1997 fuel tank module questions

[John Byers]

Dropped the tank tonight and removed the fuel module. I am chasing a pressure drop under load, wondering if someone can give me a good explanation (or link to a diagram) for how this thing is supposed to work. I see that the return goes into the well, but the supply side is confusing me. The tube for that sticks into what appears to be a filter in the bottom of the well, but it doesn't seem to have passages from the filter to the tube. There is a small rubber check valve , and under that is what appears to be a float that would cause the check valve to close at very low tank levels. Under the float is a screen that snaps on to the bottom of the module. What I'm confused about is that it appears that all the fuel has to go through that little rubber valve, which I'm finding hard to believe. Also, are there replacement filters available? I have not seen any listed anywhere. Thanks, John

 

[fest3er]

The basket is indeed fed from the bottom through that float. When there is enough fuel at the float, it opens and lets fuel flow inward. When there is not enough fuel there, the float closes and prevent the fuel in the basket from returning to the tank. (This is why you can usually climb and descend hills when the tank is nearly empty: the fuel in the basket supplies the engine and overflow fuel returns to the basket.) If the tank is full, the level is above the top of the basket and there should not be any problem with the fuel supply at that position.

I would sooner suspect a pinhole in the supply line to the fuel pump, a weak fuel pump, a weak overflow valve on the p-pump, and a clogged fuel filter.

 

[NIsaacs]

Be sure and check the secondary screen in the suction line.

 

[GAmes]

Nick, the '97 doesn't have a screen like that.

OP, yes for the most part all the fuel used goes through the valve but return fuel goes into the basket as well. The basket is designed to keep fuel available when the quantity is low and the truck isn't level i.e. climbing and descending hills. The screen is easy to clean, if it needs replacement is is likely the entire module needs replacing. I've had to replace mine twice. This is the last one.

I removed it because the fuel level sender had quit working. The first module (original equipment) was made of metal and had broken in two where the two pieces mate. The reason I dropped the tank in the first place was the same. Fuel sender had quit working correctly.

 

[NIsaacs]

Nick, the '97 doesn't have a screen like that.

Oops, my bad

 

[John Byers]

Thanks all for the replies. I am going to clean the module and filters up tonight and see if I can spot any problems. One that I know of already is one of the metal arms on the sides is bent, causing the whole thing to hang crooked and not slide up and down. Speaking of which, what is the purpose of it being able to slide? I am thinking it might compensate for different depth tanks and allow that basket to sit right on the bottom. The truck would start easily and idle at about 25 psi, but anything more than very gentle acceleration will make it drop it to almost zero. The mechanical pump "looks" recent, it has a tork-tek adjustable OFV, the screen in the heater housing by the pump was just cleaned (heater removed but was not leaking), and the big filter has maybe 5K miles on it.

 

[GAmes]

It's just a matter of time before it will become unserviceable. My guess is the same as yours, a module to fit more than one size tank. Genos sells a replacement. If the filters are clean and the fuel lines are good the loss of pressure is due to a failing lift pump IMO. A member from long ago wrote this up. https://www.cumminsforum.com/threads/joe-gs-fuel-system-writeup.58777/

 

[crabman]

The link that GAmes posted is the best information about how this fuel system works and its components. I have printed this out and keep it on file.

 

[surfbeetle]

You wrote pressure drop under load. How's your fuel pump? I'm on my third one in 240,000 (and 25 years). Also, the fuel lines as they come up by the transmission are known to crack, lose pressure and suck air. BTW, they are real pain to replace. Also, there is a hose that goes from the turbo to the AFC on the injection pump that can cause issues when that line breaks.

 

[John Byers]

Well, it's fixed. Both filters in the tank were very dirty, as was the big filter by the engine. The big one was black and slimy. I fixed the bent up fuel module, cleaned both filters, took time to understand how it works(It's ingenious!) and reinstalled. I replaced the hard plastic lines with the QD's by the tank with hose with clamps, also the braided stainless ones by the bellhousing. I did not trust those QD's, so now they're gone. After I pumped the primer about a thousand times, it started right up. It was idling at about 30 PSI, so I turned the OFV down to 25 PSI. When I drove it, the pressure went up to about 35, and never dropped below 30 even when accelerating in 5th gear. That is better than I've ever seen, so The ones in the tank were probably dirty since I've had it. BTW, I did add some algae treatment to the tank, and will continue to do so for several fillups.

Thank you all for your replies, there is no substitute for hands-on experience.

 

[GAmes]

When algae gets involved a lot of bad things happen. Hopefully you have it licked.

 

[DavidC]

Looks like you better stock up on filters for a while.

 

[John Byers]

I will certainly be keeping an eye on my fuel pressure.

 

[fest3er]

The black stuff is asphaltenes; they're normally present in diesel fuel. They do build up over time.

 

[John Byers]

The fuel in the tank looked very clean and clear. Would the asphaltenes also make the filter slimy? Also, are there any fuel additives that have proven benefits for a truck that is not driven real often? I did read that keeping the tank full is helpful for preventing algae. Thanks again!

 

[DavidC]

I do keep algecide in mine as it is not driven often.

 

[GAmes]

The fuel in the tank looked very clean and clear. Would the asphaltenes also make the filter slimy? Also, are there any fuel additives that have proven benefits for a truck that is not driven real often? I did read that keeping the tank full is helpful for preventing algae. Thanks again!

Asphaltenes are normal in all diesel fuels. They do not make the filter slimy, they just turn it black. Can't help you with the additive question, my truck rarely sits for more than a day. I don't use any additives except for the fuel I buy in the winter in the south if my route takes me north. The last thing we did when I was flying was to top off the tanks before we parked the helicopter. Jet fuel is prone to algae too.