I have an old 1994 Volvo 940, non turbo. Bosch LH 2.4 fuel management. A while back I started getting a check engine light and codes for a lean mixture. Only symptoms were bad, but not terrible, fuel milege and a high idle. The codes would return pretty quickly.

Last weekend I tore down the entire intake side of the engine and thorougly inspected, cleaned and reassembled everything (lots of gunk and oil buildup). The old intake gasket was in a bad shape so I thought I had found the issue. Idle returned to normal after replacing the idle control valve.

This time it took a bit longer (about 500 miles) before the same codes returned. Car still runs great, I haven’t fueled up yet so don’t know if there is any improvement there. I can’t see, feel or hear any exhaust leaks whatsoever.

My last few ideas:

• Bad O2 sensor (could be the factory one for all I know)?
• Bad coolant temp sensor, it would cause it to run rich when hot but I can’t see that causing a lean code?
• Bad MAF? No MAF codes or other symptoms though
• Time to refresh plugs, rotor and cap? But again, the engine runs perfectly fine?
• Fuel injection related somehow?