rough idle, bad noises

[julians999]

Hi All,

My car has recently started making some bad sounding noises upon startup from cold. When you start it up, you get a banging/knocking noise for the first 30-60 seconds, then it quietens down and seems fine. During this period it seems less willing to rev.

It only does it when its been left for 8-12 hours, if you start it up after say 3-4 hours of being sat it starts fine with no noises.

There are no warning/lights on the dashboard, and it seems to drive ok once you get past this stage.

See end of post for a you tube video, you'll have to turn the volume up because my phone didnt capture the sound very well.

I've got a few ideas of what it could be , but would appreciate any ideas you may have.

Car is a 2008 m3 with 37000miles, and is totally stock, no modifications at all.

[Rapture]

Sounds like the bottom end is done, not uncommon for 2008 models

[julians999]

I suppose it could be bottom end, but generally speaking problems with the bottom end get worse as the engine warms up not better I thought?

Anyone got any advances on the bottom end guess?

[M3PO]

Oh wow, that sounds bad. How does it run otherwise? You say that it starts and idles fine when warm but does it run fine when driving around?

[julians999]

yeah, it runs and sounds fine once you've got the first 30-60 seconds out of the way, pulls hard to the rev limit, sounds smooth with no issues. You wouldnt know there was a problem if you hadnt heard it start when stone cold.

Its going back to the dealer tomorrow, so will see what they say, its under warranty for 1 more month.

My gut feel is that its caused by a failed hydraulic lifter, but we'll see.

[julians999]

ok, on the way to the dealer the car started making the knocking noise when in 6th gear at lowish revs (~1500rpm) under a heavy throttle.

So I now think its the main bearings that are shot.

Will update in due course.

[mdosu]

good luck, let us know the diagnosis, lucky for you regarding warranty timeline, dodged a bullet there.

[julians999]

ok , dealer thinks the engine is fine.

They think the knocking noise is being caused by the exhaust which has failed internally in the back box, along with some gaskets.

i remain to be convinced, but I'll be very happy if its not the engine, as it will restore my faith that the s65 isnt a ticking timebomb.

[M3PO]

It *could* be from the exhaust. Were you able to tell roughly where it was coming from (front, back, or middle of the car)? When I first installed my Fabspeed x-pipe and HFC, they were rubbing on the trans cross member and it sounded kinda similar. Maybe your header gaskets have failed allowing the exhaust to rattle around until it heats up and seals.

[julians999]

its pretty hard to tell, but it sounds to be coming from the engine area, and somewhere around the passenger side on a right hand drive car, so driver side for most of you on here, underneath the air intake on the bonnet (hood)

[julians999]

So just to bring this thread to a close, I picked the car up yesterday, it absolutely flies now, I must not have noticed it gradually losing power.

Started it this morning from stone cold and no knocking noise, so it looks like the problem was the exhaust. Will update again if the problem comes back.

For info for anyone else who might experience similar, the detailed job sheet from the dealer states that the passenger side (drivers side in US) downpipe gasket had failed , and that he baffles in the back box had collapsed.

It really did sound like it was something terminal in the engine ( as you can tell from the you tube clips)

[Mike van D]

Glad it worked out for ya! Enjoy the Beast again.