We have a bush hog for ROW maintenance. It's a former Highway Department machine, mounted on a Case-International diesel tractor. I've become the machine's usual operator. The tractor has a chronic fuel problem. I have no clue what's wrong, so I'm seeking advice from more knowledgable folks here.

The tractor will run fine for a couple hours, then will start losing power. The engine will slow down over the course of 5-10 minutes, begin surging, then finally quit. The fuel tank is full. When I check the two fuel filters (they're right next to one another just below the injector pump), they're both a little over half full of fuel. Both filters look very clean -- no discolored fuel comes out when they're dumped. Priming the filters with fuel up to the brim and reinstalling them results in the tractor running fine for another couple hours, then the cycle repeats.

I've checked the tank and there doesn't seem to be any water in the fuel. The tank was removed for access to a wheel bearing repair last year, and was cleaned out at that time, so I don't think there's any trash in the bottom. Today when the machine acted up, I noticed that the fuel line, which passes from the fuel tank between the rear fenders under the seat to the engine, was hot enough that it was uncomfortable to touch it. This on a relatively cool day (65-70).

I'm stumped, and no one else at SCRM has any ideas, either. Suggestions?

Where is the fuel pump in this system? Your comment that the fuel line was hot leads me to suspect it is mounted in the bottom of the fuel tank as on many contemporary automobiles. Thus the fuel pump is losing 'push', heating up the fuel, and potentially causing a vapor lock type symptom. Note the half empty bowls on the filters. I would try to do some diagnosing of the pump if any of the above makes sense.

There might seem to be little bypassing of the fuel where the fuel pump works on cold fuel, but as the fuel gets hotter and hotter from leakage past a seal or fit of a worn diaphragm, it vaporizes, and once there is vapor downstream, the fuel no longer acts like a non compressable liquid, resulting in problems.

Bob Kutella

Is this not a diesel powered machine. And then if so does it have fuel injection as diesels seem to have. If so then in an injection system the excess fuel from the system is cycled back to the fuel tank via the injection return line. The fuel then is warm because it has been to the injectors. what was not used goes back to the tank. The heat was picked up from the injectors which are screwed into the head.

Look for a bad fuel pump checked by a fuel pressure gauge, look for clogged fuel lines or look for an airleak especially on the filter canister gaskets. if you are sucking air then you won't suck fuel. That goes for all the fuel line connections as well.

Sounds like your fuel transfer pump is shot.

One thing you might try, to verify this, is when she starts to cut out, crack open a fitting on the fuel return line. When loose the fuel should flow freely out it.

You might want to also check the fuel return line for pinches, and double check that the line hasn't partially clogged, if there is a partial clog, it will act as an orifice and cause to fuel pump to work harder then it needs to. Higher pressure, more work, hotter pump.

Good luck,

Rich Cizik
MoW Foreman
Ct Eastern RR Museum
Willimantic, Ct 06226
www.cteastrrmuseum.org

Matt,

You might try posting the question in one of the construction equipment enthusiast's forums. ACME (Antique Catterpillar Machinery Enthusiasts) is one:

http://www.antiquecaterpillar.net/ACF_v2/index.php

These guys work on all sorts of machinery (not just Catterpillar). Also a good source of info if you run into any problems with your crane.

I have found it best to always start with the easier stuff first. I have seen fuel filters be a problem in both gas and diesil motors. While it might indeed be your fuel pump, I would recommend changing your fuel filters first. Some of the newer diesils do have much more electronic things to complicate matters as well. I know our newer Macks gave us all sort of problems do to some new innovations that were supposed to improve effiecency. My company sent back a whole bunch of less then one year old tractors on the lemon law because of some of the problems. But back the original questions, I would start there to eliminate a rather inexpensive place to look. The bypass hose thing is another area to look as already mentioned. Good luck, John.

I'm pretty sure the pump is downstream of the filters. There's definitely no pump in the tank. The fuel in the tank was normal temperature, but up by the engine it was pretty hot. The temp gauge doesn't work, but now I think about it the engine seemed hotter than it ought to be.

Would an overheating situation cause these symptoms? I had to replace some rotted radiator lines on the tractor a couple months back. It's possble it was low on coolant yesterday, although I refilled the cooling system at the time. I was focused on fuel and didn't think to look at the coolant.

The filters are the only thing I can definitely rule out. The tractor has been acting up like this for some time, and until recently my first response was exactly that -- change the filters. I've eliminated the filters as the problem for two reasons -- one, they look clean and no dirty fuel comes out when I dump them, and two, the thing keeps doing the same thing even after replacing both filters.

Is it a closed system or is there any kind of vent for the tank? If there is and it's stopped up or not venting, that may cause a problem after a while.

Vapor lock? If the fuel line is too hot to touch, that might be a cause. Is routed close to an exhaust line or something equally as hot? Just food for thought... I don't know for sure.

The one thing that interests me is when you remove the fuel filter it is only half full. What I have found in normal P.M. is when everything is running good the filters should be full, because I always seem to wear fuel when I change them. Depending on the type of system on the tractor you may have several things to look at. On some systems the primary fuel pump is before the filters and if it is weak it may starve the system of fuel. The other may be a cracked fuel line or a lose fitting allowing air into the system again starving the system of fuel. I don’t believe that the heat is your problem mostly because diesel fuel has a higher flash point than gasoline, if you got the machine hot enough to flash off the diesel I think you would have other more serous engine problems.

The fuel system seems to be pretty simple, which is part of why the problem is so puzzling. The fuel line from the tank seems to go directly to the filters, which are right under the injector pump. There's a short hose from the filters up to the pump.

The part that's throwing me is that it'll run like a top for a couple hours before acting up. Refilling the filters results in another couple hours of flawless operation. If it's sucking air or the pump is bad, wouldn't the problem show up immediately? What would cause it to run fine for so long and then crap out?

Look for a hairline crack in the filter housing or mount. It is possible that as the hotter fuel heats the filter housing, it changes shape enough that it starts to slowly suck air. When you shut down, open the filters and fill them, it cools and closes up again. Or it always sucks air, but at such a slow rate that it takes a couple hours to get sufficient air to get air bound.

I had a hydraulic filter on a new molding press that started to leak after about 5000 hours of operation. Changing the gasket didn't help. Pulling it off and changing the O rings around the ports on the manifold didn't help. I finally cleaned it up really well, and watched it from startup. I was amazed to see a drop of oil appear in the middle of a clean surface of the casting. The oil was leaking through porosity in the casting (scary, as this is a 3500 PSI system). When confronted, the machine builder said, "Oh we thought we had all those out of service. Didn't you get the letter?" Yeah, right.

No reason a fuel filter couldn't do the same thing, except you'll never see where the air is going IN.

First thing I thought too. Check the vent on the fuel filler cap. If it gets clogged, as the fuel is used a vacuum is created in the tank and the pump starves for fuel. Always try the cheap stuff first.

I would look to see if there is a vent in the fuel cap too.

My second guess would be rubber fuel lines (if there are any) are leaking or contracting / kinking as the machine warms up. How old is the rubber on the machine?

Chris