New Slapping Noise

[113gray]

Recently noticed a new slapping noise for the first time at cold idle. Goes away on raising RPM's, after warm-up & on pushing clutch in. Located somewhere in engine compartment, but can't localise. (However, my hearing not good. Asked the Mrs. to listen & she confirms above, can't localise either.) Suspect a clutch or tranny problem because it goes away with a disengaged clutch. Any ideas as to what to do with it now? Car runs, stops, shifts fine. TIA.    -JP- '66 230SL w/ 5 spd ZF tranny

[tuultyme]

If the sound has a metalic clatter check the exhaust maniford bolts for looseness when the engine is cold.  Mine keep working loose so I have to periodically tighten them up.  You might also check the exhaust seal rings at the front pipe connection.

Bruce, Green 1970 280SL.

[113gray]

Thanks for reply Bruce. Will check it out. Not sure why clutch disengagement would make that noise disappear however.   -JP-

[George Davis]

JP, it might be piston slap, due to a worn piston or cyliner.  It goes away as the piston warms up and expands.  Pressing the clutch moves the crankshaft forward a little bit, this may be enough to reduce/eliminate the slap, too.

I don't think a bit of piston slap is a terrible thing, unless it continues when the engine is warmed up.

It may be possible to check this out by pulling the plug wires one at a time while the engine idles.  If it's piston slap, the noise should decrease or disappear when the plug wire is pulled on the affected cylinder.  If you do this, avoid getting shocked by insulating your hand well, or using insulated plug boot pliers.  Ignition system shocks are nasty.

Hoping I'm wrong, good luck!

George Davis
'69 280 SL Euro manual

[Cees Klumper]

Bruce - on the exhaust problem: my exhaust manifold bolts also keep working themselves loose. One thing you might want to give a try is to install two or possibly even three gaskets per pipe (they are pretty thin rings and adding one or two may tighten up things) and, of course, fitting all new bolts and locking nuts.
I think my problem is in an improper fit between the downpipes and the manifold, so I will be replacing the downpipes when I replace the exhaust system (going to stainless steel) in a few months.

[Naj ✝︎]

JP
I didn't want to be the bearer of bad news but George could be right!
I had a similar piston slap problem on a 10 month old W124 where the block had been bored out to its max. tolerence. Luckily, it was replaced under warranty.
naj

naj
'Kloines Scheisserle'
65 230SL
68 280SL

[113gray]

Gentlemen: Thanks for the replies. Nobody seems to think the noise is transmission related which is the most worrisome possibility to me because of hard to find parts. An engine rebuild would be easier to manage if it comes to that.    -JP-

[n/a]

Oh, it's clearly your ZF. You need to take it out and send it to me right away. Just kidding, I'd love to have a 5 speed.

I just recently had my clutch replaced. When I was sitting at a stop light, I noticed a strange noise. It went away when I engaged the clutch. Turns out, the clutch pedal or linkage or something needed to be adjusted. There was not enough play and the clutch was just barely rubbing when sitting in neutral. Don't know if this could be your problem, but the fix seems pretty easy. Would be worth eliminating as a potential problem.

There is nothing I hate more than strange sounds you can't easily put your finger, or ear, on!

Shawn Rock
Philly, PA USA
1968 280 SL 4 speed

[113gray]

Hi Shawn-Thanks for the response. Be great if that were the problem! You nearly gave me a heart attack reading your 1st comment though! Regards-    -JP-

[Richard Madison]

James:
To help locate the sound, maybe one of those engine stethoscopes will help...they sell fancy ones with rubber ear thingies and a long thin metal sound probe...or you could try what Joe Alexander used at the Ohio tech session, a thin 2 foot long piece of pipe; put one end near your ear (carefully) and put the other end on the suspected engine part to amplify and pinpoint the sound.

We offered to buy Joe's pipe, but he wouldn't sell!  )
Richard M