Komatsu PC78MR-6 Stalling When Move Stick

[Sumeet]

Hello,

I recently purchased a 2008 Komatsu PC78MR-6 Excavator. The excavator runs well and is overall in good shape. The issue I am having is the engine will bog down only when I am moving the stick away and in (Left Joystick forward or backward) and the engine will stall out when moving the stick from full away to full in or vice versa without letting go of the controls.

Every other function like swing, boom up/down, curling bucket, dozer, thumb being done will not cause any issue, even if it's a combination of moves. But once I move the stick in or away it starts bogging/drooping engine.

There is no smoke from the exhaust when the engine is bogging down.

Could anyone provide some insight of what to start looking for to diagnose this issue?

Thanks in advance!

 

[uffex]

Good day
I interpret that the issue is isolated to the arm only,I suggest you check it out by measuring the pressure for use free of any load. My instinct is that the dampener has jammed in the cylinder restricting fluid (seeNo.5), check the filter house for any witness parts to failure.
Kind regards
Uffex

 

[uffex]

A Contact just posted this to me may be helpful.
KR
Uffex

 

[Sumeet]

Thank you very much Uffex. I was about to logon to update the forum within an hour. After playing around with the machine for a while longer I have been able to confirm that the issue also occurs with curling bucket, stick in and out, boom up and down, and thumb in and out. It does not occur with swinging or dozer. I'm not sure why I was only noticing it with the stick at first... Would the same troubleshooting guide apply.

Thank you very much for the reply!

 

[uffex]

Good day Sumeet
That is a different situation, let me get hold of a hydraulic schematic and come back to you, often such complaints are a result of low engine power, so please be sure you are getting max. revs and no smoke of an unusual nature.
Kind regards
Uffex

 

[Sumeet]

No smoke occurring at all. I will check the RPM, but audibly the engine sounds like it is running at full RPM.

 

[uffex]

Your remark regarding slew & blade is easily answered as the pump which supplies those functions is a low power fixed displacement. The remaining functions are powered by a variable high pressure load sensing pump.
I am sure not all the guys will agree but hydraulics are not self adjusting, load sensing will if the delta P pressure is tampered with will cause engine overload.
Your most recent post intimates that the problem exists in all main pump circuits if there is no error displayed my thoughts will come back to the engine.
Modern excavators will reduce the engine load as the revolutions decrease under load which can hide more obvious defects.
Check the engine speed Hi Idle = 1970 – 2070
Service the fuel system change injectors according to the recommendations
Clean out inter cooler where applicable
Check the turbo
Check the compression
Kind regards
Uffex

 

[Sumeet]

Thank you Uffex. I will get working on checking those things.

I also noticed the auto idle function on the left joystick will idle the engine when turned on and it will not Rev back up when doing any function and will remain at idle. Would this be an indicator of the underlying issue. Further, searching through the Web says it could be the TVC controller, would you agree with this?

 

[John C.]

First issue of "overloading" or pulling the engine down in rpm is nearly always fuel supply or air supply.

I have been hearing of the pull down issue being caused by the hydraulics on that series of machine. I have not been on one long enough yet to figure out what is going wrong. I would suggest checking all the adjustment screws on the main pump and the valve body looking for broken paint on the locking nuts or the screw slots. If you find damaged paint there, could you take a photo and post it here.

I don't see any remarks about how many hours are on the machine. I also don't see where the monitor panel has been checked for fault codes.

 

[Sumeet]

Thanks John. I will look for broken paint on the pump tomorrow.
The machine has 7000 hrs.
I checked the monitor for fault codes and none are present.
I also determined that the boom raise pressure switch is not lighting up the bit in the monitor today.
I checked to see and TVC current also changes with engine throttle.
Engine RPM are 2070 at full throttle.

 

[uffex]

Good day Sumeet
I suggest that the auto idle issue is isolated to the left RCV this I would suspect is just an sensor problem and unlikely to have a relation to the overload.
My train of thought is still with the engine at 7000 hrs. it must be due for a top to oil pan check. I will check the achieves for engine information and post it on if applicable.
Kind regards
Uffex

 

[uffex]

Good day
This is what I have on file hope it is of help.
Kind regards
Uffex

 

[Sumeet]

First issue of "overloading" or pulling the engine down in rpm is nearly always fuel supply or air supply.



I have been hearing of the pull down issue being caused by the hydraulics on that series of machine. I have not been on one long enough yet to figure out what is going wrong. I would suggest checking all the adjustment screws on the main pump and the valve body looking for broken paint on the locking nuts or the screw slots. If you find damaged paint there, could you take a photo and post it here.

I don't see any remarks about how many hours are on the machine. I also don't see where the monitor panel has been checked for fault codes.

Attached are photos of the valve bank. Only rusty part I noticed is on the top right second fitting.

I played with the machine again and this time the engine was only stalling with moving the stick in and out like my initial post. The machine maintained full power for boom, thumb, curl bucket. Not sure why it is not stalling for those functions now as I haven't changed anything.

I don't think it's the engine as I was giving it full load digging without using stick in/out and it performed well.

Also I operated the machine at different RPMs and found that the machine will stall with stick in/out from 1420-2070 RPM but at 1360 RPM the engine would not stall from stick in/out. I used the machine for 5+ minutes at 1360 rpm and it would never stall out. One notch up at 1420 rpm and I was able to get it to bog down rpm with moving stick out by starting with stick fully in.

 

[John C.]

I don't recognize that valve bank. The top cartridge valve has the paint knocked off and there are mash marks where someone used an inappropriate tool on it. The rusty cartridge below I would guess is for an auxiliary function like a thumb or breaker. Since the arm seems to be the most affected, have you checked it for drift?

 

[Sumeet]

I don't recognize that valve bank. The top cartridge valve has the paint knocked off and there are mash marks where someone used an inappropriate tool on it. The rusty cartridge below I would guess is for an auxiliary function like a thumb or breaker. Since the arm seems to be the most affected, have you checked it for drift?

.

 

[Sumeet]

Hello. I have had the local Komatsu mechanic come look at the machine. He was able to get the pump to build full pressure after blocking the sprocket on the tracks and running them. However arm functions were never making full pressure. He determined that there was likely an issue with the valve block, potentially stuck valves or spools etc. I had the valve bank removed and inspected/resealed by a hydraulic shop. They found some bad o-rings and an issue where both pump flows combine. Unfortunately after reinstalling the valve bank the problem was not fixed and I have now spend a significant amount of money on the machine.

I am hoping someone could be advise me if I need to be looking at getting a new pump or is there potentially something else wrong with the machine? Could it be collapsed hose, relief valve, solenoid, cylinder, anything else? I have adjusted the PC and LS valve to a point where the machine will not stall out but now the arm functions are quite slow. Also put a new solenoid on the pump (TVC-EPC).

Thank you in advance!

 

[Pipeman1848]

Hello. I have had the local Komatsu mechanic come look at the machine. He was able to get the pump to build full pressure after blocking the sprocket on the tracks and running them. However arm functions were never making full pressure. He determined that there was likely an issue with the valve block, potentially stuck valves or spools etc. I had the valve bank removed and inspected/resealed by a hydraulic shop. They found some bad o-rings and an issue where both pump flows combine. Unfortunately after reinstalling the valve bank the problem was not fixed and I have now spend a significant amount of money on the machine.

I am hoping someone could be advise me if I need to be looking at getting a new pump or is there potentially something else wrong with the machine? Could it be collapsed hose, relief valve, solenoid, cylinder, anything else? I have adjusted the PC and LS valve to a point where the machine will not stall out but now the arm functions are quite slow. Also put a new solenoid on the pump (TVC-EPC).

Thank you in advance!

Did this issue ever get resolved?

 

[uffex]

Good day
If all the main pump circuits are affected, suggest that the load sensing signal is wrong or not arriving at the pump to reduce the flow. If you can check this it is a reduced pressure coming back from the main valve's. Unfourtunatly adjusting the value requires both experience and tooling.
Kind regards
Uffex

 

[Pipeman1848]

Good day
If all the main pump circuits are affected, suggest that the load sensing signal is wrong or not arriving at the pump to reduce the flow. If you can check this it is a reduced pressure coming back from the main valve's. Unfourtunatly adjusting the value requires both experience and tooling.
Kind regards
Uffex

Thanks