Q: I have a 2007 Acura MDX. It was purchased new, is serviced regularly and is never abused. It now has 53,000 miles on it. Everything I'm reading states the timing belt should be replaced at 105,000 miles (60,000 for severe usage). Age is not mentioned. The dealer is telling me it should be replaced because the car is 8 years old. It's a lot of money if it's not necessary. Your opinion please.


A: This question comes up a lot and everyone has a differing opinion. Some carmakers do specify a time interval along with the mileage, and you'd be coming up close with most of them. My concern is you have a magnificent engine that could incur significant and expensive damage if the timing belt was to break. This is known as an interference engine, where the valves will collide with the pistons if the crankshaft and camshaft lose synchronization due to belt breakage. In order to deliver high performance, many modern engines are designed this way.

It's logical that most timing belt wear occurs due to use, but environmental conditions such as airborne ozone and oil contamination could be taking their toll as well. Methodically inspecting the belt's inner cogs for cracks or deterioration makes sense, but isn't easy to do on many engines without some disassembly. If it were my vehicle, I'd renew the timing belt at 60,000 miles, along with the water pump and belt tensioner. These additional parts are recommended as they could derail or destroy the belt if one of them fails.

Q: How many miles should a catalytic converter and two sensors last? I have a four-cylinder Honda Accord with 161,000 miles. What's best: a new or replacement converter?

—Bob Seashore

A: In a perfect world these parts would last the life of the vehicle. Catalytic converters can gradually loose efficiency due to contamination and excessive heat. Oxygen sensors can also become biased or sleepy, again from contamination, and occasionally suffer from internal heater failure. Phosphorus and zinc deposits due to oil usage and silicone contamination as a result of coolant leakage — from head gasket leakage, a cracked cylinder heard or the wrong engine sealants, can slowly poison both the cat and oxygen sensors. Newer motor oils and some coolants have greatly reduced levels of these substances. A mode six scan tool reading (challenging to interpret) can indicate how close or far these parts are from reaching the failure threshold.

Major failure of a catalytic converter can occur due to engine misfiring, an excessively rich air-fuel mixture, impact damage and possibly a cold quench from a really deep puddle. The normally high temperature within the catalyst can soar with unburned air and fuel passing through, resulting in melting and breakage of the honeycomb substrate. Besides spoiling emissions treatment, this, along with impact or quench damage can cause a clogging effect, dramatically reducing engine performance. Greater heat caused by a rich air-fuel mixture fuses pores in the converter's active surface, reducing oxygen storage capacity. Assuming upstream and downstream oxygen sensors are functioning correctly, the dreaded P0420 diagnostic trouble code usually makes the right call that the cat has seen better life. If one plans to keep a car for the long term, I'd opt for the greater durability, better performance and longer warranty of an OEM cat instead of a less expensive aftermarket replacement.



Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood(at)earthlink.net; he cannot make personal replies.


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