Latest update, the WT/BL and BK/YL wires from the ECM to the Vapor Solenoid were ohmed out as good, and I used a "THIKPO Universal Fuel Injector Tester " from amazon to prove that both my old and new Vapor Solenoids operated properly, you could audibly hear the solenoid open and pulse.... Looks like the ECM is the only thing left, so calling the local authorized komatsu repair facility....

The Lp is a liquid , the vaporizer takes the liquid and hot water from engine flows through vaporizer turning it to vapor which then goes into mixer (carb ) if the thermostat is not opening properly it will cause the water from the radiator to the engine to not heat up which in turn doesn't change liquid to a vapor which in turn causes too much fuel to enter carb
Resulting in a too rich environment and causes engine to die or falter if you let it run a minute the vaporizer should be warm to touch and after a few minutes it will be hot enough to burn you that is a sign that it is getting flow from engine to vaporizer

OK, if that is the chamber / pressure regulator looking thing, I can check, but it is flowing propane gas to the vapor solenoid, as every time you remove the solenoid, it purges the space behind it with a nice whiff of propane.

Can you explain how the thermostat would stop propane flow, I am lost on that one. No leaks in the gas hose to the throttle body...

The vaporizer is not an electrical part it is where the hose from LP tank hose goes into
There will be a lock off valve and vaporizer the the line feeds into from the tank there is a hose from it to mixer (carb)
The thermostat is in the engine where the upper radiator hose ends at engine
Also look at hose from vaporizer to the mixer (carb) make sure tight and no holes

I have already replaced the vapor solenoid you are speaking of after the place it was purchased from many years ago diagnosed that for $450... and wanted $1100 additional to replace. And the tech boogered the wires with regular meter probes instead of piercing probes... I did replace it with a brand new one for much less, but the vapor solenoid is not bad, it is not getting the PWM voltage, I tested next to my neighbors fully operational FG15HT-20, theirs gets a PWM signal during cranking and at idle, but does not get it for initial key on like some have said..


RPM sensor - I have changed both the cam and crank rpm sensors.
Thermostat? do you mean temp sensor? I guess a faulty temp sensor could tell the engine not to give fuel... I can check that.

Next steps:
1 - hook a PWM injector pulse tester to the vapor solenoid and see if it passes fuel, it should, it may even start and run a bit
2 - trace the vapor solenoid power and ground wire to the ECM and verify not a bad connection


Since mine is a 2007 model, I believe it does not have the separate LP controller, and that function is built into the PCM, but a replacement PCM is $2000. I would send mine in for repair, but I have a funny feeling about all the places promising to fix for $4-600..

For a $2000 new part, I think I will pay to have a Komatsu authorized repair center replace it and make sure they give me back my old part and I can try to have it fixed for a spare...

That solenoid only gets voltage as you turn on key , and only long enough to open and let enough fuel in to start then as it starts it opens and closes to keep it idling.you can turn off LP remove four screws and work that solenoid
Out there is an o ring on shaft that may be bad
As you have it in your hand turn key switch on
You should feel it click that is the initial burst of fuel to start , after it starts the ecm sends voltage to that solenoid to keep it idling
The long one that screws in is high speed fuel solenoid it opens after the rpm rate increases
You may have a faulty vaporizer or the base below the mixer (carb) could be leaking
Internally if heat affects it I would 1- check the vaporizer and see if it is freezing up if so check coolant in radiator and lines to it as well as thermostat 2- look at the r p m sensor 3- look at any relays that are in line with said solenoid
It may be getting too much fuel instead of starving. The vaporizer turns the liquid into vapor as you know but if the vaporizer isn't getting enough heat then the engine will get liquid and not a burnable vapor which leads to the things listed above

I replaced the cam and crank sensors with Hitachi CPS0003(23731-6n206) and CPS0008(23731-6j906) ones and there is no change.

I wanted to ask if I have a separate LP gas controller (22710-fu460) or if mine is built into the ECM? I looked and looked on the lift for a separate controller that I found while fumbling through the sxkomatsu web parts list, that thing is confusing...
My lift is FG18SHT-20 sn 675820
which page should I be looking at in the parts manual?
this is the one I am using now
>FORK LIFT TRUCK(LOAD CAPACITY 01 TON)
>> FG15H/18H-20 S/N 650001-UP [fg15h_el]

Look at rpm sensor , it is what the secm
Reads to send voltage to lock off
It is usually in front of engine by crank pulley
As engine spins the rpm sensor reads the crank revolutions and sends info to secm to power the lock off , the secm is not a cheap item

so a couple days after the last reply, big forklift place near me here in Florida charged me $440 to diagnose the vapor solenoid, I had just paid them to do a service a month earlier, before the problem happened. They they brought no parts which I specifically asked them to bring common parts, as they are 2 hours from Clearwater, and they wanted $1100 to replace the solenoid, I am not made of money, so I bought the exact oem solenoid on amazon for $250 and replaced, noting a lot of oil foam inside the cavity (I checked oil level and they had overfilled about 1/4" in the dipstick, grrrr..) I also noticed they had really boogered up the wire leads to the solenoid and the wiring connector instead of using piercing probes to troubleshoot, I could see the copper was shiny, so the work was fresh. i was frustrated.... Then Hurricane Helena put 4' of water in my house and forklift repair was put on the back burner since the neighbor has a FG15 I could borrow. 195 days later, I am in my house and I put a DMM and piercing probes on it and I get no voltage on my unit whatsoever. I walk over to my neighbors and their FG15 gets maybe 0.5v about a half a second after cranking then drops to a few tenths at idle... I am guessing this is PWM and my unit is not putting it out... I am guessing I need a new ECM/PCM now, but to verify, does someone have the ability to enlighten me on the wiring from the vapor solenoid back to the PCM?

Usually the signal comes from the secm
If you can you might check the voltage coming from secm to solenoid both positive and negative wires I do not have a diagram for that unit but it is same set up as a Nissan just for sake of eliminating the fuse check both sides of fuse holder for voltage I had one that the fuse holder was faulty

I do not get the initial voltage, does it come straight from the ecm/secm or go through some circuit with relays and fuses? If i take the fuse out to protect the ecm and give the vapor solenoid a little juice via an ammeter and the battery, I can hear the solenoid and a bit of gas flow and see a short burst of.5a or so..

I believe that is a low idle solenoid if it is round and has 4 screws
The small one is the High rpm solenoid
The voltage comes from small engine controller
Or secm the secm sends 4.6- 4.8 volts to solenoid. When key is turned on
It should engage for a second then shut off
That small amount of time fills the cavity with lp for starting after the start it opens and lets lp into mixer and the rpm sensor sends signal to secm that it is running and keeps it open
If it is dying I would check 1- lock off solenoid
2- rpm sensor 3 - vaporizer
The solenoid will show voltage only if engine rpm is reading