I have a 1994 Saab 900s that every two years fails smog. I would like to see if there is any additives that will help me, or put me in the best position when getting it tested. Thanks for your help.
Methanol or denatured Ethanol found in most hardware stores, or an additive found at Napa stores called Fuelon. Schedule your test so you can go in when the car is running HOT (maybe 30 mintues drive time just prior) I've heard that an oil additive may help too since the PCV allows oil pan gasses in and can cause a richer reading, and old thinned oil can get past seals and rings. If you're over due for an oil change do it. Contaminated oils are high in Hydrocarbons and will present a rich mixture to the engine chambers. Good luck.
No 'additives' will help you. What has it failed on previously? Co2, HC or NOX? Elevated levels in each of these categories will have different causes. Is your O2 sensor working properly? Is the EGR working properly? When was the last time you had a tune up and a fuel injector flush? Is the catalytic converter working properly? Is the vehicle burning oil? There are too many possibilities to go though. You need to specify. -4th year automotive apprentice in Canada and a certified drive clean inspector for the Province of Ontario.
If you have a check engine light on, that will have to be taken care of or you'll never pass. If there are no light and no codes then all you can do is take care of the basics. Throttle body service/fuel injection flush if hasn't been done in the last 25,000 miles, air filter if more then a year old, oil change, check PCV and plugs, wires, cap and rotor have to be good. I always like to fill the last 2 tanks of gas prior to test with the highest grade and take it for a good long freeway run. good luck
