Keep getting air in diesel lines.

[John80y]

I have a Bobcat with Kubota diesel engine and its started a few weeks ago to get air in the fuel lines.. I bleed it out and within an hour the tractor is stalling out again. Fuel tank is always 1/2 full or more.. I am at a loss why this is happening.

[Treemonkey1000]

Just a wild guess but have you checked fuel filters lately? Is there one in the tank? Thinking that if there is a blockage or restriction and the fuel pump is pulling pressure if there isn't enough fuel available it is going to suck air in at a loose fitting or clamp..

[Soundguy]

If the fuel pickup is under the level of fuel you have, and you are not chronically on slopes, then that leaves a suction side leak.. and that can be anywhere from the tank tap all the way to the injector pump seals..

soundguy

[TCBoomer]

If you can substitute the fuel feed to the injector pump with some clear hose, see if air bubbles are present. If none present, injector pump is suspect.

[irwin]

Here's a similar problem, the thread was recently active. Don't think he found out why, but he did rig it so it would stay running..http://www.tractorbynet.com/forums/y...lows-down.html

[Farmerford]

John80y:

If Treemonkey's suggestions (replace fuel filter and check inline screens at tank outlet and injection pump) don't solve the problem, next check to be sure there is a working vent on the fuel tank to prevent a vacuum in there.

If the problem persists, Soundguy is right about where the problem is likely to be. The problem is that it can be very difficult to locate the leak. In my experience, there is usually a very small leak of air into the lines between the tank and the injection pump. If the leak is upstream of the filter, the invading air may accumulate in the filter until it uncovers the outlet port, at which point some of the air moves on to the injection pump where it again may accumulate for a bit in the injection pump case before it reaches the intakes for the plungers. That's why the machine runs a while after you bleed it: depending on the size of the leak, engine speed, etc. it may take several minutes or more for enough air to accumulate in the injection pump to stop the engine.

Check all the fittings in the lines to be sure they are snug.

If that does not cure the problem you may have a cracked line. A couple years ago I had the same problem with a Cat D3B. I finally discovered that the banjo fitting welded onto the line between the fuel filter and supply pump had a tiny crack on the back side that resulted from many 1000 hours of vibration. You could not possibly see the crack without taking the line off.

I am not familiar with the bleed plugs on your engine, but it may be possible to at least isolate the leak to a part of the line by paying attention when you remove the bleed plugs. For example, if there is air in the fuel filter it is a good sign that the leak is between the fuel tank and the filter.

As a last resort, put low air pressure on the fuel tank and look for leaks by putting soapy water on the entire fuel supply system. An air leak will make a soap bubble. It may be sufficient pressure just to blow as hard as you can into the tank fill spout (of course, you'll need your mate to do that while you check the line with soapy water). I take an old inner tube and cut out the valve stem with several inches of tube around it. Then clamp the valve stem onto the fuel tank fill spout with a hose clamp. Then pressurize the tank with air. Don't put more than 1 or 2 psi pressure. Usually just enough to make the rubber around the valve stem bulge is sufficient to push air through the leaking fitting/line and make a bubble in your soapy water.

The key is to carefully and thoroughly eliminate the possible leaks by checking every part of the fuel supply system.

Good luck

[638]

Farmerford,

You won't get any air bubbles out of a leaky fuel line using soapy water IF there is any fuel in the tank. You should be able to see a fuel leak though.

ron

[Iron Horse]

If it has a plunger type prime pump that you unscrew the knob and pump . It may be worn out and letting in air . If it is loose and sloppy when unscrewed it is highly likely .

[Farmerford]

Ron:

OOPS!

You are right. I forgot to mention that you have to drain the tank and the lines first. That happens automatically on my older machines because you drain the tank to get at the screen inside the tank outlet.

You are also right that diesel fuel may escape from the leak, but often the leak is so small that a liquid will not pass through it, but air will.

[gatractorman]

Likely the trouble is the fuel pick-up in the tank has trash around it and wont let an adequate amount of fuel pass thru the screen filter. Very common problem on Bobcats. Lift boom up and secure in place there, flip cab back, look underneath the drive pumps which will be the top of the fuel tank, there will be 2 fuel lines exiting the tank. You may need someone to pull on the line at the engine to determine which one you need out, pull it from the tank, it'll just be an aluminum 90 degree elbow with a 9-10" long piece of tubing with the fuel screen on the end, there will be a grommett in the tank that may come out when you pull the fitting but if not it will need to come out before the screen will. The screen can be cleaned and re-used but the tubing will need to be replaced it gets hard and brittle inside the tank I have saw it crack and pull air in there if it's above the level of the fuel. None of this stuff is terribly expensive at the B/C dealer less hassle to replace the whole lot instead of cleaning screen.