Another flavor of air in fuel problem

Replaced fuel sending unit (with proper sock) due to original not working. Replaced fuel lines from lift pump to fuel sending unit. Also replaced lift pump and OPS.

Now: engine will only stay running with fuel filter air bleed valve opened. Lift pump running normal. Fuel coming out of air bleed valve while engine running looks sort of 'foamy'. If I close air bleed valve, engine seems to run fuel out of FFM and stalls. After engine stalls, when I open air bleed valve a burst of air comes out. If I leave air bleed valve open, engine restarts and runs normal. Also, opened water drain valve and good clear fuel comes out and engine stalls.

I knew I was introducing a lot of air into system replacing fuel lines, but it's obvious something else is going on. Fuel tank started about 3/8 fuel, now with all attempts to bleed air down to about 1/4. I rechecked all connections that were made during parts replacement and everything is tight.

I just finished up some similar work on the K2500 - new fuel sending unit, new fuel lines up to lift pump, new lift pump, new fuel lines and hoses up to fuel manager, new OPS, new fuel manager, and new hoses to drain and IP.

With everything finished, I jumpered the lift pump with the bleed valve open until fuel ran out. Closed the bleed valve and after a series of attempts until it cranked over, once it caught, it ran a bit rough for a moment or two, evened out, and is in good shape.

If you replaced the fuel lines from the tank to lift pump, the lines from the lift pump forward are also likely suspect - particularly the hoses from the fuel lines to the fuel manager. I ended up replacing the fuel manager because the seal on the base was leaking and the water sensor was suspect and the parts were more than half the cost of the whole unit. With 24 years and nearly 200K on everything and all the other work I was doing, it seemed advantageous to just replace it while I was in there.

Are there any signs of leak around the fuel manager or the fuel lines from the lift pump forward?

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Originally Posted by trbankii

Are there any signs of leak around the fuel manager or the fuel lines from the lift pump forward?

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The FFM had been replaced at an earlier date due to a obvious and pronounced leak. I got a good deal on a complete unit and that fixed the only leak. The fuel lines I replaced were in pretty bad shape as far as wear and corrosion but they were not leaking and engine was running with no air in lines.

The other thing I should have mentioned in the OP is that even though the engine was running decent before the fuel line/sending unit repairs, the engine was running without the lift pump working. I'm thinking it was a OPS problem, but like you, I was down there and the lift pump was pretty old so I replaced both the OPS and the lift pump. I did NOT replace the fuel lines from the lift pump to the IP. They are not leaking and were functioning fine before the other lines were replaced.

I'm interested to know what the fuel should look like coming out of the air bleed valve when it is opened. On mine like I stated earlier, it is coming out in spurts and look foamy, not a clear heavy stream like when I open the water drain valve.

Could the fact that the lift pump is now working have introduced this current problem?

PS I have the old FFM if anyone is interested. It needs a rebuild kit but I think the heater and water circuits are ok.

When I primed things with the lift pump, I got a clear, steady stream out of the bleed valve.

From discussions I’ve read here on the forum, things will still run without the lift pump, but it puts an added strain on the injection pump and can lead to its demise. Also, it makes it that much more difficult to prime the system after a fuel filter change or such. I don’t see how a properly functioning lift pump would cause the foaming and spurting you’re encountering.

To prime things, I just took a piece of speaker wire, clamped the one set of ends to the battery, stuck the other ends into the plug for the lift pump, and then opened the bleed valve for a moment until the fuel started pumping out. At that point I just pulled the wire from the battery to stop things. I’d start with doing that to make sure that you’re getting a solid stream from the lift pump to the FFM.

I had signs of fuel leaking from the lines along the frame rail, but when I got in there the rubber lines were showing signs of cracks and crumbling. No actual leaks, though, but I just went ahead and replaced everything. Same with the brake lines. Over the past couple years I’ve had to make some repairs to both fuel and brake lines that were leaking, so it just seemed like time to take care of everything. They were all original, so if some are going bad I figure the rest of it is getting close. I don’t see any leaks from the injector return lines yet, but I’ve got them next on my list of projects when I can find some time.

Keep it simple. From your description, there is a significant air inlet (leak) between the fuel tank level and the lift pump (to include the lift pump inlet). Any leak between the lift pump and injection pump won't cause running issues without making a BIG mess. Just for S&G's, check the actual level of fuel in the tank (sounds funny, but it's the #1 cause of facepalms).

Hey Dmax, you may be right about the air leak between the lift pump and the tank, but I personally pumped about 15 gals of fuel into the tank. And the fuel gauge is now working, which was the reason I dropped the fuel tank in the first place. I'll have to lift the bed up again and recheck all my connections. I didn't bolt the bed down JIC so using my JD 1025R w/pallet forks it won't take much time.

Thanks for all replies and suggestions. It's nice to have some support to go along with crawling around on the ground swimming in diesel. And to think I used to actually like the smell of diesel fuel!