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Komatsu PC78 MR6 low power and stalling

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TexasDan

Member
Joined
Dec 11, 2021
Messages
24
Location
North Texas
Hey everybody,

New member and first time posting here. I recently purchased a used PC78 with ~6k hours on the machine. I wasn't able to test the equipment too much at the dealership (used equipment lot), but the machine appeared to operate well. After the machine was delivered I started to put it to work and noticed it was significantly under-powered. After working the machine for 10-15 minutes it began stalling very easily under almost any load, or sometimes no load at all, with any/all combination of functions (travel, boom, arm, curl, swing, counter-steer, push dirt with blade, etc). The machine is always operated with full RPM.

The machine will immediately crank back up after stalling with no issues, and no error codes are indicated on the computer screen. The machine will put off a little dark smoke when immediately starting but after that it runs smooth (less than 2 seconds). If I wasn't operating the machine I have full confidence that it would run without stalling until the machine ran out of fuel. Also, until the machine bogs down and stalls it sounds like its running smooth. Also, when I'm operating the hydraulics there is no unusual noise coming from the hydraulic pump. When the machine bogs down and stalls it doesnt produce any noticeably colored smoke either.

All the fluid levels on the machine are good, I replaced the fuel filter and hydraulic filter, tried running the machine with the fuel cap off (I read on another post that that could indicate something), and put clean fuel in the machine and treated with dry gas and algae killer, just in case. All had no effect. The fuel-water separator had no water in it, and very little dirt/sediment. I also checked the banjo bolts going to/from the fuel filter, they didn't have any screens or signs of clogging or dirt. My guess is maybe there are some more banjo bolts in the fuel system to look into.

I'm pretty limited when it comes to mechanic skills, I have the basic skills, but if anyone has any ideas on what this could be, where to look, or things to do to test/troubleshoot or eliminate possible causes I'm all ears.

Thanks for reading and any help provided.
-Dan
 
T

TexasDan

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Dec 11, 2021
Messages
24
Location
North Texas
CapnDean

CapnDean

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Dec 1, 2021
Messages
33
Location
Gulfport MS
Yes your problem sounds to me like fuel starvation. There isnt a whole lot that can go wrong with a diesel engine that will allow it to run but not put in the work OTHER than fuel problems. A bad injector would make it run rough for example, a broken valve would be evident as well. If youre for certain that you have ample fuel supply and no air leaks - - check your intake for obstruction. Starving for air is just as bad as starving for fuel. All in all I have to rely on what a much older much wiser man once told me. "If your diesel engine wont run and you dont see a rod sticking out of the block, YOU have a fuel problem.

You have started down the right trailL Fuel filter is number one. Check to see if you dont have any fuel lines that are cracking and allowing air into the fuel supply. I am not sure on your specific machine, but if you could somehow rig up a way to feed fuel gravity wise to the injector pump you could do that temporarily and see if that doesnt change the power situation.
 
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thepumpguysc

Senior Member
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Mar 18, 2010
Messages
7,539
Location
Sunny South Carolina
Occupation
Master Inj.Pump rebuilder
If u have the inline injection pump, the banjo bolt that has the filter screen in it is the 1 under/below the hand primer..on the supply pump..
Good luck
 
D

Delmer

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Jan 3, 2013
Messages
8,891
Location
WI
Do you have one of these things bolted to the side of your injection pump?

https://www.ebay.com/itm/2009537787...efaultOrganicWeb&_trksid=p2047675.c101224.m-1

See the two bolts in the plastic bag? one of them has a white ring sticking past the threads, that's a screen that UNSCREWS from INSIDE that bolt. That bolt is installed pointing down, remove it, remove the screen, clean it, put it all back together without loosing the washers. If it doesn't run right, replace any rubber fuel line.
 
T

TexasDan

Member
Joined
Dec 11, 2021
Messages
24
Location
North Texas
Vetech63 said:
I would check your fuel flow and supply pressure 1st thing . Make sure you don't have a fuel line restriction between your supply pump and the fuel tank.
Click to expand...

All the pressures were good pre and post fuel filter, leading up to the pump. When there was a failure (stalling) the pressures would gradually go down to nothing.
 
T

TexasDan

Member
Joined
Dec 11, 2021
Messages
24
Location
North Texas
CapnDean said:
Yes your problem sounds to me like fuel starvation. There isnt a whole lot that can go wrong with a diesel engine that will allow it to run but not put in the work OTHER than fuel problems. A bad injector would make it run rough for example, a broken valve would be evident as well. If youre for certain that you have ample fuel supply and no air leaks - - check your intake for obstruction. Starving for air is just as bad as starving for fuel. All in all I have to rely on what a much older much wiser man once told me. "If your diesel engine wont run and you dont see a rod sticking out of the block, YOU have a fuel problem.

You have started down the right trailL Fuel filter is number one. Check to see if you dont have any fuel lines that are cracking and allowing air into the fuel supply. I am not sure on your specific machine, but if you could somehow rig up a way to feed fuel gravity wise to the injector pump you could do that temporarily and see if that doesnt change the power situation.
Click to expand...

Thanks for the reply, you seem to be quite wise as well.

I should probably have put this out there in my first reply, just to catch everyone up. Here are where things stand now:

After running out of ideas I contacted a local heavy equipment mechanic at White Star Equipment. The service manager I've talked through on the phone and all the mechanics that have came out on site have been great. First they noticed that fuel filter was not properly secured (it was broken off of the mounting portion of the fuel filter housing assembly and hanging up with wire). This led them to further inspect the areas immediately up and downstream of that. They noticed where the in/out lines going into the fuel filter have been rubbing for quite a bit, and they suspect it may have a small hole introducing air into the lines. They go to napa, replace the lines and restart the machine. Immediately the machine feels like it has a lot more power, and runs well, but then starts stalling again in 5 minutes.

The mechanics look back in the engine compartment, see fresh liquid around the fuel filter, and now believe it must be the fuel filter housing assembly which is bad, and that is introducing air into the system. So they're about to leave the site with the intentions of ordering the part, returning tomorrow, and then everything is well. They turn around their truck and say lets just bypass the fuel filter to make sure. So they bypass the fuel filter and fuel is going right from the tank to pump. I test the machine again and it begins to run with a lot of power, and run a lot longer, well beyond the point it would normally fail.

The mechanics were about to leave but I insist to stay just for a few more minutes. Right after is say that it feels like the machine bogs down. Ultimately it bogs down and then begins stalling again. The mechanic says he needs to come back the next day with gauges.

They show up the next day with gauges. Check the pressure of fuel system. From tank to pump it was i believe above 50 PSI. when the machine would fail it would still have good pressure, then gradually return to nothing after stalling. The mechanics advised me a sudden loss of PSI followed by engine failure would indicate something in the fuel system (dirty tank, weird blockage, etc), however, engine failure followed by a loss of PSI would indicate the problem was in the injection pump.


So back to your question. The fuel lines are in good shape, no concern for air getting in. The intake is very clean, no issue on the air side. I don't think there is a primer pump on this machine, because just prior to the injection pump the machine was getting 55psi on the fuel line.

Would I be right to be right to assume the mechanics are correct on this?
 
T

TexasDan

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Joined
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Messages
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Location
North Texas
Delmer said:
Do you have one of these things bolted to the side of your injection pump?

https://www.ebay.com/itm/200953778789?_trkparms=amclksrc=ITM&aid=1110006&algo=HOMESPLICE.SIM&ao=1&asc=20200818143230&meid=6c5479287f274af38344fac9cda2f963&pid=101224&rk=1&rkt=5&sd=190863113553&itm=200953778789&pmt=0&noa=1&pg=2047675&algv=DefaultOrganicWeb&_trksid=p2047675.c101224.m-1

See the two bolts in the plastic bag? one of them has a white ring sticking past the threads, that's a screen that UNSCREWS from INSIDE that bolt. That bolt is installed pointing down, remove it, remove the screen, clean it, put it all back together without loosing the washers. If it doesn't run right, replace any rubber fuel line.
Click to expand...


Thanks for the reply. I don't believe my machine has one of these on the injection pump, but I can try and get back to the machine on sunday or monday to make sure.
 
T

TexasDan

Member
Joined
Dec 11, 2021
Messages
24
Location
North Texas
thepumpguysc said:
If u have the inline injection pump, the banjo bolt that has the filter screen in it is the 1 under/below the hand primer..on the supply pump..
Good luck
Click to expand...

Thanks for the reply, I will try and get to the machine on monday to get a good look, and then get back to this board to update everyone.
 
CapnDean

CapnDean

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Dec 1, 2021
Messages
33
Location
Gulfport MS
Gutsy move to bypass fuel filter when chances you have crap in your fuel tank are high. But now that it’s done here is my take: if you do not have a lift pump and you are counting on the injection pump to suck fuel….. then you’re looking for a vacuum between the tank and the pump. Not pressure. Have you removed the fuel cap to see if perhaps the tank is not properly venting?
 
John C.

John C.

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Your machine serial number does not come up in my sources. I suspect it to be a gray market machine. Does the engine have one valve cover or individual valve covers? The engine with individual valve covers is a Cummins clone which does not have a strainer in a banjo bolt. It came with an inline fuel strainer that looks like an in line gas filter. If you find that filter, I would suggest you get a new strainer from the Komatsu dealer. The paper gas filters will swell and choke off the fuel flow and the engine will die and usually stay dead. The brass strainers will also do the same thing but you can take it out and blow air pressure through it backwards and clean it out the engine will run fine after that. One other thought is that on some machines the strainer is under the tank under the cab. Follow the fuel lines all the way back to the source to be sure.
 
D

Delmer

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Jan 3, 2013
Messages
8,891
Location
WI
TexasDan said:
From tank to pump it was i believe above 50 PSI. when the machine would fail it would still have good pressure, then gradually return to nothing after stalling. The mechanics advised me a sudden loss of PSI followed by engine failure would indicate something in the fuel system (dirty tank, weird blockage, etc), however, engine failure followed by a loss of PSI would indicate the problem was in the injection pump.


So back to your question. The fuel lines are in good shape, no concern for air getting in. The intake is very clean, no issue on the air side. I don't think there is a primer pump on this machine, because just prior to the injection pump the machine was getting 55psi on the fuel line.

Would I be right to be right to assume the mechanics are correct on this?
Click to expand...

They're correct that the pressure dropping after the RPM drops is normal. But that assumes you have a mechanical transfer pump, so which is it? If the pressure that the injection pump sees is constant until the the engine slows and the engine slows before the pressure drops, then thepumpguy is going to need the numbers off the pump, unless he knows what it is from the serial number.

Fuel filters are almost always under pressure from the transfer pump. Strainers, water separators are usually on the suction side, that is, between the tank and the transfer pump. So where's the transfer pump? follow the hose from the tank to the injection pump and back.

I agree with CapnDean, bypassing the filter is a dumb move. Three reasons I don't like the guys, and I'll think of more later.
 
CapnDean

CapnDean

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Messages
33
Location
Gulfport MS
OK: I just read your mechanic report a second time. Machine runs fine, but after a while under load it stalls out. Bypassing fuel filter housing that you suspect is leaking.... allows machine to run strong much longer, but ultimately it loses power and stalls out. Could it be this? Your fuel tank vent is obstructed by a dirt dauber. So when you start the engine fuel can easily flow from the tank to the engine. As you use fuel faster than the air can enter the tank, you're creating a vacuum in the tank until the engine cant get enough fuel hence it starves. Removing the filter housing that was leaking simply eliminated that one other place that the system was getting vent air. Try this: run it til it stalls - - when it stalls, remove the fuel tank fill cap...... see if you hear a whoosh of air. What is truly telling in this situation is that after stalling....you are not having to bleed the injectors to get the fuel back to the cylinders. If it was truly a bad air leak - - in order to re start the engine, you would be having to prime the heck out of it. You know....how they say never let a diesel engine run out of fuel...
 
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