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Car Care & Repair Tips - October 17, 2003

What Your Mechanic Doesn't Want You to Know

Reader Question: My car doesn't have the power it once had, and I was told it could be due to a problem with the catalytic converter. How can I tell if there is a problem with the converter, and is there a "quick fix" that I can do?

Dear concerned car owner,

Lack of power can be caused by a few things, and a restriction in the catalytic converter or in the rest of the exhaust system can be one of them. A restriction in the exhaust system will not allow the exhaust gases to leave the engine, thus suffocating the engine in its own by-products. Just as the human body needs to exhale before it can take another breath, an internal combustion engine needs to remove burned and unburned fuel. The combustion engine "breathes" by pushing the exhaust gas out of the engine via the exhaust manifold.

The exhaust manifold is attached to the side of the engine. Valves inside the engine open to allow the hot exhaust to escape through the manifold, then the valves close to seal the engine so that it can perform the combustion process again. So the engine is constantly taking in fresh, clean air and fuel to start the combustion process, then expelling the remnants of this process as exhaust gas.

Now, what could go wrong with the exhaust system that would cause a lack of power complaint? A restriction that is not allowing the exhaust gas to escape the engine will also not allow the fresh air and fuel to enter the engine to be combusted. An example of this scenario is trying to run a 100-yard dash with your mouth taped shut. Walking around with tape on your mouth is not that much of a problem, but when you exert your body to do something more strenuous, you need to inhale larger amounts of air at a greater frequency. If your body cannot exhale the spent air from your lungs, it cannot take in a new supply.

The inside of a catalytic converter is made up of a honeycomb-type of material that can become restricted with soot from the engine exhaust. In some cases the material inside the converter can break apart and actually cause it to restrict itself. When this happens, you can usually hear a rattling noise coming from under the vehicle when you accelerate. I actually think it sounds like popcorn popping.

If the engine is performing correctly, there really should not be much soot or debris entering the exhaust system. An engine that is running rich (too much fuel) will produce visible black smoke and soot from the tailpipe. This soot will eventually plug up the honeycomb material inside the converter and cause a restriction, and this restriction will cause the lack of power complaint you are having. In this instance, the converter is a symptom of a poorly running engine, and is not the cause of the lack of power.

What can cause the engine to run rich and eventually plug up the converter or exhaust system? It could be an engine that is misfiring due to a bad spark plug, spark plug wire, or other tune up related item. The problem might also be a faulty engine control sensor or emissions control sensor that is not properly regulating the engine fuel and air ratio. Or you may have an engine that is "out of time" or having an ignition timing problem.

So now that we've covered the possible root causes of a restriction in the exhaust system, how can you tell if the converter or exhaust system is restricted? There are special testing tools that your mechanic can use to test the back pressure of the exhaust system, but there is also a simple test you can do at home to check for a restriction problem. With the vehicle in park and the engine running, quickly mash down and release the gas pedal to "goose it" while someone is standing next to the exhaust tailpipe. Caution: exhaust gases will be very hot, and will burn you if you get too close.

If the engine is running properly and a restriction in the exhaust is not present, the engine will accelerate quickly without having to labor. There should be a drastic difference in the amount of exhaust coming from the tailpipe when you allow the engine to idle as compared to when you "goose" the gas pedal. There should also be no visible signs of soot or black smoke coming from the tailpipe, or at least not a significant amount. If there is a restriction, you won't notice the drastic difference in exhaust volume, and you may also detect soot, black smoke, and even actual debris or pieces of the catalytic converter coming out of the tail pipe. You will also notice that the engine has to really labor or struggle to accommodate the request for increased performance.

What should you do if a restriction is found? Take the vehicle to your mechanic and have him determine if the engine is running too rich. If the engine is found to be too rich, the problem must be corrected first before you spend the money on a replacement converter. If you don't fix the root problem, the engine will just stop up the new converter. Once the engine is running properly again and the restriction has been removed, then the problem should be corrected.

Most muffler shops offer aftermarket replacement converters at a fraction of the price of a new one from the dealership. Talk to your mechanic to see what he recommends for your make and model of car.


Austin C Davis

Austin C Davis

Interested in an e-book about everything your mechanic doesn't want you to know? Sound advice from Austin Davis. Click Here!

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