Rough idle

    Hi,
    I have a problem with rough idle, It doesnt do it all the time but sometimes when i sit for a while i feel the car start to shake as if the revs were dropping to stall and then it corrects itself, It also sometimes does it when coming to a stop.Also i noticed while driving from any kind of speed and taking my foot off the gas the revs go down as they should but sometimes they surge up and down by about 500rpm which makes the car rock very slightly.Any ideas guys

    Regards
    Derek

    Hunting to find the correct idle speed is usually (nearly always) due to a fuel/air mixture problem. This can be due to a popped PCV valve, inlet manifold gasket leaks or a MAF that fails to measure the air mass correctly.

    The usual plan is to spray brake cleaner around the inlet manifold where it meets the heads, where it meets the PCV valve and where it meets the throttle body. A change of RPM indicates a leak, and can pinpoint which gasket is the problem.

    The PCV valve has a bleed hole at the bottom of the diaphragm housing, you can put your finger over the hole to check if the diaphragm is split. If it is, the revs will change. Once all of the gaskets and the PCV valve has been eliminated it leaves the MAF as the most likely candidate.

    Hi,
    Thanks for your replys, Dont want to sound completely stupid but where can i find the pcv? And is the inlet manifold easy to get to so i can test them.Just cleaned the maf sensor with crc cleaner yesterday and made no real difference so perhaps is goosed although i will try the other two before i go down that road.

    Cheers

    Here's the PCV and how to get to it - this routine is on an E32 but the E31 is similar:

    http://www.meeknet.co.uk/E32/PCV/Index.htm

    And a few pictures of the E31 suspect parts from Gareth on the UK Forum











    Derek,


    You did not say if you had a V8 or a V12. I just had the same issue on a V12, and this is what we ended up doing on it. We finally got it solved when we changed the plug wires and the Crankshaft Position Sensors(CPS) (#5 below).


    1. Changed the O2 sensors (made no difference)
    2. Rebuilt DK motors (one was sticky - made no difference)
    3. Changed the Distributor caps and Rotors (made no difference)
    4. Checked MAFs - switched side to side (made no difference) - switched back to original sides, but cleaned inside with MAF cleaner
    5. Changed plug wires and sensors, changed both CPS. IT WAS FIXED!


    There is another friend here in the US chasing the same problem, and has done all above except the CPS. He is going to do this this weekend and will see.


    Good Luck.

    BMW 850 CSi
    BMW 8er.org Clubs International (B8CI) Board Representative, USA and North America
    Editor, E31 NEWS International E31 Newsletter



    Hi,
    Sorry its a 4.4 V8, i changed the O2 sensors, Cleaned maf and have ordered a CPS (only one on the 4.4 V8?) as i had a problem years ago which also was not happening all the time with my E39 535i and it fixed it. After that is fitted i will go down Timms route, Was going to do that this weekend but its monsoon time up here in Glasgow. Will let u know if the CPS helps.Just to add to the headache i hit a pothole so hard today that i now have a clunk when i turn right:banghead:


    Thanks guys

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