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%.