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High HC, new Cat

How quickly will high HC damage a new catalytic converter? Back and forth between smog shop and carb shop, need to use the car in between. Also, any ideas in general? 88 Chevy Nova/Toyota Corolla. Thanks.

High HC is seldom the carb…It’s an ignition problem or a mechanical problem. (burned valve). Over-advanced timing can cause it too…

A carb problem will give you high CO…Unless it’s lean enough to cause misfire, then the CO will be VERY low.

To answer your question, YES, high HC will overheat the converter as it attempts to burn the unburned fuel…

You are missing details like miles on the car. Some hc stuff on this car is easy. You could have a bad bypass valve on the carb or a bad temp sensor on the engine. These are really tough systems because you still have a lot of vacuum controls mixed with poor electronics. You really need a specialist in this era. It did not last long but it really was the low point in fuel control. And you may still have valve issues, but that is not where I would start.

Car has 240K miles.

High HCs are caused by oil consumption, rich running, or both. A worn engine can contribute to high HCs and seeing as how it has almost a quarter million miles on it the first thing I would do is run both a dry and wet compression test on it. A low cylinder (or more than one) can cause incomplete combustion and high HCs.

If compression is fine and oil consumption is not an issue then things like spark plugs, plug wires, retarded ignition timing, or an internal carburetor fault should be considered.
Many cars of this era that used carburetors often relied on a handful of items such as thermovalves, vacuum switches, air bleeds, and so on to work in conjunction with the carb and a hiccup in any of it can throw the carb out of whack.

I haven’t messed with one of these in many years so the advice above is only general in nature.
Will take a look at a manual and try to refresh both brain cells… :wink:

In emissions terms, HC’s are unburned hydrocarbons. Key word, unburned…No combustion. Gasoline vapor somehow reached the tailpipe in an unburned condition.

CO=Carbon monoxide. Results from incomplete combustion. Too much fuel, not enough oxygen

NOx= Nitrous oxides. Combustion temperature too high. Nitrogen in the air combines with oxygen in the air. Suppressed by EGR valve and broken down by catalytic converter

The converter is designed to remove small quantities of these combustion by-products produced by properly running engines. If the HC, CO or NOx exceed normal limits, too much heat is generated by the reaction in the converter and it can destroy itself…

How high are the HC’s? Slightly rich can be tolerated for a short time. Very high, like watering the eyes near the tailpipe may burn out a cat in short order.

Google “How to pass an emissions test”. There’s a couple links there that have some useful info addressing your particular question about high HC’s.

First off: Make sure you engine maintenance is all up to date. New oil. New air filter. New Spark plugs, new cap and rotor, ignition timing verified, spark plug wires checked for cracks, all that, etc.

High HC’s are often caused by a problem w/the fuel/air ratio. Too much fuel and not enough air yields unburned HC’s because there wasn’t enough air for the remaining HC to burn with. And too little fuel and too much air can cause the mixture to not fire correctly, and can also result in unburned HC’s.

Vacuum hose, vacuum device leaks would be the first place to look. Any unmetered air entering the intake manifold. Clogged air filter would be next. Faulty PCV valve is a possibility. A sticking EGR valve is a remote possibilityk, but this usually results in very noticeable performance problems too. And carb problems are possible, especially if the cold-start choke system is malfunctioning. At 240K, it could easily be a valve or compression problem too.

Thanks, everyone, for the input. Today I went to the referee to request a cost repair waiver, which is a 2-year smog deferral in California. They performed a complete smog test, and all emissions were well within the allowable range. I failed on a broken hose on the vapor canister, and the timing, which had been set wrong by the smog mechanic. Neither of these had been detected in three official tests and several visits to technicians. But I did get the deferral. So I remain baffled by this whole process - many tests over two months, readings all over the map, HC 430 just three days ago (although it wasn’t an official test).