dvno,
actually you didn't solve the real problem, all you did was compensate for the air/fuel imbalance being created by another issue.
That solenoid valve you disconnected is the Fuel control solenoid valve and it is supposed to rattle, it pulses to help regulate the lpg fuel flow.
In most cases when your situation happens and it is a air/fuel mixture issue it causes a rich environment which saturates the O2 sensor and can cause the MIL light to come on after idling for a short period.
The Dwell setting in the mixer is generally what causes this, this dwell setting is basically an air/fuel mixture screw behind a small freeze plug in the side of the mixer that's used to regulate the fuel control solenoid valve.
Several things can cause the mixer to get out of wack throwing the dwell setting off, a dirty diaphragm, dirty mixer/throttle plate, worn spark plugs, poor air flow due to dirty air filter, a vac leak in the vac hoses, especially the vac hose to the ecm (if it has an older model ecm with the internal map sensor.
The best way to set the dwell is to use the mfcadiag service tool and set the dwell setting to the recomended setting of 38 to 40%.

After struggling with the dash light turning on and the unit going into limp mode, we changed the following items:
LPG regulator
Carburetor Diaphragm
O2 Sensor
Temperature sensor
Spark plugs, wires, rotor, and dist.cap.
Air filter
And after all this unit was still doing the same, after about ten minutes idling the light on the dash would turn on an no more throttle, unit would drive really slow.
After paying attention timing the light and idle we found out that the little solenoid valve next to the regulator with the vacuum lines would start making noises and the light would turn on.
We decided to unplug the ground wire (white wire) and tested again, unit started working perfectly fine, we left the unit idling for 40 minutes no more light, no more limping.
We test drove unit for about two hours and worked fine.
So getting rid off the solenoid would solve the problem.
I hope this is helpful, too much headaches, so I just wanted to share.

Check Engine light for GC25K. Any Advise?

-The Mixer has been replaced with new Unit. (New Gaskets/diaphragm)
-New Spark Plugs
-New Universal 02 Sensor
-New Timing belt (timing is spot on)/ was off by one gear before new belt was installed.
-New Diaphragm on top of Carburetor/Throttle body.
-Pedal Sensor is verified Good and in range.
-Engine Temp sensor appears to be working fine as Temp gauge appears to warm up and cool as thermostat opens to allow cooling fluid to cool engine.
-flywheel sensor appears to be working since technician could read RPMs when computer was attached.
-All vacuum lines appear to be in excellent condition with no leaks or cracks.

Technician with computer attached could not seem to get the dwell to adjust properly via the throttle body adjustment screw. Dwell drops to zero when idling for a long time causing the check engine light to illuminate and motor to go into limp mode. If you full throttle the engine, the dwell pegs out at 100 and after 1 minute 30 seconds, the check engine light comes on and engine goes into limp mode.

The computer does not give a flashing code. it appears to be the older computer that has a vacuum line attached.

To summarize the issue, Check engine light comes on if unit idles for to long or is revved for to long. When the light comes on Engine goes into limp mode. You can turn the key off and back on again quickly, and this seems to delete the check engine light until it comes back on again due to the idle or high revved motor condition.

What are we missing/overlooking?

there are a few things that can make the warning light illuminate
i made a post on another thread here about this light and how it works.

begin quote

there is no specific code chart for these systems like newer systems.
they do display warning or diagnostic codes by blinking the 'check engine' light.
When the key switch is turned on, the check engine light is illuminated to show that the lamp is operational. If the engine is not started, the light will turn off after 20 seconds. If error codes are detected (RPM=0), the light will begin to blink codes sequentially after an additional 15 seconds.

here are how they work:
key on
engine not running (RPM=0)

- lamp turns off after 30 seconds - no error codes
- after 45 seconds, lamp flashes - shows error codes
- lamp off - normal operation
- lamp blinks slowly - charging system problem
- lamp blinks quickly - overheat/over speed
- lamp on steady - other error detected

System Faults
- 3 flashes - Fuel System Error
- 4 flashes - Pedal Sensor open
- 6 flashes - Engine Temperature Sensor
- 8 flashes - Battery Voltage
- 9 flashes - TACH. (rpm input)

this is how it is explained in the check engine light blink faults in the troubleshooting manual.
There is no way to force it to blink codes, it is all handled in the ecm and if a fault is present it will display the code by blinking the light or staying on solid.

If you want more definitive diagnostic info then you may need to get the LS20/S15G software program and see what else it may show you.

i would first try removing the mixer cover and pulling the diaphragm and cleaning it and the mixer throat out, if it is dirty this can cause problems with starting and running.
- if the pedal sensor is out of range that can cause limp mode when engine is running but usually does not keep the engine from starting and running.
- if the 02 sensor is faulty the engine should start but when it senses a problem with it the engine will go into limp mode, but this does not keep engine from starting and running.
- if there is a problem in the throttle body this might cause the symptoms you are having, check the harness at the plug and where the harness enters the body.
- if the engine temp sensor is faulty one of 2 things can happen depending on the level of temp difference, it will either flash a code and go into limp mode, or it will flash, go into limp mode and then shut down after 45 seconds.
- if it detects a flywheel sensor (RPM TACH) fault the check engine light is illuminated, engine enters limp mode. If there is no signal from the flywheel sensor the fuel lockoff will not operate and engine will not start and run.

hope this helps

end quote

Having the same issue on a GC15k. any resolution?