2006 Z4M Roadster @ ~ 62,xxx miles
Went for a drive tonight with the wife. Nice cool evening in Florida (think Spring everywhere else). Car was warmed up from a short commute home. We drove about 3-4 miles, then sitting at an intersection waiting to do a U-Turn.
I feel the steering wheel vibrate quickly once and engine sounds like the RPMs dropped almost to a stall (we're at idle already). I stare at the tach and grip the wheel to see if it happens again. RPMs visibly drop and vibration hits again. Wait...then a third time and wife can also notice it's happening. It's very brief, but it's noticeable.
I pulled into the lot across from us and it's immediately apparent there's a loud tick/knock/whatever coming from engine. I pull into the lot and pop the hood (engine still running). The sound is very apparent. I close hood and rush home as slow as possible (rushing slowly
).
I drove the 2 miles home (more direct route back), never going over 2500 RPMs and trying to keep it under 2k. Clutch in as much as possible, just rolling in idle whereever possible. It's obvious the sound is there whenever a car passes us or we go by a wall/building (something for sound to bounce off).
Got home and shut it off immediately. After 5 minutes I realize I should record sound for you guys and my mechanic to hear. I pop the hood and record as I start it. Sounds 100% normal. Rev it a few times, all normal.
20 minutes later, I start it again so wife can confirm (she ran out as soon as we got home). No noise. I revved into the 5k range and I can't make the sound come back.
Thoughts? My fear is Vanos bolts, of course. But, I just had the Vanos bolts replaced 5 weeks ago. I've driven about 200 miles since then (maybe 300, definitely not much). So only way that's an issue is if they didn't do it or did it wrong. I went to a reputable shop in town, not the dealer, so I'm quite confident in their work (wouldn't have asked them to do it otherwise).
i'm pretty apprehensive to just "drive it until it happens again", but what else do I do?
Last week or so, I've noticed a 'clunk' or 'pop' going in/out of driveway. At first I feared the engine bolts, but they look ok (I have replacements, but they haven't gone in yet - I can confirm I don't have the 10.9 bolts in now). From reading, it seems this sound may be the strut towers need torqueing. Been cold here (for Florida) and I've noticed a lot more creeks in the roadster, I assume due to the cold. Probably unrelated, but just mentioning it.
While the bolts were being replaced, I had them shim the valves. There were a lot of valves way out of spec. Car sounds, and feels, much better since they did the work. Considerable valve 'ticking' went away after they did the work.
I have not replaced plugs. I don't know if/when an inspection was done on car or if the plugs were replaced at that interval. For all I know, they're original.
Only other data point is that I filled up gas yesterday. Was down to 2 miles range on the computer, so I got a very full tank. 93 octane at one of three Chevrons I frequent (this car sees a lot of the same gas). It's always possible I got 'bad gas', I suppose, though I'm not sure what that would sound like.
Any thoughts/suggestions are welcome. I don't know what else to do than try to reproduce it, but I feel that bring inherent risk of killing it.
-Ian