Fuel-air mixture may be out of adjustment.

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Dear Car Talk: My 1997 Mercedes E420 leads a very leisurely life, sitting in the garage for months before being called on for the occasional road trip. Lately, I’ve been experiencing a rough idle condition, where the engine rpm varies between 550 and 700, like a bad engine miss. Acceleration, driving around town or driving on the highway doesn’t create a miss, with the car performing as normal. No check engine light has ever appeared, although it does illuminate at startup, so I know it works. When I had an OBD scan, two codes came up. One code is an oxygen sensor, and the other a secondary air-system code. Can you provide any insight about how to get my baby running well again? — Martin

A: Well, if I had to take a wild stab at it, I’d say you need an oxygen sensor and some work on your secondary air system, Martin. How’s that for clairvoyance?

Actually, even though the scan identified those two problems, they may not be responsible for your rough idle. A rough idle is more likely to be related to the secondary air system, which is responsible for maintaining the right fuel-to-air mixture when the engine is cold.

But the rough idle could be caused by something else entirely. And since you drive the car so infrequently, it could be that your injectors are just gummed up. That can come from not driving it enough, and having the same tank of gas in there for six to eight years at a time.

So I’d start by trying a fuel-system cleaner. We use a product in the shop called 44K, which is made by BG. But if you can’t find that, try Chevron Techron, which is widely available. Use it for a few tanks of gas in a row and see what happens.

If that helps, then try driving the car more often and not filling the tank before you store it.

You’ll still have to take care of those emissions codes that came up during the scan before the next inspection.

Got a question about cars?
Contact Ray through the website cartalk.com.