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1994 Mazda MX-3 - Help request - smog

What is the best way to solve High HC and NOX problem for smog. Car runs great but has 160K and is a V6. Has new O2 Sensors, new VLF and new Plugs. Thinking CRC Guaranted to Pass, Water De-Carb, or C4 (HaHa)

High NOx readings are a result of elevated combustion temperatures.


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You can’t get a mechanic in a can (or bottle)

C4 would definitely be a way to not have to worry about this cars smog tests ever again… :smiley:

Many things come in a can that do not require a wrench. One of these things are various decarbonization chemicals however sometimes water will do the trick as I understand it. Not being familiar with either was the motivation factor for part of the question. thanx

Thanks for all the answers so I guess I will go somewhere else for the solution.

Marvel Mystery Oil in the fuel may decarbonize the combustion chambers. It stopped the pinging in my 1984 Cavalier a few decades back. Supposedly it softens the carbon, which then gets expelled out the exhaust. I used it at the ratio recommended on the bottle. I don’t remember how many tankfuls before the pinging stopped, but it did and I kept using the stuff. Some time later I had the head at a machine shop and they commented on how clean it was compared to what they usually see at that mileage.

Good luck and please let us know what you find out.

Carboned up combustion Chambers, while common years ago, are fairly rare now. The only time I’ve seen one work was years ago when I dumped a can of Marvel mystery oil into the crankcase of a Chevy V8 whose owner didn’t like to spend money on oil changes and sludged up the hydraulic valve lifters. They are cheap to try, so give it a shot. Good luck

Post a picture of the smog results printout, and we’ll try to help you

There are smog techs among the regulars, and the more information you give us . . . such as that picture I requested . . . the better chance you’ll some good advice

Is the Check Engine light on?

Good comments above, espcially about CEL, diagnostic codes, and posting the emissions measurements. Beyond that, for high HC, look at my old thread on HC emissions problems for Corolla. Googling ‘Corolla HC emissions’ should find a car talk thread about the 4AFE engine near the top of the list. For high nox, there’s a pretty good recent thread (within the past 2 months) about a Scion here as I recall. Might provide some clues anyway. You’ve already replaced the engine air filter, right? Fresh oil and oil filter before testing?