Can a catalytic converter cause an engine to burn oil?

November 3, 2023 10:13 AM

I have a 2016 Hyundai Elantra GT that is burning oil. I've taken it to mechanic and they say the engine needs to be replaced.

I bought the car used with a multi year / 100k warranty which is almost up due to time as it is only at about 75k miles. The company (CarMax) is denying the claim because they say the damage to the engine was caused by the catalytic converter, and the catalytic converter is not a covered part, and damage from a non covered part (catalytic converter) to a covered part (engine) is not covered. Does this make sense or are they trying to find a way out of paying for my engine?

posted by ridogi to Travel & Transportation (8 answers total)

Response by poster: My mechanic says it is the opposite, that the engine damaged the catalytic converter and that they are doing it to get out of the paying the claim.
posted by ridogi at 10:18 AM on November 3, 2023 [1 favorite]

Best answer: Your mechanic is right. There's no way a catalytic converter would cause an engine to burn oil: the gases go FROM the engine TO the cat converter, not the other way around. And yes, oil can damage the cat converter. CarMax are spinning you a line. Good luck
posted by anadem at 10:24 AM on November 3, 2023 [8 favorites]

Best answer: Hyundais of that era have a known issue where they eat oil. I have a 2016 Sonata and I have to top off the oil almost every time I get gas.

Some people have had some luck with free repairs, up to and including a replacement engine. I don’t know the particulars, so you’ll need to research that route and see if you have any options.
posted by sacrifix at 12:00 PM on November 3, 2023

Best answer: Generally speaking, cat got damaged by leaking oil into exhaust is far more likely than plugged cat causing damage to engine.

There are a lot of places in an engine that can leak oil and ALL that oil have to go somewhere, such as into the exhaust, and thus, to the catalytic converter.
posted by kschang at 6:00 PM on November 3, 2023

Best answer: It seems more likely that an engine consuming oil damaged the catalytic converter. The inorganic ingredients can clog the catalytic converter and the oil can accumulate on the internal surfaces and oxidize/combust there.

However, the notion that because the engine exhaust gas flows from the engine to the cat means a damaged cat couldn't damage the engine is not entirely correct.

There are sensors upstream and downstream of the catalytic converter that affect the air fuel mixture to the engine. It's unlikely but not inconceivable that a damaged catalytic converter could create conditions that would lead to engine damage (e.g. excessively lean combustion causing high engine temperature or peak pressure). However there would have been numerous check engine codes for weeks or months. You'd also probably notice power or fuel economy issues.

I think this is a Hyundai engine problem that caused cat damage. I'm mentioning it here because a mechanic should be able to read a clear fault code history and use that to dispute the cat --> engine direction of causality.
posted by KevCed at 10:43 AM on November 4, 2023

Many post-2010 Hyundai engine types have been prone to catastrophic failure. The company has been somewhat accommodating in replacing these cruddy engines, though it certainly helps the owner to be able to demonstrate that the recommended service (oil changes, etc) has been followed to the letter.
posted by Jessica Savitch's Coke Spoon at 3:48 PM on November 4, 2023

Response by poster: There was never a warning light. I never missed an oil change and have a record of all service work that I've had done. This isn't the warranty with Hyundai. It is a warranty through CarMax which is a used car dealership.
posted by ridogi at 6:04 AM on November 5, 2023

Response by poster: I never had power of fuel economy issues. It was running louder so I took it early for an oil change and was informed that it was very low on oil and it was either burning oil or leaking. I park in a driveway and have no history of visible oil on the driveway ever.
posted by ridogi at 6:24 AM on November 5, 2023