Hmmwv fuel system issue

[NCarolinadawg]

Hello, 1989 M998 6.2 a few days now seems to crank/idle/run fine for about 10 minutes but then starts to surge and sputters and will eventually shut off. Will only then restart after cranking about 15 to 20 seconds but will then immediately die again. Will not restart. Come out in Morning, same thing, runs fine, then acts up as above. Fuel line to filter seems to pump fine into a bottle, pumps like the old lever handle water pumps, not a solid stream, but I assume that's correct. Replaced fuel filter and separator, cleaned housing, started just fine amd purged air with screw on housing, them Same scenario as above repeated with issue. Just don't understand how runs so well then issue.

 

[papakb]

The IP will draw fuel from a full tank but then fail when the tank level gets low if the mechanical pump on the block is failing/bad. Check the fuel pressure with a mechanical gauge on the top of the water seperator housing where the STE/ICE transducer normally lives. You should see around 6 psi there.

 

[Coug]

If it was a gas lawnmower I would say the symptoms exactly describe a plugged/malfunctioning fuel tank vent, and the unit is starving for fuel as it builds up suction.
Does it still do it if you remove the fuel cap?

 

[NCarolinadawg]

If it was a gas lawnmower I would say the symptoms exactly describe a plugged/malfunctioning fuel tank vent, and the unit is starving for fuel as it builds up suction.
Does it still do it if you remove the fuel cap?

I have not tried that, with gas cap off. I thought of buying a diesel can and make a line from that to the fuel pump therefore bypassing the tank all together to see if it will stay running.

 

[NCarolinadawg]

I have not tried that, with gas cap off. I thought of buying a diesel can and make a line from that to the fuel pump therefore bypassing the tank all together to see if it will stay running.

So what should happen if leave cap off?

 

[papakb]

If the cap on the fuel tank isn't venting or the breather isn't functioning it will create a vacuum in the tank that the pumps can't overcome so the engine starves. By removing the cap your testing whether or not that's happening.

You didn't include much information about the truck other than it's an M998. If it was Army in usually didn't have a fording kit installed. If it was USMC it might have. The fording system shifts the vent system from externally venting to venting inside the snorkle tube. If something there was wonky the fording system could be pulling a vacuum on the fuel tank. Things to look at.

 

[osteo16]

Sounds like partially clogged vent line...fuel tank not bringing in air as fuel is depleted.. If it sits awhile, the air will slowly make its way back into tank, equalizing.. Then process starts again when fuel is drawn out faster than air can get in ( creating a vacuum). Like above have said, open fuel cap and run... If runs fine, start checking vent location. Like sucking on a balloon full of air, eventually the air runs out and there's nothing left to suck

If you're really bored, stretch a balloon over gas tank nozzle and run engine. You'll be able to see vacuum forming as balloon gets sucked into fuel opening if there is a vent restriction somewhere in fuel tank.

 

[Coug]

So what should happen if leave cap off?

The fuel cap fully seals the fill tube opening when tightened down.
If you leave the cap off, all you are doing is allowing the fuel tank to be equal to atmospheric pressure, meaning you're letting air into the tank to take the place of the fuel being sucked out of the tank.
From how short of a time it is taking, I am going to guess your fuel tank is nearly full at the moment. The more fuel in the tank the less air space, and the faster it creates a vacuum (or low pressure zone to be more accurate) as the air will expand somewhat when the pressure changes, but the fuel does not expand at all in liquid form.


edit: Not knowing more about how your truck is set up, I know mine has a vent line for the fuel tank up near the air filter housing, with a small filter on the end to keep out dirt. Unfortunately the opening on the filter is about right for a small mud wasp or similar insect to plug up.

 

[Valor]

Air in fuel lines. Sucking air with fuel?

 

[NCarolinadawg]

The fuel cap fully seals the fill tube opening when tightened down.
If you leave the cap off, all you are doing is allowing the fuel tank to be equal to atmospheric pressure, meaning you're letting air into the tank to take the place of the fuel being sucked out of the tank.
From how short of a time it is taking, I am going to guess your fuel tank is nearly full at the moment. The more fuel in the tank the less air space, and the faster it creates a vacuum (or low pressure zone to be more accurate) as the air will expand somewhat when the pressure changes, but the fuel does not expand at all in liquid form.


edit: Not knowing more about how your truck is set up, I know mine has a vent line for the fuel tank up near the air filter housing, with a small filter on the end to keep out dirt. Unfortunately the opening on the filter is about right for a small mud wasp or similar insect to plug up.

My tank is almost full. actually. I will try amd run without gas cap when I get home and report.

 

[NCarolinadawg]

The fuel cap fully seals the fill tube opening when tightened down.
If you leave the cap off, all you are doing is allowing the fuel tank to be equal to atmospheric pressure, meaning you're letting air into the tank to take the place of the fuel being sucked out of the tank.
From how short of a time it is taking, I am going to guess your fuel tank is nearly full at the moment. The more fuel in the tank the less air space, and the faster it creates a vacuum (or low pressure zone to be more accurate) as the air will expand somewhat when the pressure changes, but the fuel does not expand at all in liquid form.


edit: Not knowing more about how your truck is set up, I know mine has a vent line for the fuel tank up near the air filter housing, with a small filter on the end to keep out dirt. Unfortunately the opening on the filter is about right for a small mud wasp or similar insect to plug up.

OK so I ran truck without cap on and same result. Has not been cranked for about 24 hours. Started up, drove great and sounded great in the neighborhood, Parked and left running in my driveway amd then as above began to sputter and act as not getting fuel. Filter housing cleaned and separator/ filter replaced, I get great flow all the way to the injection pump.

 

[NCarolinadawg]

OK so I ran truck without cap on and same result. Has not been cranked for about 24 hours. Started up, drove great and sounded great in the neighborhood, Parked and left running in my driveway amd then as above began to sputter and act as not getting fuel. Filter housing cleaned and separator/ filter replaced, I get great flow all the way to the injection pump.

Should I bypass fuel tank using a Jerry can or 5 gallon can to go directly to fuel pump amd then see if stays running to isolate fuel tank out of the equation?

 

[NCarolinadawg]

OK so I ran truck without cap on and same result. Has not been cranked for about 24 hours. Started up, drove great and sounded great in the neighborhood, Parked and left running in my driveway amd then as above began to sputter and act as not getting fuel. Filter housing cleaned and separator/ filter replaced, I get great flow all the way to the injection pump.
Should I bypass fuel tank using a Jerry can or 5 gallon can to go directly to fuel pump amd then see if stays running to isolate fuel tank out of the equation?

 

[papakb]

Before you start replumbing things you might want to tap into the #54 wire on the top of the IP and monitor the voltage (+24v) there to see if it's steady and not dropping out. That voltage shutting off is how the system shuts down the engine and a bad solenoid or bad wiring will do the same thing.

 

[NCarolinadawg]

Before you start replumbing things you might want to tap into the #54 wire on the top of the IP and monitor the voltage (+24v) there to see if it's steady and not dropping out. That voltage shutting off is how the system shuts down the engine and a bad solenoid or bad wiring will do the same thing.

I am getting 24.5 to 25v DC on my multimeter at 54A

 

[NCarolinadawg]

Fuel System testing. I am also trying to follow the TM LOL. In the image, if you need to do as it says and make sure 54A is properly connected to the IP, then it says turn to run and test 54A to see if has 0.5 amps DC flowing? How the hell do you test for that if it is "properly connected?"

 

[TOBASH]

Fuel System testing. I am also trying to follow the TM LOL. In the image, if you need to do as it says and make sure 54A is properly connected to the IP, then it says turn to run and test 54A to see if has 0.5 amps DC flowing? How the hell do you test for that if it is "properly connected?"

Disconnect, clean the leads, apply dielectric grease, and visually confirm it is properly connected.

Proper connection verification means look and see. My unit had wires out of place in the dash so gauges didn't work. I needed to do a visual and a continuity test on each circuit.

 

[Milcommoguy]

You really can't... But you need to know to measure current with a multimeter one needs to break into the circuit. One could use a clamp on DC amp probe too without getting in line with the solenoid feed. TM are good to follow and it helps to have a little exposure to basic electricity. I'm guessing you caught that boo boo in the book.

Add that procedure is for very old M998's with glow plug controllers. MOST likely you don't have one.

Batting 3 and 2. No strike out yet, CAMO

 

[Action]

Fuel System testing. I am also trying to follow the TM LOL. In the image, if you need to do as it says and make sure 54A is properly connected to the IP, then it says turn to run and test 54A to see if has 0.5 amps DC flowing? How the hell do you test for that if it is "properly connected?"

Make or buy a 3-way wire. male one end. female another end. whatever for the third end. hook the male and female to the truck. Hook your multimeter to the third end.

 

[NCarolinadawg]

So I DC checked 54A and that 569B/569D. I have 24V at 54A bit not reading anything at 569B/D? Should it read the same as 54A, in the same testing manor? Also I pulled the fuel return line at IP, I then tried to crank. It cranked strong, nothing coming out of that fuel return line and where the return line was connected to IP it was spitting fuel a little bit. But it never cracked so not sure if this test works if foes not run. It will crank tomorrow morning as it has been and will probably start to sputter bad 10 minutes in. Again, I am getting good clean strong fuel to IP.