More than likely the problem your vehicle is facing is a defective head gasket. A working head gasket should not allow seepage of exhaust into the radiator fluid (water) chambers within the engine; used to cool the engine down.
In the said state, your vehicle is more than likely producing high NOx and will do so during the smog inspection. High NOx is created when combustion temperatures rise above 2500f. High NOx will cause a smog check failure.
The reason your temp gauge drops to normal off idle is because once the water starts circulating (caused by the water pump which is driven by the engine’s crankshaft), more water is able through flow through the radiator vanes; effectively cooling the engine and dropping the temperature gauge. During the smog check, if your vehicle must be driven on a dynamometer (1999 & older gasoline powered) it will not receive the same amount of air flow through the radiator as it would if being driven on the road. The smog station will more than likely place a fan toward your vehicle’s radiator; however the air flow might not be adequate enough to lower engine temperatures and reduce the formation of NOx.
Furthermore, should a smog technician detect the failure of a major engine component which is certain to increase emissions, the technician must mark the engine component as defective and fail the vehicle.
posted by SmogTips Support
