Aftermarket O2 sensor not likely at fault

Hey there, time traveller!
This article was published 14/08/2009 (6233 days ago), so information in it may no longer be current.

QUESTION: I have 2000 Toyota Camry with 190,000 kilometres. Every year, I replace the primary O2 sensor. Every time, the check engine light comes on, my mechanic connects the computer and this sensor is faulty. I buy the aftermarket part because it’s cheaper than dealer’s price, about half. I was told that if I buy the Toyota original part, it wouldn’t happen. Please advise.

Paul

ANSWER: Original equipment replacement parts are designed by the manufacturer to be exactly the same or better than the original part on your car. Aftermarket parts are sometimes more generic, in that one part number may fit several applications. However, I haven’t run into your problem with repeated oxygen sensor failure with aftermarket ones.

The oxygen sensor can stop working properly because it becomes contaminated by carbon, engine coolant or silicone. Carbon comes from burned oil and rich fuel mixtures. If your engine is using oil, this could be the problem. A small head-gasket leak into the combustion chamber can allow coolant into the exhaust and contaminate the sensor. Finally, if you have used non-automotive silicone as an engine gasket, the curing silicone fumes will be pulled through the crankcase ventilation system and contaminate the sensor. It can take years for the silicone to cure completely.

I doubt that the problem is with the oxygen sensors, although you could try a different brand. Instead, look for other faults that would make the engine run improperly, such as a vacuum leak, dripping fuel injector or a leak in the exhaust system before the oxygen sensor.

 

QUESTION: I have a 2001 Chevrolet Malibu with only 82,000 km. Since last winter, it makes some noise and the sound is like a broken fan belt. It is much noisier when the car is first started, less noisy after warm-up. I took it to the dealer and the mechanic said the car is OK and the noise is from the engine due to the piston overlap. He said that if I don’t mind the noise it is safe to drive the car. Otherwise, I have to replace the engine and the cost is about $4,000.

I talked to two other car mechanics and they both said the same thing. They both said my car is in good shape and I should keep it for couple of years because if I trade for a new car I won’t get much on it and I use it less than 10,000 km per year. I would like to have your opinion about my car. Do you think it is a good idea for me to keep it for a couple of years? What is piston overlap and what causes it? Is is true a lot of GM cars from 1999 to 2004 have the same problem of piston overlap?

Peter

ANSWER: I believe the "piston overlap" you are referring to is really "piston slap." Overlap generally refers to valve timing, not pistons, and overlap doesn’t cause any noises. Piston slap can cause a knocking or deep ticking sound in the engine, usually when the engine is cold.

The slap or knock is caused by clearance between the piston and cylinder wall. When the cylinder fires, the piston is forced against the side of the cylinder wall by the pressure of combustion and the angle of the connecting rod. We hear this as a knock. As the engine warms up, the piston expands and there is less clearance to the cylinder wall, so the noise disappears.

Engines built in the past decade from many manufacturers, not just GM, are more likely to have piston slap because of smaller and shorter piston skirts and different design pistons. Although these designs reduce internal friction, they are more likely to make noise. If the noise is there cold but disappears within a minute or two of running (GM allows up to five minutes of warm up for warranty purposes), then there isn’t any problem. If the noise is there continually or under load, then the piston or pistons need replacing.

You have had three mechanics say the car is in good shape, so I would defer to their opinion, as there is much more to a car than just the engine. If the noise disappears as the engine warms up, I would likely keep the car. Just be sure to let the engine warm up before putting high loads on it.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net