Q: Q: Misfires while idling

asked by on

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?

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1,000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. Q: Misfires while idling

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: My Check Engine Light was on, so I tried the paperclip trick to get a code and the light went out. Tested the bulb and it works

Since I cannot display an image on here, I will describe it best as possible. When looking at the diagnostic connector, is is basically a rectangle with flared tabs on one side. Consider that side of the bottom when viewing...

Q: The engine ticking on a '95 Lexus LS 400

The first thing you'd want to do is get someone to check the valve lash--it may need the valves adjusted. Those things have pretty bullet-proof engines but they do have the valve adjustments on them that are little, stainless steel...

Q: Stalling tottering 50 mph

Hello there, thank you for asking about your 1991 Geo Storm. There may be an issue in the vehicle's ignition system. Hire an experienced technician, like one from YourMechanic, to inspect the distributor assembly for any faults. The ignition coil...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.