Fuel gauge issue after changing fuel filter

[Myrtleman]

Hi all. Changed out my fuel filter in my 1097A2 yesterday and used procedure outlined in the technical manual. I used the bleeder screw on top to purge air. Today I wanted to be super sure that all the air was out so I used a vampire pump at the fuel filter outlet on top of the housing to draw out fuel. I reconnected the fuel line, started the truck, and it stumbled for a second. I gave it some throttle, it recovered and is running really smoothly. However I noticed that all of a sudden, my fuel gauge is reading half the amount of fuel that it was reading prior to this procedure. I assume that the vacuum affected the sending unit in some way? As a sidenote, I know these are 25 gallon tanks, however after putting in 10 gallons of fresh diesel in my empty tank a few days ago, it’s only reading 1/4 tank. Is this normal? Not sure if the sending unit was on its way out and I finished it off with the vacuum pump? Thanks in advance for your help.

 

[CapePrep]

I think you just proved you have a bad sending unit. The system couldn't be more simple. Time to break out the CC!

 

[Mogman]

Pulling a vacuum on the filter would not affect the sending unit, if you don't have many miles on a truck that has been sitting awhile I would drive it some before condemning the sending unit, it could be quite normal for your truck, I have the new style fuel sending unit and it was on empty today but only took 18 gallons to fill it up, I think having a little "reserve" when showing empty is not a bad thing.

 

[Myrtleman]

Pulling a vacuum on the filter would not affect the sending unit, if you don't have many miles on a truck that has been sitting awhile I would drive it some before condemning the sending unit, it could be quite normal for your truck, I have the new style fuel sending unit and it was on empty today but only took 18 gallons to fill it up, I think having a little "reserve" when showing empty is not a bad thing.

Mogman thank you. I will drive it around for a while before condemning it. It was reading a quarter tank consistently for the past few days. As soon as I got through with the vacuum procedure the needle immediately sank to right above E and despite re-bleeding with the screw, opening and re-closing the fuel door, disconnecting and reconnecting the batteries, and draining some fuel out of the bottom with the petcock, it stayed right there. I’m thinking maybe the unit was getting ready to go and that suction that the vacuum caused maybe pulled the ball within the sending unit a little lower and it stuck there? I’m just speculating. I’ll give it a little bit of time and see what happens. I’m kind of constrained to driving around my neighborhood because of course my Sf97 has not arrived yet and it’s been over 130 days. GP blaming the army yada yada

 

[Mogman]

It is possible you have the late style sending unit, it only has 4-5 possible readings, (not continuous like the resistor style sending unit) it is a magnet that floats up and down a tube closing reed switches so it will "jump" about a 1/4 tank at a time, normal for that kind of sending unit, put another gallon or two in it and see if it jumps to a 1/4 tank.

 

[Coug]

agree with the others.

My tank still has at least 5 gallons when on empty. Every 1/4 tank mark equals 5 gallons. Fuel gauge sometimes seems to move normally, but usually just shows one of a few positions.
Float in tank is unaffected by vacuum.

 

[Myrtleman]

It is possible you have the late style sending unit, it only has 4-5 possible readings, (not continuous like the resistor style sending unit) it is a magnet that floats up and down a tube closing reed switches so it will "jump" about a 1/4 tank at a time, normal for that kind of sending unit, put another gallon or two in it and see if it jumps to a 1/4 tank.

Mogman you are correct. I added 2 gallons and it jumped to 1/4 tank. There’s at least 11 gallons in the tank. Maybe in the future I will upgrade to the more modern sending unit just for a little bit more accuracy. Thanks again

 

[Mogman]

Mogman you are correct. I added 2 gallons and it jumped to 1/4 tank. There’s at least 11 gallons in the tank. Maybe in the future I will upgrade to the more modern sending unit just for a little bit more accuracy. Thanks again

You have the more "modern" sending unit, I am not sure you can still get the old style as NOS.
EDIT, except some universal crap I see out there.

 

[Myrtleman]

You have the more "modern" sending unit, I am not sure you can still get the old style as NOS.
EDIT, except some universal crap I see out there.

Just out of curiosity what was the purpose of moving to that newer style sender? I’m assuming so the soldiers wouldn’t wait until the last minute to fuel up and not run these things dry?

 

[Mogman]

Just out of curiosity what was the purpose of moving to that newer style sender? I’m assuming so the soldiers wouldn’t wait until the last minute to fuel up and not run these things dry?

I have no idea, the reed switch type should be much more dependable, especially for vehicles that site for extended periods of time, the "variable resistor" tends to varnish over when sitting, or maybe the variable resistor type does not like kerosene based fuels (JP)
There are companies that custom make the reed switch type with as many reed switches as you want (up to a point) that would give you a more "resolution" but I don't think too many folks really care, any time I get under 1/2 tank I look for someplace to top it off (figuratively as many leak if you actually top the tank off)

 

[Mogman]

In any case drive yours a while before trying to change things, it is possible you have the old style and it actually has an issue, if you have the new style when you fill the tank slowly with the run switch on you will see it advance about 1/4 tank at a time.

 

[Myrtleman]

In any case drive yours a while before trying to change things, it is possible you have the old style and it actually has an issue, if you have the new style when you fill the tank slowly with the run switch on you will see it advance about 1/4 tank at a time.

Will do. Being that I’m still waiting on the sf97 and will just be taking it around my neighborhood for now, do you have a suggestion for a fuel additive to keep the diesel fresh or is it even worth it? Thanks for all your help

 

[Mogman]

Will do. Being that I’m still waiting on the sf97 and will just be taking it around my neighborhood for now, do you have a suggestion for a fuel additive to keep the diesel fresh or is it even worth it? Thanks for all your help

Diesel is pretty stable so keeping it "fresh" is not an issue, most do use a fuel lubricant additive because the ultra low sulfur diesel does not lubricate the Stanadyne IP as well as it was originally designed for the diesel with sulfur.
I use the Stanadyne "lubricity" additive because they build the IP so I figure they know what it needs but there are several good additives on the market.

 

[Coug]

If you're going through more than a couple tanks a year, then no real need for treatment. As long as you buy clean, dry diesel and don't let it sit for more than 3 or 4 months, you should be fine without treating it.
If you're buying and storing diesel longer than a year or so, then I'd start to worry about treating it. Less than that you don't usually need to worry, and even if the fuel is starting to oxidize a little, the HMMWV engine will still burn it.

going slightly off topic below, but since it was asked about:

back in 2007 The Diesel Place and A. D. Spicer did a diesel fuel additive lubricity study and had a lot of useful results. The Stanadyne that Mogman likes was one of the better additives. Personally I like the Optilube XL, which wasn't on the list, but is directly related to the #2 finisher in the study the lubricity additive without the other components so it goes a lot further than the XPD does. I buy it in the gallon jugs and use a HDPE dosing bottle to measure it out, which comes out a LOT cheaper than buying the smaller dosing bottles of the additives.

Here's a link to the study

https://www.jatonkam35s.com/DeuceTechnicalManuals/Diesel_fuel_additive_test.pdf

A link to the additive I'm using. Last jug is just running out and I bought it over 2 years ago, and running it in both the HMMWV and my diesel car.

https://www.amazon.com/Opti-Lube-Xtreme-Lubricant-Diesel-Additive/dp/B00MAU9PV8

And last but not least, the refillable dosing bottles. I bought a 12 pack because it was less than buying 4 bottles individually, and gave half of them to my buddies for their rigs

https://www.amazon.com/Consolidated-Plastics-41394-Graduated-Dispenser/dp/B06Y2FDYZW/ref=sr_1_3?crid=L1CWBA4TUQ8W&keywords=dosing%2Bbottle%2BHDPE&qid=1707367480&sprefix=dosing%2Bbottle%2Bhdp%2Caps%2C858&sr=8-3&th=1

if you don't want multiple then singles are available

https://www.amazon.com/Portable-Dispenser-Mixologists-Beverage-Recyclable/dp/B0CN7X11BZ/ref=sr_1_6?crid=L1CWBA4TUQ8W&keywords=dosing+bottle+HDPE&qid=1707367480&sprefix=dosing+bottle+hdp%2Caps%2C858&sr=8-6