I have a ck 3500 with a 5.7 litre engine running on a 300a propane system. I have recently failed 4 etests. I've changed the plugs to Bosch Platinum 2, new cap, rotor, plug wires, egr valve and solenoid, map sensor, tps, iac, catalytic, air filter. The engine is still putting out high hydrocarbons when drive at an idle. It seems to have a misfire that I beleive is causing the hydrocarbons. Any idea what it could be?