Air bubbles in (diesel) fuel line from tank

[KarlH]

The fuel tank and lines on my truck are new in the last 2,500 miles, but in the last 200 it began to stall occasionally with air in the lines.

So I spliced in a section of transparent hose where the line connects to the engine to see what's going on, and the bubbles start after about 5 miles of driving and rapidly worsen until it's just froth in the line (causing the engine to stall).

Driving with the fuel cap loose doesn't seem to make a difference, so I don't think the tank vent is plugged. And it will stall even when the tank is full.

So it has to be a bad seal in the line and possibly something plugging it, right?

 

[bullfrog !]

Air has to be let into the system so yes there has to be a loose fitting, pin hole or defective seal letting the air in. I’m not familiar with modern systems but I would assume air is being sucked in by a low pressure feeder/supply pump somewhere between the fuel pickup in the tank and the inlet to the pump or by a defective pump seal itself. Often holes or leaks are so small air can go in but fuel doesn’t leak out as it is much denser making it hard to locate.

 

[KarlH]

@bullfrog ! That makes the most sense, thanks.

I was just confused that it takes 5 miles before the bubbles show up, and goes from clear fuel to froth in a mile or two. But maybe the air is getting cycled back into the tank through the return line...

(The engine on this truck is 30 years old and the tank and sender are no-frills.)

 

[Happy Camper]

Vapor lock possibly? Your said the fuel lines were recently replaced. Did your run then closer to the exhaust than the original lines? That would account for the 5 minutes as the exhaust heats up.

Maybe a quick double check would help. Even if it's not vapor lock, the lines will be warmer once the exhaust heats things up under the. The slight expansion/pliability of the lines might "open" a pinhole leak that might be closed when colder.

Just things to think about.

 

[tx2sturgis]

I would check the seals/gaskets on the fuel filter housing. Also check the fuel filter, making sure it is tightly screwed down on the fitting.

Also, since the lines are new, double and triple check ALL the hose clamps from one end to the other.

Good luck.

 

[KarlH]

Somehow I missed the new replies... Yeah, those are good ideas, I'll check.

 

[KarlH]

Well I finally pulled the return line and ran it into a clear fuel can, and a few seconds after starting the engine there was froth coming out. So my best guess is that the tank is gradually filling with bubbles and that's why it takes 5 miles or so for the problem to show up in the fuel line feeding the engine.

Edit: Now I just have to find the [gosh darned] leak It's not between the lift pump and the filter (already have a clear tube there), but it could be anywhere between the filter and the return line. The filter is the easiest to check (the housing has a 6mm port for my pressure/vacuum gauge) and most probable (lots of bubbles when I purged it), so I'll start there.