.

 

[Max Mini]

Mechanical lift pumps are very durable, but nothing is forever. Filters decrease PSI, moreover as they get dirty. A tired lift can go from borderline to less-than easily in that situation.

Have you cut open a filter to see if there is crap in it? If not then it's definitely not a filter issue, and just working on the filter is a sign of another symptom. That would have been step one if you are still having issues or have access to one of the first "bad filters".

Any rubber hoses between the tanks and filter? Its rare but I've heard of guys who used rubber to the steel lines, the rubber looked fine but internally delaminated and pinched shut. Removing the filter could give it a chance to reopen. Just speculating at this point.

You can get a pressure gauge on the filter house for cheap, get some readings at idle.

Yes, today was the first time I got a chance to see the fuel coming out of the filter and it's clean. And yes, it does have rubber hoses. OK, thanks, this helps.

 

[Max Mini]

I have a mesh pre-filter on my facet lift pump with clear bowl. The rear tank has rust and algae probably, or just algae. Going to drop it and swap to a bigger tank but it only takes a couple hundred miles TOPS before the fuel flow is cut too much and it starts surging before stalling. I've had to pull it out on the side of the road and give it a rough cleaning.

Maybe the front tank is worse than you think?

Again, cut up a filter and see if it's actually dirty. It would have to be filthy


Couple other oddball ideas:
Have you changed gas caps? I've had ones before that were too sealed, the system would create a vacuum cutting flow at the lift pump. Popping the cap would cause a hiss, performance restored and would start declining again. Drilled a small hole through the caps fixed it.

scraped the inside of the front tank with a screwdriver, and it came up with nothing on it. The scratch is metal on metal, it really doesn't seem like anything. BUt I guess it's possible it's worse... then again fuel in the filter is clean. Have not changed gas caps, but that's interesting to know!

 

[Big Bart]

Yes, today was the first time I got a chance to see the fuel coming out of the filter and it's clean. And yes, it does have rubber hoses. OK, thanks, this helps.

RHK may be on to something here.

1) Maybe you have a pressure issue, opening the fuel filter is releasing the pressure or a vacuum.
a) Try opening the two gas caps next time it has the issue to see what difference it perhaps makes.
b) Test the fuel pressure at the filter head schrader valve after a new filter and when it is acting up. Does it go up or down?
c) Mechanical pumps are $40 and only take 45-60 minutes to replace. Never hurts to put a néw one in.

2) Rust usually does not clog a large filter like ours. But tank algae would, look at the filter, you won’t necessarily see the algae in the fuel. But it should discolor the filter as it builds up over time. Maybe add bio bore a couple times and treat both tanks. It kills the algae but does not remove it.

3) Maybe your fuel tank has something in it that partially clogged the pick up line. When you stop to put the filter in it moves out of the way. Maybe the check valve in the lift pump is bad. When you open the fuel filter the fuel flows backwards and unclogs the pick up or tank transfer valve.

 

[Max Mini]

RHK may be on to something here.

1) Maybe you have a pressure issue, opening the fuel filter is releasing the pressure or a vacuum.
a) Try opening the two gas caps next time it has the issue to see what difference it perhaps makes.
b) Test the fuel pressure at the filter head schrader valve after a new filter and when it is acting up. Does it go up or down?
c) Mechanical pumps are $40 and only take 45-60 minutes to replace. Never hurts to put a néw one in.

2) Rust usually does not clog a large filter like ours. But tank algae would, look at the filter, you won’t necessarily see the algae in the fuel. But it should discolor the filter as it builds up over time. Maybe add bio bore a couple times and treat both tanks. It kills the algae but does not remove it.

3) Maybe your fuel tank has something in it that partially clogged the pick up line. When you stop to put the filter in it moves out of the way. Maybe the check valve in the lift pump is bad. When you open the fuel filter the fuel flows backwards and unclog the pick up or tank transfer valve.

There are a lot of great suggestions here, thanks!

 

[Jesus Freak]

It's out there but, I thought I had clogged filter issues it turned out that my IP was going. And another thought, it is a "short bus" it might just be special and want attention.....I'm kidding....it had to be said tho.

 

[chris142]

Are there any sections of rubber hose in the system? If so could it be sucking shut?

 

[catbird7]

New mechanical fuel pump is about the same price as a fuel filter. I'd replace the pump before buying another filter. It would also be wise to check fuel pressure at the Schrader before and after the new pump install.