Detroit 12.7 Sputtering/losing prime after changing fuel tanks, ‘99 Peterbilt 379

I'd start by removing both pickup hoses from the top of the tanks and put them into a 5 gallon pail of fuel. You are getting air bubbles from somewhere just have to find from where. If it was fine before you replaced the tanks, perhaps there is a pinhole somewhere on the pickup tube on one of your new tanks. Do you happen to have a davco filter? If not, I'd also suggest plumbing in a clear tube for diagnostic purposes, if you are getting no air bubbles going to your engine then the problem is... well, beyond me.

*edit to add*
I am assuming your pickup is at the top of your fuel tanks rather than connected at the bottom.


Have valves on both tanks supply and return lines yet?

 

Are you sure you don't have a supply and return line backwards?

Were these tanks new?

 

I can't see if you mean the draw line on yours is from the bottom of your tank or from the top.

Needless to say if you're out on the road at the moment I'd just keep topping it off so you can at least get to your home or shop.

 

I'm stumped.

Hope ya get home. Please update when you get it figured out.

 

You replaced tanks, did you replace the lines? If they are old and brittle the rubber could have cracked....

Try lightly pressurizing the tank (5-10 psi max) if possible and look for leaks

Also, if you have a fuel restriction guage, try and block that off before pressurizing, those typically end up spilling diesel all over the cab if theyre cracked in the dash

 

Surprised draw line is on bottom. I thought that was only allowed before 99. I was going to suggest the top draw lines have a high spot. Supposed to run on a steady rise towards the T junction and fuel filter. Only other thing is the lines themselves getting disturbed. Old brittle steel reinforced lines with Eaton reusable fittings will easily leak when disturbed. Been fighting this myself since an overhaul. My check valve on back of head was also suddenly bad afterwards. Replaced it, quit losing prime. Been getting sporadic sputtering. Replaced all my lines and fittings, now it’s losing prime again. Took check valve apart. All it has is 1 O ring and 1 spring. Replaced O ring. Still loses prime. It’s somewhere in my lines/fittings. There’s 2 different size fittings/lines that are very close in size to each other. If you’ve changed either the fuel line or fitting. Might be the problem. I think that’s what’s wrong with mine. Just can’t pinpoint it. Won’t leak when pressure is applied. Still sucks air though. I’ve re done the fittings, made some improvements. It was real bad. Still loses prime. My Freightliner has a dual shut off valve. Might be the culprit. I don’t think Peterbilt used them. Freightliner replaced them with just a T fitting. Each tank fuel line in, and one line to the primary filter. That’s my next move. Eliminating the valve. Can’t figure my own out! Lol. Just make sure everything’s tight, and lines are running at a steady rise. No high spots to lose siphon effect. Good Luck. It’s aggravating,