Deere 710B Backhoe Fuel Woes

[SLK001]

My 710B has been giving me fits with air getting into the fuel system. The unit will run fine for about 20 seconds, then bubbles will start to appear in the fuel. This thing has a glass fuel filter, so I can see the air bubbles thru the glass. The fuel system trace to the filter is tank, fuel petcock, soft fuel line, fuel pump, hard fuel line, then filter. It is my belief that the suction side of the fuel system is pulling air into some joint. I have replace the fuel petcock, the soft fuel line and the fuel pump. I tested the petcock with compressed air in water to make sure it wasn't leaking. All the fitting connections are tight (maybe too tight). There is plenty of fuel in the tank. All fittings have sealer on them.

I'm spinning my wheels and I don't know why. Am I even on the right track? Am I correct in thinking that a leak on the pressure side of the pump would result in fuel leakage and not air intrusion?

 

[Coaldust]

The pencil nozzles are notorious for allowing combustion gas to leak through the nozzle tip into the fuel system. That’s an exaggeration. It happens often enough to throw you for a loop when it does happen.

It’s hard to troubleshoot, but you can temporarily put clear tubing on the return line (s) and usually find the leaker. Or, install a new set of nozzles.

But, your mileage may vary.

 

[mg2361]

Has the fuel strainer in the tank been checked?

 

[SLK001]

Strainer was checked/cleaned a couple of days ago - it was okay. Also, the inside of the tank was pressure washed. Are you suspecting a vapor lock caused by a clogged strainer?

As for the nozzles, I can definitely see air bubbles ENTER the filter from the pump side. The return line goes straight to the tank and couldn't be injecting bubbles into the intake side even if it wanted to. Am I wrong on this?

 

[mg2361]

Are you suspecting a vapor lock caused by a clogged strainer?

Yes, if the strainer was clogged you would see what looks like air bubbles, but they would in fact have been vacuum bubbles.

I think a 20 second run time until air shows up would be too fast for this, but did you loosen the fuel cap when the air starts to see if it goes away?

I would suggest placing a clear line between the tank and the fuel transfer pump, and another one on the return line back to tank. It will help narrow down where to look. I have already had hose shops make me clear hose with fittings to place clear lines on the pressure side of transfer pump systems to help isolate air sources.

The return line goes straight to the tank and couldn't be injecting bubbles into the intake side even if it wanted to.

If there was compression getting in through an injector, it could start to aerate the tank. Can't say for sure if it would be enough to enter the suction side as quickly as air shows up in your case, but still worth a look. Did you look in the tank when the issue is occurring?

 

[highwayghost]

I’ve seen the suction tube crack the solder at the fitting that screws into the top the tank, likely from vibration.

 

[SLK001]

Well, I fabricated a clear line for the soft line from the tank, installed it and fired the machine up. As soon as I got down to look at the line, there were bubbles marching down the tube in perfect order, one right after the other. Well, at least I don't have to buy another fuel pump (about the only remaining option).

So I drain the tank, remove the output fitting... and remove the tank filter. I reassemble everything, fuel up and start the engine. Finally, no more bubbles. I run it for about 10 minutes and still no bubbles. I can't drive it right now, because resent rains gave me mud everywhere.

I know that it is sacrilegious to run it without the filter, but I would rather have to change the external fuel filter every 20-30 hours as have it die every 10-20 seconds. At least now I can get some work out of it.

 

[Coaldust]

Good work figuring it out. Thanks for the update.