Hello all, new to the forum and just bought first mini ex on marketplace. It’s a Komatsu pc27r-8 and is the original green color. The machine will crank and run but hydraulics are really weak. All work attachments work slowly but drive motors will not move at all. The pump is new and previous owner stated that pump was replaced and worked for about 1 hour and then the machine quit moving and has been sitting for 2 years. I have checked main relief valve and even removed main valve assembly. It had been taken apart as well. It had pin marks on the blocks but everything looked ok according to manual. I ordered a pressure test kit and the main outlet port on pump only shows 2000psi and manual says it should be over 5000psi. I also removed filter in bottom of hydraulic tank to pump and it all looks good. The pump does look new and it is a Komatsu brand. The pump does not make any abnormal noises but obviously something is wrong. Also noticed there is a whooshing noise at main valve entrance and it can be felt in main pressure line running into main valve. The fluid seems to heat up really quick within just a couple minutes but pump remains cool. Is there any other things I should do before removing pump and looking for some obvious damage? Any help would be greatly appreciated
I'd guess that a relief valve is stuck open (the whooshing sound & excessive fluid heating). More specifically, a relief valve not fully seated (partially open) will make a variable-displacement pump operate at maximum displacement while struggling to achieve rated output pressure. Closed-center hydraulic systems shouldn't be whooshing audibly unless the machine is moving under power.
2000 psi vs. 5000+ psi spec at pump outlet = massive internal bypass somewhere, NOT the pump itself (since it's new and runs quietly). That "whooshing noise" at main valve entrance is your smoking gun - you have a catastrophic leak inside the main control valve, likely from: 1. Main spool housing cracked/damaged: Previous owner disassembled valve, may have damaged sealing surfaces or reassembled incorrectly 2. Missing/damaged seals or o-rings: When valve was apart, seals not replaced or installed wrong 3. Valve body warped/scored: Allows massive internal bypass from pressure to tank Your valve reassembly is suspect - pin marks mean alignment, but doesn't guarantee sealing surfaces are good or parts assembled correctly. Block/cap the main valve inlet temporarily - measure pump pressure again. If it jumps to spec, valve is 100% your problem. Disassemble main valve CAREFULLY - inspect every sealing surface, replace ALL seals/o-rings. Check manual for proper torque sequence/specs on valve reassembly.
About your Komatsu PC27R-8. From your description, a few points stand out clearly: Hydraulic pressure only 2000 psi (should be 5000+ psi) — since the pump is new and not making noise, it’s very likely not a mechanical failure inside the pump itself but a flow or control issue . Whooshing noise and fast heating oil — this often indicates internal leakage , possibly through the main control valve or relief valve staying slightly open. Pump remains cool but oil heats up quickly — that confirms that the pump is working but the oil is bypassing somewhere (flowing under restriction, generating heat). Here’s what I’d check, in this order: ✅ 1. Check suction line for air leak or blockage – even a small air leak can cause cavitation and pressure loss. ✅ 2. Inspect the pilot control line – sometimes a stuck or damaged pilot line prevents the swashplate or control spool from moving fully. ✅ 3. Recheck main relief valve – even if it “looks” fine, it may be weak or contaminated internally, causing it to open too early. ✅ 4. Test case drain flow – high case drain flow from the pump means internal leakage (bad compensator or worn seal). ✅ 5. Check for incorrect pump installation – make sure rotation direction, pump control connection, and compensator setting match the system. If all external causes check out, then yes, it’s worth removing and bench-testing the pump. Given the story that it worked for 1 hour and then failed, it could be an assembly or calibration error inside the replacement pump. If you end up replacing it, I’d recommend using a high-quality aftermarket gear pump that matches Komatsu’s specifications. We’ve had good success with Fairy Deer hydraulic gear pumps — they’re built with tight tolerances, high-pressure aluminum bodies, and proven reliability in compact excavators like the PC27R-8.
Ut oh.....now there is 2 of them
Thanks for the feedback,last week I ended up taking pump off and apart. The brass end plate (opposite side of squash plate) had small grooves where it was riding so I used a flat stone 600 grit to sand on flat table until all grooves were gone. Also had some of that brass in the gear side of pump but it was just in sealing area and no scoring was seen. Put pump back together and bled system and started up. Pressure increase to around 5000psi and bucket boom work good. As soon as I pushed lever to move tracks it started bogging engine down to barely an idle even when I released lever. If I move the boom up or down or in and out the engine speed increases as if the flow to cylinders is actually releasing some pressure on the engine and pump. Now after replacing the return filter and continue to try to further diagnose the pressure is now once again at 2400psi hot. It’s back to the same issue with bucket curl moves at a snails pace. Also the tracks wills moves super slow when it’s off the ground. I guess at this point I can try to cap the main inlet to the main valve or at pump and see if pump is putting out the 5000psi and if not then I know it’s pump. Even when pump was putting out 5000psi it’s still wouldn’t move the tracks or curl the bucket. Not sure what the person done before me with the compensator setting or the squash plate stops. Not sure how I go about setting those correctly the shop manual does not go into great detail.
That valve plate has small creases that are used for lubrication. If you ground everything off to a flat surface, you may have burned up your valve plate now.
It has small machined surfaces from the lubrication holes and they were still there. I just removed a very small amount of material to make it smooth again where it was scored. I can hear a bypassing noise in top of main valve and fluid heats up super quick. The previous owner apparently added a service valve plate in the main valve and it has nothing attached to it. I will remove it and see if that makes any difference. According to the service manual this can be added or removed. It is top part of valve so you don’t need to remove the whole assembly. I did find that the bucket cylinder is bypassing so everything seems to work other than having the correct pressures and flow from pump or bypassing in main valve. Again I appreciate any and all thoughts.
If fluid is rushing through the valve body with a 2000 psig differential pressure & rapidly heating, you should shift your troubleshooting efforts to the valve body and stop messing with the pump. Pay particular attention to pressure relief valves. You're looking for a relief valve that's sticking, or partially blocked open (aka find out why your closed-center system is behaving like it's an open-center system)
The picture below shows the main valve. The second valve was added I found out for unknown reason. The 3rd valve is for aux control. Is there any reason to have non working plate on the machine ? The top half unbolts so would be easy to remove. When machine is running and using stethoscope it is much louder in the upper main valve than the lower. Not really sure if that means anything but thought i would throw it out there. View attachment IMG_6458.jpeg
I’ll start by checking for air leaks, pilot line issues, and the relief valve first, then test case drain flow before considering a pump bench test.
Finally getting a chance to pull main valve apart and replace all seals and looking for anything obvious. One thing I have found is a spring in the travel motor main valve circuit does not show in service manual or the parts manual. I also called dealer and they have looked in multiple models and see no springs call out for that location. The attached pictures show what I’m talking about. Any advice is welcome View attachment IMG_6559.jpeg
The spring is in location 30 on the picture attached
I think the manual/picture tries to say that some serial numbers have the two springs; pos 34 and 34. And some other serial numbers have one spring; pos 36 instead of the two springs, pos 34 and 35.
Yes, that is correct. My serial number does have only 1 spring instead of the 2. The spring location I am talking about is in location 30.
Ahh, ok. I now see there is several identical O-rings (pos30), sorry. Then I understand it like there is a surplus spring between restrictor(?) 33 and plug 29 right next to the arrow. Someone have misunderstood the picture and put a spring there which should not be there acording to this pic I would say.
Yes, that is correct. There is a surplus spring between plug 29 that seats on valve 20. See picture. The red mark shows the extra spring.
Been a while but I finally rebuilt the main valve assembly. I found one valve completely backwards and a spring missing. The machine now moves but hydraulics still weak even though pump pressure is within standard. When cranking the pump pressure is maxed out and you have to pull a lever to drop pressure so it will crank. I have found the original owner put the wrong pump on the machine. The original pump on machine only has 3 hoses and the new pump that is on the machine has 4 hoses connections ( not counting suction hose). I think this is the problem. See attached pump diagrams. Can I install a T on one of the 3 hoses on the machine and make this pump work ? The hose that has a plug on its location is the pressure pump input that is marked in red. Any help is appreciated.
Anyone know if the newer pump will work on my machine. All hydraulics work. Tracks speed is within spec and both Hydraulic pressure at main port and pilot pressure within spec. Bucket takes about 10 seconds for full stroke and spec is 2.5 seconds. When I try to dig it is really weak and just stops moving. No bogging of engine or anything. Main pressure remains at 3000 psi spec. Also tested one side of bucket and it went to 3000psi as well.
First of all can you drop the entire schematic here? Now please bear with my reply. Based on your symptoms and the specific pump mismatch you described, the stalling issue and the weak performance are almost certainly caused by the Load Sensing (LS) logic failing to "destroke" the pump . Because you have a 4-hose pump on a machine that is designed for a 3-hose pump, the pump is mechanically locked in "Full Flow" mode because it is missing a critical internal pressure signal that the original pump generated for itself. Your machine uses a Closed Center Load Sensing (CLSS) system. In Normal Operation, when you start the engine with levers in neutral, the pump should sense "Zero Load." It should immediately angle its swashplate to near-zero (minimum flow). This is called Standby Mode (usually ~300 to 450 PSI). But in your Situation, The engine stalls because the pump is stuck at Maximum Swashplate Angle . It is trying to pump 100% flow against a closed valve (deadhead). The engine cannot overcome this massive hydraulic load at idle, so it dies. The swashplate is moved by a "Servo Piston." This piston needs hydraulic pressure to push the swashplate back to neutral. Original Pump (3-Hose): Had an LS passage that fed system pressure to the piston. New Pump (4-Hose): Likely lacks this LS passage and expects an External Pilot Supply (the 4th port) to feed the piston. Since that port is plugged, the servo has no pressure, the spring pushes the swashplate to Max, and the pump locks up. (my guess - need to see the entire schematic) Even though it's at max stroke, the control valves in the pump (LS and PC valves) are not balanced. When you pull a lever, the pressures equalize incorrectly, and the pump may be mechanically washing out or failing to build differential pressure because the control spool isn't floating correctly. You likely have a pump meant for a slightly different variation (e.g., a PC27MR vs. PC27R, or a specific serial number break). So in total, the pump is stalling because it cannot destroke without a pressure feed to its servo.
Thanks for the help, you are correct the pump looks to be from a PC27MR-2 while my machine is a PC27R-8. The first 3 pictures are for the pump (PC27R-8)that is supposed to be on the machine. The last 3 pictures are for from The pump that is actually on the machine. If the pictures are not readable I can email you the manuals I have. I ended up buying one for the PC27 View attachment IMG_6759.jpeg View attachment IMG_6760.jpeg View attachment IMG_6761.jpeg MR machine just to see what the pump differences were
View attachment IMG_6764.jpeg
It's hard to view schematic due to reflections. Can you share manual on: [email protected] ?
I sent you the files
Your system is weak, means no enough power is building up > means no enough pressure/ pressure+flow is building up > after the right pump update, flow issue should get resolved > this narrow down to pressure issue > system couldn't build enough pressure, when it tries to buid pressure, restriction opens (relief valve), so when system try to build pressure, relief valve gets open (partially) and due to that not enough pressure get build to do work. start checking relief valve.
As previously discussed my machine has the wrong pump on it. The drive motors seems to work normal and the machine will lift off ground and seems to function pretty well but bucket is really slow with a 10 sec stroke vs spec of 2.5 seconds. Also system is overall weak when trying to dig. I will also mention that when cranking the engine turns slowly until I pull a lever and give the hydraulic system relief. It makes sense that for sure it’s on the the LS side and something is miss-matched with the wrong pump. Is there no way to pipe in hydraulic pressure on the 4th plugged port on my pump or is it just completely different set up and I need to replace the pump on my machine with the correct one ?