Mercedes CLK Check Engine Light and Idle Problems

I had similar issues with my Mercedes CLK. The check engine light and rough idle suggest a possible air leak in the secondary air injection system. Check the hoses connected to the air pump and air valve, especially the one going to the manifold. Cracks or loose connections can cause idle problems and trigger the light. Have you inspected the hoses for damage? A smoke test can help find the leak.

Thanks, that's helpful. I'm not sure I can do a smoke test myself. Where are the air pump and air valve located? I want to check the hose connection, but I don't want to mess with the wrong thing. Is it easy to spot under the hood?

The air pump is usually near the front of the engine bay, often on the side. The air valve connects to the exhaust manifold. The hose runs between them. It's usually a black rubber hose. Look for cracks or disconnections. If unsure, a mechanic can diagnose it accurately. A defective hose connection is likely, and a replacement should be around 175€, including labor.

Thanks a lot for the advice. I took it to the shop and you were right. It was the hose connection from the pump to the air valve. They replaced it, and the check engine light is gone. Runs like new. Really appreciate your help!