# Kenworth problems



## pallis (Jun 15, 2005)

Are there any truck drivers or mechanics here who can help me? First the questions, then I'll explain. Does the compressor on a diesel truck --85 Kenworth with Cat engine-- have anything to do with pressurizing the fuel? Next, what would cause the air pressure to build up in the air tanks until the relief valve opens and bleeds the pressure out? Now the explanation. I'm thinking about buying the Kenworth to haul trailers that I make, and to use for hauling wood. I haven't driven a truck in for more than twenty years, and I never did much work on them. I won't get a chance to see the truck until Thursday, but my son has driven it and says it runs and works great except for the air pressure problem and the fact that it has to crank for a while to build up fuel pressure. The compressor works fine, so the problem is elsewhere. I'm not sure what that could have to do with the fuel problem.

Further information that might help: The truck has been used as a water truck at a mine where it sat more than it ran. The engine was rebuilt last November, along with the transmission (13 speed). The guy who is selling off the equipment at the mine thinks the problem with the air and the fuel pressure are due to a bad air line. I'm guessing it's a regulator problem. Can anyone give me an idea of what to look for before I go check it out? Could there be just one thing causing the fuel pressure and air tank problem, or are there two different problems I should look for?

Most important, can anyone understand a thing I've just said? It's late. I've been working and frying in the sun all day, and my information on the truck is second hand. 

Pat


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## geofore (Jun 15, 2005)

*Trucks*

1) No, the compressor does not pressurize the fuel. That is done by the injector pump, which is fed by a fuel pump that should pass 28-48 gallons of fuel past the injector pump per hour. Any fuel not used is sent back to the fuel tank. When was the last time the fuel filter was changed or the filter assembly vented? If the engine was rebuilt it sounds as if they passed on rebuilding the injector pump. (read not cheap to rebuild) replacing the injector pump's pistons and cylinders are about $200-$350 each set. Every cylinder in the engine has a mated set/ piston-cylinder in the injector pump to feed the injector for that cylinder. The injector pump is worth about $550 trade in but costs $4k-$6k new. A rebuild can be had for $1,000-$3,000. New injectors are less. Bench test for the injector pump should run about $900, injectors less and should be done if you have the pump rebuilt. Having to crank for a while to start is normal for an old engine not for one that is recently rebuilt. 
2)The air pressure is supposed to build up to open the relief valve/bleeder valve. This is so the moisture is bled off when the relief valve/bleeder opens so the air lines don't fill up with water that would freeze in the winter or cause the brakes or any other air operated equipment to waterlog and cease to work. It should cycle every miniute or so while the truck is running to regulate the air pressure.. If the air pressure drops more than 10-20psi when the relief valve/bleeder opens automatically, replace it. There should be a seperate relief valve on the bottom of the air tank to bleed water off this should be opened manually to drain the water that gets past the bleeder. I would suggest you put a bucket under the relief valve on the air tank when you pull it open as you may find there is water and oil mixxed in the air tank and it hasn't been opened/drained in sometime. The air tank should be checked by bleeding it manually. It is a seperate valve from the automatic bleeder that regulates the air pressure. 
3) The guy at the mine knows you know nothing about a diesel engine if you believe the compressor pressurizes the fuel lines. If the rebuild was done ask to see the paperwork or for the name of the shop that did the work. The mine is supposed to keep maintanence records on the truck, written records. It sounds like the injector pump is a little tired or the injectors are a bit worn if has to crank over for a while to start or it could just be air in the fuel filter canister needs to be bled off. Engine rebuild does not include injector pump rebuild or injector replacement. I used to have my own fleet of trucks before I retired and have rebuilt Bosch injector pumps. I can do more than just run a chainsaw. Google inline pumps or Bosch.


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## pallis (Jun 15, 2005)

I knew I would get lined out if I asked the question here. I didn't think the compressor had anything to do with the fuel. It was something my son was told, or got confused, and I know so little I didn't want to argue the point. Thanks for the quick and detailed answer. You are a life saver. I drove trucks for Halliburton years ago. Other than checking oil, water, tire pressure and bleeding the air tanks every time I parked in the yard, I didn't touch them. I'll print out your response and follow it to the letter. The truck is priced right, but not if I have to spend a few thousand to make it work right.

Pat


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## kf_tree (Jun 15, 2005)

those injector pumps can get pretty pricey......not too bad for my old international....a rebuilt was around 900.00. but i think a new injector pump for a cummins is around 3500.00 not too sure about cats though.....but cat diesels are the cream of the crop.


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## pallis (Jun 16, 2005)

The starting problem is fixed. It starts right up. The brake problem is a pinhole (so I'm being told) in the air tank. The guy is throwing in two new air tanks, so I think I'll get it.


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## PWB (Jun 16, 2005)

Two different problems.Air compressor is controlled by a regulator right on the compressor. Compressor should kick in at about 90 pounds and kick out at 120. There's probably a line from the regulator that goes to an air dryer, to make it blow as the compressor kicks out, assuming there's a dryer. If the thing is just blowing off at a check valve on an air tank, (150 psi?) the problem is either a bad regulator, or a bad line to the regulator.

The fuel thing might be just a bad shutoff solenoid on the fuel pump. Is this a 3406, or something newer? Should be an inline pump with 6 injector lines, expensive to work on but last a long time. In a truck that old with a cummins, the pump is just a gear pump with a few controls, cheap to work on but doesn't last quite as long.

(Former Kenworth owner)


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