Extreme Idle Knocking In The Morning

[caroholic]

Hi,

I have a 1992 1.8 carina sx. I have been having this problem for years now and could not find a solution yet. The car is octane/petrol run but was converted to run on CNG(compressed natural gas) as well. 99.9% of the cars in here are converted to CNG due to low fuel cost.

The problem is - When i first start the car in the morning (when the engine is cold) it knocks like hell as if everythging will fall apart. The idle RPM drops very very low, approximately around 200. I press on the gas pedal to rev up a bit and reduce the vibration and heat up the engine. After some time when the engine starts to heat up the vibration reduces significatly and stops completely when the engine is fully in it's normal operating temperature. Same thing happens if i start the car in octane mode, but the problem is somewhat less severs on octane as octane is much more powerful than CNG.

Everytime i went to a repair shop they cleaned up the throttle body and air filter or did engine tuning. But all failed to solve the issue. It is so annoying to wait for the engine to heat up every time i go out with the car.

Any suggestions what could be the problem? One more thing i should mention here; i keep the throttle position sensor unplugged because with this sensor connected to the ECU, the car would stall whenever brakes are applied fast.

Thanks,