air getting into fuel lines

[hawkeyepoole]

some months ago my M35A2C (1979 AMG) started hiccupin badly suddenly, and began acting like a gas engine with someone removing power to the spark. I thought I had some bad fuel. It was a combination of JetA and WMO that I had filtered. I took the tank pump out, cleaned out the fuel tank, and flushed clean diesel through everything. I pulled all three filters and checked them, then cleaned them in Gasoline. They had nothing on/in them. Looked like new.

After having other problems and then remembering to purge the air from the top of the two filters on the driver's side, the truck seemed to run fine.

However, any time I run any distance, say 10-15 minutes, at speed, meaning not between redlights, the truck would start to 'hunt' at idle, and then die. It would take a rest of 5 minutes and about 1 minute total of cranking to get it restarted, and then off we go again.

Well today I picked up the truck and headed to my domicile to get it ready for some 4th of July activities. I got to a major intersection and it started 'hunting' then then acted erractic and finally died. On the side of the road I turned the pump on and opened the relief valve at the top of the fuel filters. It spit a bit of fuel and then nothing for about 1 entire minute. Then finally fuel began to pump slowly and then more forcefully out.

I repeated this scene twice more in the next ten miles of surface street driving.

How is air getting into the fuel system? The entire system is pressurized and any leaks should be fuel spraying out, and not air coming in, right?

What can I look for? Anybody had this before?

P. S. I have turned the fuel up a little, but I'm still at 7psi wide open on the turbo, so I don't think that is it, or could turning up the fuel cause issues like this?

 

[onegmjack]

Could fuel pump be weak???Or on the way out?

 

[hoop]

You are sucking air in from the Primary filter. Clean the filter and replace the gasket with a new one and tighten to specs.

 

[WillWagner]

On the in tank pump there is a hose between the pump and the outlet of the tank. It can rot, crack, break. Check that too.

 

[hawkeyepoole]

You are sucking air in from the Primary filter. Clean the filter and replace the gasket with a new one and tighten to specs.

OK. I'll bite. How can I suck air from the primary filter, when the tank pump should be pressurizing that filter along with the other two?

 

[Heavysteven]

I have getting air into my fuel lines as well. I think it's my flame heater. I came to this conclusion because I having a hard time starting the truck and my fuel pump not working all the time.

If you did want to check the fuel lines and find a problem they are easy to replace. There compression nuts, flex tubing(I use commercial truck air line tubing from mcmaster), and a sleeve.

 

[hawkeyepoole]

Well it's definitely NOT the primary fuel filter. I took that apart, cleaned, checked and reassembled it. I got about 5 miles with three stops to purge the air from the top of the second and final filter. I think there's a blockage somewhere, as there's no fuel coming out of the top of the filters now.

I'm going to have to have it trailered back to storage and begin taking it apart. Please let me know if anyone else has experienced this kind of issue.

P.S. I don't have a flame heater, or any heater.

 

[nk14zp]

Check the line under the radiator to see if it's crushed.

 

[hoop]

Did you replace the primary filter gasket?
This is not a trick answer....What Will and I have said is usually the problem.
The injector pump pulls fuel and pressurizes it to the secondary filters, and then pressurizes it to the injectors.
If the injector pump wants more fuel, than that which will pass thru the primary filter, it will suck air in around the gasket.
You ARE sucking air between the IP and the Fuel Level.
Now You have to find out where it is.

 

[RedbeardTheZombieHunter]

Also, the pump in the tank is referred to as the "lift pump" because that's all it basically does. It lifts the fuel from the tank and feeds it at low pressure so that the injector pump (IP) can pressurize it to the cylinders in firing order. If the hose from the lift pump has a crack or split, the IP will suck air in and put bubbles in the fuel feed and NO diesel in the world likes that.
That short hose from the lift pump to the top of the fuel tank bracket is something that you should just replace whenever you take the pump bracket out of the tank. It's cheap enough that you're better off throwing in a new one since you're into it that far. It may look OK at a glance, but when you take off the clamps and start moving it around, you'll find they tend to crack and split right at the hose clamps, especially if the clamps were over-tightened the last time it was worked-on.

 

[Blueduce]

I would suggest eliminating one possibility at a time. I would start with the fuel lines then tank/pump. Put the rig on flat level ground with at least a 1/2 tank of fuel. Turn your master power on and see if you can hear the fuel pump humming in the tank . If that checks out leave the pump running and check all fuel lines, filters and filter gaskets for leaks or crushed lines. If you have a soldier "B" check each fuel line individually for blockage with pump running. Tank to filter 1 line, etc. If you find no leaks get a funnel, long tube and a 5gal bucket. Make a beer bong to catch and drain fuel into the bucket. With the pump on open the fuel purge above the filters and see what kind of flow you actually have. Is it strong or weak and intermittent. If it's not strong and continuous then you need to bust into the tank.

 

[RedbeardTheZombieHunter]

(At the risk of sounding white and nerdy)
WOOMP, THERE IT IS!
(sorry, I couldn't resist) I'm with Blue on that one.

 

[hawkeyepoole]

Once I get 'er towed into the hangar, I'm going to do what has been mentioned about fuel lines. I'm almost believing there's a blockage somewhere downstream of the primary filter.

the primary filter gasket is new/clean/tight and I know there's no blockage between the tank and the primary as it dumps fuel like a water hose with the filter off.

There is a short rubber hose right near the steering gear. It looks original and it has a plastic fitting on it. I suspect that as well.

When I stopped getting full flow fuel from the top of the secondary filters, I began to suspect a bad pump or blockage.

Unfortunately my 4th is ruined as I can't have her towed until Friday. Right now she's in the Hobby Lobby parking lot at Heritage Trace and I35W in Fort Worth. Anybody want to volunteer to tow me home?

 

[Blueduce]

pm sent

 

[hawkeyepoole]

Well at least I've met two local MV owners! I'm P O'd I can't drive the deuce on Wednesday, but stuff happens. Once I get her to where I can work on her I'll let everyone know through this thread what it takes to fix her..

 

[hawkeyepoole]

OK. I managed to get her to the hangar. Took FOUR stops in 8 miles to get her there. Each time it seems like the tank pump wasn't lifting fuel into the filters, but after a minute or two it would begin to push the air out and then push fuel out of the vent from the top of the two final filters.

I pulled the rubber line apart that's under the steering gear and got lots of fuel at pressure. I pulled one of the two lines that go into the filters from the bottom of the pump and got lots of fuel at a lower pressure.

I noticed that the fuel line goes into the bottom of the injector pump and then up into the final filter?

Today I replaced the rubber fuel line that's below the steering gear, and tonight have replaced the rubber line on the in-tank pump. Tomorrow I'll put the tank pump back in the tank and see if I've done anything.

I have already removed, checked cleaned and replaced the primary fuel filter canister. The only thing else I can to to it is use gasket sealer around the edge of the canister.

 

[hawkeyepoole]

I replaced the rubber line on the pump in the tank, and have replaced the rubber line under the steering gear. I have removed and reinstalled the primary filter. The only other thing I can do to the primary filter is try to find another gasket.

The symptoms are like this: I purge air from the top of the secondary fuel filters, start the truck; it runs for about 5 minutes at idle, then dies. Open up the air vent at the top of the secondary filters, and get pressurized air, then nothing. It's almost as if the tank pump has lost prime. It will take a couple of minutes for fuel to start pushing fuel out of the filter. When it does it comes out very slowly and then builds up to full pressure.

Every line I break loose pushes out fuel easily. I've pulled both lines that come from down around the injector pump.

Can the injector pump itself introduce air? The fuel line from the primary filter goes up into the bottom of the injector pump and then out of the bottom of the pump up to the secondary filters.

This is killing me. I want to drive my truck!

 

[gungearz]

Your pump can be weak and going out and/or you have to figure out where the air is being sucked in at. Also check the fuel filter bracket gasket. Sometimes they leak from under the filters. Hopefully you can get it fixed soon.

 

[hoop]

Is the bottom of your primary filter indented? Or is the nut protruding? If it is indented, someone has tightened it up too much and it will not make a good seal. I know this from experence....I did it to mine chasing an air leak. Primary filter always gets a new square ring. I went 1 step beyond and installed another primary filter to make sure my IP gets enough fuel.

 

[212sparky]

could the bottom of the lift pump screen be clogged and the injector pump is draining the bowls?