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Discussion Starter #1
Hello,

I found this forum and it seems to be a great source of info on the 4 cyl Cummins motors. I have a piece of heavy equipment that has a 3.9L Cummins diesel in it. I believe it to be the 4B motor as it is not turbo charged. Built around 1998-1999.

Problem I am having is the motor will start cold and run ok, and soon it begins to bog down under a load and will eventually die. Becomes hard to restart or will not restart. I replaced all the fuel filters and no change. I am very mechanically inclined but usually only work on gassers. I would assume I am looking at a lift or injection pump issue, or possibly injectors. I cracked each injector line at the pump one at a time while it was running and the motor would bog down as I opened each one. Each one exhibeted the same change so it did not seem like an issue specific to one cylinder.

Is there a way to test the lift pump? How can you test the injection pump without pulling it off? Is there something else that I should be looking for here? Motor has about 2500 hours on it and has always run great.

Any help would be greatly appreciated.

Thanks,

Brad
 

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How long does it take for the engine to "bog down"?

Is the tank vent open?

I've read, in the tech reference (thank you Bob S :) ) that you can use a see through hose at least temporarily between the lift pump and the injection pump to check for air leaks between the lift pump and tank.

IIRC it includes how to test the lift pump.
 

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It does not take to long about 5 or 10 minutes of run time varied rpm's. Problem has definetly gotten worse over the past few days to the point I cant really use the machine.

To do the clear hose test I guess you remove the hard line from the lift pump to the filters?

How sensitive are these motors to bleeding? If it is running and ideling fine after a filter change will they self bleed what ever residual air got in the system? The condition didnt improve any after the fuel filter changes.

Thanks,

Brad
 

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As a brainstorm, bounceI would say you have one of three problems:

1. The afore mentioned issue of fuel supply to the injector pump. Definitely make sure the IP is receiving enough fuel as this is an easy problem to diagnose and fix. Generally, when the IP is partially starved for fuel, the engine will idle fine, but will not have any power; under severe starvation, the engine will barely idle. I would say this is your most likely problem.

2. Your IP is worn out. Your IP works well when things are cold, but as things warm up, the viscosity of the fuel is less, and the internal spaces in the IP grow which both would prevent the IP from generating the necessary pressure. There are several reasons why this would not be the cause of your problem: IPs do not wear out overnight, unless the engine sat for a long time with water in the IP causing rust; also, an engine with a highly worn IP is generally hard to start all the time.

3. You have a blown headgasket/cracked head. You engine starts fine, but as the engine warms up, pressure builds in the cooling system. As the temperature nears boiling, the pressure increases and starts to flood the cylinders with enough water to cause the engine to lose significant power/stop running. I had this occur on a vw 1.6 rabbit diesel. Of course there would be other symptoms such as pressure in the coolant system, and elevated water temps.

I cannot think of any other reasons that would effect the operation of your engine as a function of time, or engine temperature.

I would definitely closely monitor engine operating parameters to try and determine a pattern. Does the engine start acting up at a certain temperature? Does it always take the same time? Is it a constant volume of fuel that runs through the engine before it acts up (does it act up quicker when the engine is under a load)? Does the ambient temperature have an effect? Is there any exhaust smoke, and if so what color?
 

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if I were just trying to figure out where to look first, I would Take the fuel tank cap off, take the radiator cap off, Start it and see if it bogs down. If it runs fine, I would check fuel tank vents. If it dies, check to see if you have exhaust coming out of your radiator, If not I would do the Clear tube on the lift pump. Then at least you can rule out some easy things.
 

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How often is the eqipment run? Has it set on down time for awhile?? If so fuel may have gotten contaminated,or has water in it.
 

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I was on the same track as 2dodges But I was going to ask also if the screen in the bottom of the tank is not stopped up.. After it set and runs fuel back into system it runs at idle for a while OK but in a 5 minutes it begans to starve out .. I had an old tractor do this to me once. But I'M going to place my vote on the Injection Pump... Sorry.
 

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algae in fuel usually wouldnt be my bet,but i just came down with a bad case,so my moneys on algae/delivery problem
 

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Might check fuel return line to tank.....
 

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Could be the fuel shut-off especially if it a mechanical one. Have seen the linkages wear out on them and cause it to start closing or if it is partially closed the motor will be laggy. You can try to test this by going from idle to WOT and see if the motor slowly builds up RPM's or if just jumps right up to full speed.
 

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i wonder If he got this piece of equipment running .
As I found this post on the WEB because mine is running like his tractor.
 
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