Smog check hydrocarbons



My 1983 Toyota Supra is due for another California smog check. My hydrocarbons at 15 mph have increased over the last 4 tests (8 years) and are are approaching the max allowed limit (130 ppm). What would cause the HC to increase and how can I get it down?


Usually, the presence of excess hydrocarbons indicates that there’s unburned fuel in the exhaust. New spark plugs might help, or maybe replacing the catalytic convertor if other things are also reading higher than before.

A 25-year-old car is still being smog checked?? Here in MD, that thing would be five years past the need for that.


If there’s no oxygen in the exhaust it’s running rich, and could be a bad oxygen sensor or leaking injector, fuel pressure regulator, etc. If there is oxygen in the exhaust it indicates a misfire, allowing unburned fuel and oxygen into the exhaust…


Being a Supra owner myself, I can tell you that the best thing to do is tune it up, and replace the O2 sensor with a Factory Toyota replacement. This will help keep the HC number in check. Also, if the numbers are still high, check the coil as stated in a lot of repair manuals to see if it is still in tolerance. A weak spark can bump up the numbers.


Another possible source of HC is engine oil. Make sure the PCV system is working properly. But the engine could just be getting worn out, allowing excess blow-by.

Those are great cars. If it’s in good shape, I’d try to do what’s necessary to keep it alive.


Make sure the passages to, through, and from the EGR valve are clean of carbon build-up. You want the EGR to flow freely to help lean the engine. Check that the voltage signal (or, resistance value) of the engine coolant temperature sensor (cts) is correct. A misreading cts can cause the engine computer to demand too much fuel. It can even keep the computer from going into computer control of the engine (closed loop).