Mitsubishi FGC25K 0R FG25K:
Very Slow Idle on Start Up

Looking for some help with a Mitsubishi FGC25K (s/n AF82D01804). Sometimes when it starts it runs at a very slow idle. I don't know the speed but it seems to run slower than the starter motor turns the engine over at. Usually within 5 minutes or less it eventually will come up to a normal idle speed all of a sudden. The time frame at which it does this varies. It runs fine after that and will typically start and run fine if it is used on a regular basis throughout the day, but not always. I haven't had any issue with it not starting it just won't ramp up to speed like it should.

Thinking it was a fuel related issue I replace the Impco Cobra vaporizer and the filter in the lock-off valve. That did not solve the problem. Put a different LP bottle on it as well with no improvement.

I've tried various things like squeeze and/or remove vacuum lines, wiggle wires, press the gas pedal and the only thing that seems to make a difference is squeezing the outlet of the vaporizer. The engine will pick up speed just a tiny bit (still not anywhere near a normal idle speed) if you squeeze it just right or it will kill the engine if you squeeze too much.

I'm looking to know what to try next??? Thanks!
  • Posted 8 Aug 2018 02:55
  • Discussion started by eaugie
  • Kansas, United States
Showing items 1 - 2 of 2 results.
Did you ever get this problem resolved? I have the same truck with the same problem and having a hard time getting anywhere with it. Was hoping you may have gotten it figured out and could shed some light on your resolution for me.

Thank You
  • Posted 4 Aug 2023 05:26
  • Reply by MrChris
  • Oklahoma, United States
So the problem went away for a while but is now back and more consistent. It seems to run fine on start up in the morning but once it gets a warm and you try to restart it the engine will start but not idle up. I've done the following. It has a S15G-2-N039-002 (91165-15800) ECM. I have tracked down all the I/O wires and they have tracked out fine. The engine intake manifold temp sensor, throttle position sensor, fuel lock off, FCSV and throttle body controller all are connected to the ECM.

What I have found is the throttle body valve gets about a 50% duty cycle PWM at 180HZ (about 4.6 - 5 VDC read on a DMM) as the engine cranks and then runs at a very low idle. Foot pedal input will not change this PWM nor is the FCSV chattering. I can hear a slight hum over the throttle body. If I remove the connector going to the throttle valve the hum goes away and the engine speed drops a little bit more. I have applied a variable DC power supply to the throttle valve and once I get over 4 volts the engine picks up speed and I can control the engine speed in the 4 to 5 Volts DC range. Higher the voltage equals higher engine speed. Once the engine speed gets high enough I can see a change in voltage from the ECM wires that should be going to the throttle valve. If I press the foot pedal it will increase the PWM signal as well. You can also feel a vibration in the FCSV valve too. It seems to me that the input voltage to the throttle valve is not high enough on start up to open the throttle enough to increase the engine speed high enough to think it is running. I looked at the PWM signal to the throttle valve while the valve was connected and I was getting a good 14.2V pk output which would lead me to believe the ECM is in good condition.

Is the throttle valve actuator going out? Are they repairable or can you open them up and clean/lube them back up? The throttle valve is a Barber Colmen 91165-05400

Thank you for any input.
  • Posted 3 Feb 2019 10:56
  • Reply by eaugie
  • Kansas, United States

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