Ideas on air in fuel

[JDave]

My truck has the classic air in the fuel return line, surge at idle and will die on opening the throttle too much. The short history is that the truck was running okay and died at the top of a long hill pull acting like it was out of fuel. I towed it home and replaced the filters and the mechanical pump. It had nearly 15 gallons in the tank, so it wasn't out. It fired up okay but has had the air bubbles in the return line and surging ever since. I have done the pressure at the tank, and replaced every rubber hose on the supply and return. It will hold 10 psi on the tank no problem with no leaks evident. I just replaced the primary filter with another today with the same symptoms. I ran a temporary tank with the same symptoms, and decent flow from the return line. Is there an easy way to check the pressure from the mechanical pump? Could the main injector pump have failed and exhibit these symptoms? Let me know what I may be missing. Thanks, Dave

[crashz]

Sounds like similar issues I've seen with my truck.

Have you run a clear tube from the IP to the steel return line (~3")? That seems to be the best test for air issues.

Let us know how it goes.

[JDave]

Yes, that's how I see the bubbles. Search has revealed others saying the pump can do this or the injectors. I can see the pump maybe but the injectors shouldn't be able to put the air bubbles in the line coming out of the top of the pump. I think I will crack the line at the fitting going into the secondary to see if the pressure is there out of the mechanical pump. Other than bypassing the current system with an electric pump I can't see what else to try.

[crashz]

I've been told that leaking return lines on the injectors may cause air problems too. Mine are leaking, especially the end caps, but I have no sign of air at the pump. Same symptoms as you.

The secondary is a bear to change out on the early trucks. I always hated that filter location. Maybe the air has worked its was through yet?

Do you see any fuel leaking around the pump or in the valley at all? This could be a sign of the seals in the IP going.

What is the mileage of your truck? If its the original pump, you could have the ring failure (looks like coffee grounds in the pump). Have you tried taking the return fitting off of the pump housing? This holds a glass check valve that can get clogged with plastic from a failed reluctor(??)ring. If you see any in there, the pump could have failed.

Considering the age and questionable service, I was thinking about having the IP and injectors in my truck rebuilt.

[twaddle]

An injector can cause symtoms of air in fuel. If the valves in the injector allow pressure from the cylinder during compression stroke it will allow air into the fuel system. I have only ever seen this once in all the years that I have worked on diesels, but in can happen and it can have you running around in circles if you do not consider it.

Do the air bubbles you see show up when you just crank over the engine or is only when the engine is running? If the air bubbles appear when just cranking then the easiest way to find which injector or injectors are possibly causing a problems is loosen the injection pipe at each injector one at a time, crank the engine over and see if the bubbles dissappear. Check this with each injector pipe loosened one at a time just is case you have more than one faulty injector.
It may not be a faulty injector, as I said I have only seen this an one occasion but it is worth checking for, just in case.

Are any of your steel lines that may not have been removed to visually check perhaps corroded or rubbed through?
I had a rubber hose on my old ex Military K30 that had rubbed through and sometimes would run ok then give symptoms of low fuel pressure plus surging as well. If you have checked all rubber hoses that rules them out but double check the steel as well.

Good luck.

[JDave]

Thanks guys for the tips and ideas. I am in the process of checking the pressure at the outlet of the secondary to the pump. One symptom is that when you crack the throttle it is very slow to rev, similar to the bog on a gas that doesn't have fuel, and will send a large surge of air through the return line. It starts easy on all tries, even when it just died. I have good fuel through the primary with pressure at the tank, I will insert a gauge on the output of the secondary filter.

[twaddle]

Have you tried testing the lift pump vacuum. Is yours the diaphragm type pump on the lower side of the engine block or an electrical pump?

If you remove the suction pipe from the pump, put your finger over the suction connection on the pump and crank the external lever if it is a diaghphram pump, get someone to crank the engine over, you should feel a vacuum on your finger. The vacuum should hold your finger to the suction inlet for a little while if the check valves and diaphragm are good.

Also, one of those cheap (or more expensive) hand operated suction pumps with a vacuum gage for bleeding brakes is handy for doing a suction/vacuum test on lines, hoses and connectors. Usually a vacuum will find leaks easier than pressure.

Good luck

Jim

[JDave]

I put the gauge on today, I have a solid 9 psi with a drop to 8 when you blip the throttle. Held pressure well, still at 7 psi 15 minutes later. Still has air in the return line and still will die when you goose it.
The truck has 85K on the odometer, I just bought it in May and haven't run a full tank through it yet. It had one injector that would smoke when cold so I will change the injectors and see if that makes any difference. I pulled #1 and I have the coarse thread set.
I didn't try the vacuum on the suction side, I think the new mechanical pump is working fine.

[DieselDavy]

Dave,
I don't have an answer for you but I'd like to say hang in there. It will be worth it when you get this fixed. I'm hoping some more of the experts here will jump in with help.
Good luck and keep us posed on your progress.

[JDave]

I changed the injectors for a reman set but no change. I will be changing the pump and let you know.

[More Power]

You could temporarily install a clear (or less opaque) fuel supply line from the fuel filter to the fuel inlet of the fuel injection pump to look for bubbles. If no bubbles exist in that segment, then it's either the injection pump or the injectors. Installing a clear line at the housing fuel return outlet on top of the fuel injection pump, and look for bubbles will tell you whether the injection pump is generating the bubbles (assuming you're not seeing bubbles in the fuel inlet side). Jim

[JDave]

I have this running well again, but with an electric pump to take care of the air issue. I replaced the filters, reman injectors, finally a rebuilt pump but no change until the electric pusher pump right outside the tank. thanks for the encouragement and ideas, truck runs better than I remember it could.