Tuesday, September 1, 2015

Kubota Misfire


Todays repair is a Kubota M9960 with the customer complaint of an engine noise.  The driver also complained of a lack of power.
Checked engine for low power and making noise.  Hooked up laptop and found no codes present.  Engine has slight misfire with no load.  Did injector cutout test and found #4 cylinder had no change when injector was disabled. 


In the shop, I removed the injectors and checked engine compression.  Compression was low but within specifications.  I sent the injectors to the pump shop to be tested.  Injector #4 was stuck and all the other injectors were below specifications.  I ordered replacement injectors. 


I installed the injectors and ran the tractor on the dynamometer.  The horsepower was low (hard to see in the photo because of my reflection but it reads 77 hp). 
 I checked for fuel restriction and found low flow to the strainer.  The I blew in tank with compressed air to push the restriction out.  Next, I checked fuel pressure and found the pressure would drop to 0 psi above 1500 rpm.  The photo shows the setup I used.  The vacuum gauge to check for restricted inlet, a clear hose to check for air, and a pressure transducer hooked to a graphing meter to read pressure.

  Checked power supply to lift pump and found good.  I used a current clamp to measure current to the lift pump.  With the meter set to read two channels, I can graph current and pressure at the same time.  This is helpful to quickly determine of I have a power of pressure problem, and I can record the results for later use. 
 Replaced lift pump.  Rechecked fuel pressure.  Pressure now normal.  Ran on dynamometer and found 83 hp (rated at 89).  The horsepower is still lower than normal because of the low compression.  This is as far as the customer wants to go on this one.  The cure for low compression is a replacement engine, and that is very expensive to gain six horsepower.


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