justagoodolboy

What does it mean (other than the obvious) when an inline fuel filter won't stay full on the supply side of the LP? I've replaced everything from the tank with a new pickup "draw straw" type through a bulkhead to new 3/8 line all the way to my 3rd piston LP and still losing power and the FP gauge bouncing and pulling down past 6 PSI when in the go-pedal.

This weekend I plumbed in a filter to catch anything big from the tank, and was amazed to see it never get over half full, and any time checking it after driving the truck it was nearly empty. I'm betting if I had a needle valve on my FP gauge I could take the bounce out of the needle to find its pulling down to zero PSI.

Any ideas? I've had the VE rebuilt, replaced the fuel heater gasket, 3rd piston LP, new supply line like mentioned above and new injectors.

What hurts the worst is when its running right and pegging the FP needle, which only happens when you open one of the lines by way of a filter change or dropping the tank like I did yesterday to recheck everything, it hauls the mail for a few miles, then starts the erratic FP needle bounce and loses every bit of half its power.

I'm at the end of my rope, so any ideas of advice would be appreciated.

Santaclaus

You might start by putting a small section of clear tubing close to the lift pump and look where bubbles. If you have bubbles there you can move it back to where you installed your filter and from there back to the top of the tank. I have found sometimes the clear plastic in-line filters don't fill up all the way, the more they clog up the more they fill up.

Another possible flying to do is take a shop vac and pressurize your fuel tank """carefully""", and then look for leaks.

BearKiller

Reads like a stopped up tank vent.


As for the inline filter, it is next to impossible to keep one air-free.

I have one on a gas-burner with a check-valve on the upstream side and it still stays barely half full; same for my diesel trucks; it doesn't seem to affect the running however.

dzl_damon

LIke Bear Killer said, nearly impossible to get all the way full without a large bubble. Ask any Mercedes Diesel owners. They have suction filters before the LP from the factory, and it's a common newbie question of why there is always some bubble in there.

If it's BUBBLING while its running, then THAT is an air leak however. If it's static, it's fine.

Montana2x2

Where are these tanks vented? I'm still fighting what seems like air in then fuel system.

JustRamIt91

you said you installed a draw straw... is your tank more than 1/4 full??? i know you can suck air fairly easily when accelerating, especially if your straw is located near the front of the tank....

93flatbed

I'm purdy sure it's the fuel cap, I know I've had old 6.5's in the shop with sucked flat fuel tanks because the customer put one of those locking fuel caps on!

dzl_damon

I've changed my fuel caps from 1 truck to another and the dually no matter what will "WHOOSH" when I open the tank after driving a bit. The utility truck has no issues.

I dropped the tank to do my new fuel lines/pump yesterday, and the roll over protection valve was all crudded up and clogged??? Maybe it vents through there too?

justagoodolboy

Let me be a little more specific. The filter will fill up about 3/4 under idle, appears there's what you refer to as a static, but if I go drive it down the road - IE getting in and out of the fuel under load - it leaves litterally MAYBE a teaspoon of fuel in the filter, and you can see it trickling in it, but no where near the solid 3/8 hose stream I would expect.

Plus it greatly affects my power - I rebuilt the VE and put in new DDP's, and it ran like a top for a day or so, and now I am back to the same amount of power before the new sticks, which tells me obviously something is not right.

I've wondered the same thing about vents as well. I confirmed FOR SURE there is no vent on top of the pickup assembly on the tank. My fuel cap appears to be vented, but I guess it could be stopped up, but loosening the cap when the FP gauge is bouncing does not seem to affect it. I also gutted the roll-over ball valve think when I put my flatbed on.

Its almost like there is some sort of suction to return equillibrium being crossed up, as the LP draws fuel fine, especially if you'd say changed your fuel filter, or cut the line to put the inline filter in, allowing air in the system and the FP gauge will only smoothly pull down to 14 or so PSI WOT, and the truck hauls the mail, but once it runs for a while the FP gauge starts bouncing, power gone, and with the very minimal amount of fuel in that clear filter, its apparently the truck is starving for fuel running 70 because I've been hearing some weird engine noises at highway speeds, and now I think its just the truck running really lean.

justagoodolboy

I put my draw straw towards the front of the tank, but having it coming into the tank and laying flat on the bottom running past the pickup assembly about 10 inches and weighted down. I thought the same thing, but when I dropped the tank last weekend to retrace my steps, I siphoned the fuel out to make it lighter, and only put about 3 gallons back in it and it ran fine with the FP gauge steady and only pulling down to 16 PSI.

If there is a silver lining, I have become and expert at dropping the tank!

BearKiller

I could be wrong, but I am fairly certain the cap DOES NOT VENT.

There should be a vent in the top of the big sending-unit assembly.

BUT, if removing the cap does not help the problem, then it must not be a venting issue.

The rubber lines can suck together on the insides and look fine on the outside.

Plumb the lift-pump into a jug of fuel and see how it runs then.

BearKiller

Borrowed from BC847 :




Although much too small, in my opinion, there is a vent, with a roll-over protection valve, located amongst all the other business.

Here is the entire article :

https://www.dieseltruckresource.com/....html?t=151190


Myself, I would remove that tiny roll-over foolishness and install a 1/2" vent-hose in it's place.

JustRamIt91

i like your idea!!

BearKiller

Just look at that tiny orifice of a vent and think about the volume of air that can enter it, compared to the volume of fuel that can exit via the 3/8" draw-line.

Add a dirt-dauber nest to the equation and it's a wonder the engine could even run.

My Ford came factory-original with a 7/16-I.D. vent-hose through the top of each tank, which has since been replaced with 1/2" hoses.

My big auxilliary tank has a 5/8" vent-hose.

It would be a simple matter to remove that tiny vent and replace it with 1/2" hose; it would probably even fit the existing grommet.


The more I think about it, I am almost certain that the caps DO NOT VENT.

kensmachine

What kind of fuel filter are you using Ken