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Q: Misfires while idling

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My car misfires while idling and at cruising speeds with steady rpms. It runs smooth when cold or warming up. As soon at it idles it will misfire. I've done a valve adjustment, new valve cover gasket, new CTS, TTS is good, speed sensor good, reference sensor good, cap and rotor are good, new plugs, new wires, all new vacuum lines, good thermo valve, AFM cleaned, verified, working, ICV cleaned, verified, working, no vacuum leaks at intake manifold or throttle body, throttle switch correct, MEPEH rebuilt fuel injectors installed, checked, cleaned all engine bay grounds, coil checks out, cold start injector not leaking, correct voltage at fuel injectors and correct resistance on injectors. The previous owner says there was a vacuum leak in rear main seal. I have never heard of this. My car doesn't leak any oil. Why does my car misfire?

A: Hello there, thank you for asking about you...

Hello there, thank you for asking about your 1986 BMW 635CSi. It sounds like you’ve checked all of the low hanging fruit. The previous owner may be right - some vehicles (only German ones come to mind) can develop a vacuum leak, lean condition, and misfire due to a worn rear main seal. It’s not a myth - a failed rear main seal actually causes an air leak in the engine. However, this usually happens when the seal is a one-piece plate design, like those found in late model Volkswagens and Audis. Your BMW uses a traditional lip seal. The only way to know for certain what is going on with your BMW is to have it inspected for misfires first hand by a professional, such as one from YourMechanic.

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