M931A2 air in fuel

[MAdams]

Went off-roading a few months ago and on an extreme angle I think sucked some air. The truck never died but sputtered for a while. The rest of the day it would intermittently
sputter again on angles. Drove it home with a slight lack of power due to the sputter. Came home, change the secondary fuel filter and bleed all the air out and it ran fine. Drove it for a month and the only problem is sometimes it would loose prime. Pump the primer a few times and everything ran perfect.

Today the sputter came back and wouldn’t go away. Here is a list of things I’ve tried.

With engine running, bleed all the fuel lines that are after the lift pump. Two on the filter housing, inlet into the IP, IP return line. Clean fuel came out with no air

Pressurized the fuel cap and checked for leaks where air could be sucked in. No leaks

Changed the lift pump

Ran the truck from a 5 gal can of fuel

Cracked all the injector lines with the engine running

Checked the IP timing

Changed the secondary fuel filter. Cut the old filter open,
looked good.

Filled the passenger fuel tank to the very top. This is the
tank I am using

Tried using the driver fuel tank. Same results.

I drove the truck about 20 miles today. I held the pedal to the floor for about ¾ of a mile to try to get all the air out. The sputter happens at idle and while moving. I don’t think it’s a cracked feed tube in the tank because both tanks have the same results and there is a strong spray of fuel when the bleed fittings are opened while the truck is running.

 

[Fredo Corleone]

Check the fuel lines my M931A2 had cracks in the outer cover coming from the big filter on the drivers side to the engine. Put a piece of tape over it to check.

 

[WillWagner]

Could be a failed ovèrflow valve.

 

[74M35A2]

You should replace the soft fuel suction lines regardless if you have not yet, they have a near 100% failure rate at this age. Numerous members have tested them the way you have, and they don’t bleed externally, but will still suck air.

Also, the fuel selector valve is a known air leak area on twin tank trucks.

At least one member has found leaks at a fitting junction (cracked flare, etc...). I have not seen too many discovered leak areas via the fuel tank pressure method, and you are for sure sucking air, so it is coming from somewhere.

Just judging by frequency of issues, I’d go new dip tubes and soft suction lines, then see where you are at. Low cost to do vs a truck of this mass coming to a stop in traffic.

 

[MAdams]

I would change the hoses and o rings in the selector valve, but the problem didn't go away when I ran the truck on a 5 gal fuel can hook directly to the lift pump.

I changed the overflow valve with a known working unit, no change.
I checked all the ports on the exhaust manifold with a IR temp gun. With the truck running for about 3 min, the front cylinder was 100 degrees and all the rest were 170.

I think the next step is to swap the injectors around and see if the problem moves.

 

[WillWagner]

Seen a pintle stick open before, what you want to do is a valid ts step. I'd do that. You might be able to crack the line and see if it fires better with it loose.

 

[MAdams]

I swapped the #1 injector with a good used one and it smoothed out. I haven't driven the truck yet but it idles smooth in neutral and in gear. All the temps on the exhaust manifold were the same.
I tried to blow through the bad injector nozzle and couldn't but could blow through a spare one. I'll drive the truck this weekend and confirm.

Sent from my SM-G930V using Tapatalk

 

[TedCat]

Did you replace all your suction hoses? Also if your concerned about air, while running from the 5 gallon can swap your return hose out for clear vinyl and inspect while its running for air bubbles. That is how another member got on the trail to a cracked fitting. Good luck!