Primary fuel filter / water separator question

[Carl_in_NH]

Went to drain the crud out of the frame mounted fuel filter / water separator on my '71 M35, and instead of junk coming out, air went rushing in and it sounded like it was draining back into the tank through the lift pump.

I turned on the master, and I get plenty of fuel from the lift pump injected into the filter - but each time I open the drain valve (with the master off), it sucks air and drains back into the tank with only a few drops of fuel coming from the drain on the filter housing itself.

I tried blowing it out by leaving the filter drain open and turning on the master, and that blew out some small flakes of crud, but no water - only nice clean diesel. That part I suspect is a good sign - although if it's sucking fuel back, it could also be sucking water and other junk back, too.

My question is, should this drain back into the tank this way - or should there be a check-valve action somewhere?

Thanks!

 

[Boatcarpenter]

Do you have a vented cap on your fuel tank? If so, is the vent open? Perhaps you are drawing a vacuum and when you open the filter, it sucks the fuel back into the tank.
I don't believe there is a check valve in the system. I know from experience that you can siphon fuel through an in tank pump.
BC

 

[Carl_in_NH]

Thanks, BC; I'll have to go look at the cap again. There's nothing obvious in the way of a vent on that cap - but that just means I need to examine it closely.

 

[Boatcarpenter]

If it is vented, it will have instructions on the outside and the vent tab that you turn will be on the inside.
BC

 

[Carl_in_NH]

Thanks for that, BC - this cap is unvented.

 

[dobyg75]

you could loosen the cap and see if that solves the problem

 

[Carl_in_NH]

We'll take another look at this tomorrow morning. It's too late to get covered with diesel from the filter drain tonight.

Tomorrow morning is safety inspection sticker day; got my real NH plates today - which is the material for another interesting story thread.

 

[dobyg75]

good luck with saftey inspections if you have any questions ask me i have my nh inspection license

 

[1stDeuce]

Um, in order for fuel to come out, air has to get in... fuel isn't going to come from the tank, as it's lower...
The bottom of the filter is most likely higher than the fuel level in the tank, so the fuel gets pulled back by the weight of fuel in the lines below it... (siphons from the filter into the tank when you allow air to enter the filter)

I drain a jar of fuel out of the filter carefully with the in tank pump on so I don't introduce air into the system. Don't usually find much water in mine, so I usually pour it back in the tank.
C

 

[cranetruck]

You may also want to check the vent line that combines with the MC and airpak vents and ultimately exits via the crankcase breather tube...

 

[roger-wilco-66]

I thougth it's the normal procedure to drain the primary with the in-tank pump running?
I wouldn't try it otherwise to avoid exactly the described consequences, and fearing that the drain back takes the accumulated dirt and water back into the fuel tank.

I tried blowing it out by leaving the filter drain open and turning on the master, and that blew out some small flakes of crud, but no water - only nice clean diesel.

I think this is because either there was nothing like that in the filter in the first place, or it got sucked back into the tank when you openend it the first time without the pressure from the intake-pump. The flakes you saw could be what was left on the bottom filter when the stuff drained back into the tank.

My 2ct,
Mark

 

[Carl_in_NH]

Thanks, 1stDeuce - I understand what's going on, but I did consider it to be abnormal - since there's no mention of ever doing fuel system drain checks with the lift pump on in any of the TMs that I've seen. That's what was troubling me. Typically if you need the equipment in a specific state to do something, they spell it out well.

I'll also take a look at the vent line, cranetruck - thanks!

Also thanks, dobyg75 - if anything pops up I'll let you know. I'm off to a small, local Jeep place that's been around a long time to get the sticker this morning. When I made the appointment for the inspection and told the counterman what it was I wanted inspected, it didn't seem to phase him at all.

 

[Boatcarpenter]

Carl, I don't know if you got antique tags or not, but if you did, the regular inspection date for all vehicles with ant. tags (after your initial insp.) is April, not your birth month. Many inspection stations don't know this.
BC

 

[2Deuces wild]

Hey Carl, Later invest in a set of spin on adapters, Makes filter changes quicker and cleaner. Jtonka makes them. K. P.s. run tank pump when draining filters

 

[dobyg75]

Carl BTW are you registering as a regular vehical or and RV if you use the RV you can aviod the dot phisical and i think the insurance would be alot cheaper there's a guy around my town that has three deuces and all are registered as RVs