Classic cars forum & vehicle restoration.

[brinjoy@hotmail.com]

Hi
My 2.5 has developed a knock on cold startup. Once warm the noise goes away however it can be heard slightly under acceleration. Havn,t had the car that long so I am new to Daimlers. Have been told that they do suffer a bit from piston slap? Or could it be a gudgeon? Someone has suggested a Tappet?
Has anybody any clues on this.
Thanks
Brian R

[P3steve]

With out hearing it its only a guess but it dose sound piston related rather than crank main bearings, if it was the main bearings the rumble wouldnt go away and would be there every time you accelerated, I wouldnt have thought the tappets as there a simple adjustment type not hydraulic so would be there all the time too not just on start up (though there easy to get at on the V8 250 so give it a go first as theres no harm done in trying) It dose sound more like its piston related either slap, gudgeon or more likely conrod bearing big end. Sorry I've not been much help but as I say with out hearing it!
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If the world didn't suck we'd all fall off

[baconsdozen]

With some engines piston slap can be loud and another of the same make with pretty much the same clearances can be relatively quiet.I had a v4 engine that was very loud when cold but the noise virtually went away when hot,on stripping the engine the clearance in the bores was still just withing accepted limits but fitting a secondhand set of pistons from a very low mileage engine and new rings resulted in it being much quieter.
The noise on my pre rebuild engine came on quickly but didn't really get worse.
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Thirty years selling imperial hand tools for old machinery(Now happily retired).

[Inglewood]

An old engineer trick, if the noise is piston/con-rod based it will soften and may dissappear if you remove a plug lead from the affected cylinder.

If the noise is cam/tappet based removing a plug lead will not affect the noise.

Also, if cam/tappet the noise should be half the frequency of the crank - I hope this makes sense, you need an engineers ear!

[baconsdozen]

Sometimes a squirt of oil down the bore (do it when it's cold and try not to rev it when it fires) will cause the noise to lessen or alter. A stethoscope might help in finding out which cylinder is making the noise,or you'll have to try them one at a time.
If the noise lessens as the engine warms its more likely to be piston slap than anything else.
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Thirty years selling imperial hand tools for old machinery(Now happily retired).

[brinjoy@hotmail.com]

Thanks for all the info. More or less confirms my own thoughts.Will try the shorting out plug lead trick. Have heard of this one before.

[john-saab]

We had the same issue on my friends 250..it turned out to be a fautly exhaust manifold gasket..while the engine was cold you could hear it but when it warmed up and the manifold expanded it quietened down..under load the gasses (and sound) would force through and could be heard..new gasket and she was quiet again.
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Rust Junky & oil addict.

[baconsdozen]

Yes,its a good idea to rule nothing out.
I once went to look at an old diesel van,the owner was convinced it had an injector fault as when pulling hard it vibrated really badly,coasting or with minimal throttle it was fine.
He'd had the injectors redone,ran comression tests and altered the pump timing etc.
It was a UJ breaking up on the prop shaft.
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Thirty years selling imperial hand tools for old machinery(Now happily retired).

[brinjoy@hotmail.com]

Have tried removing all plug leads one at a time and the noise is still there so i am now thinking it might be valve gear related ?? Once the engine is warmed up it does sound like a tappet. I am new to Daimler V8's so not sure. Definitely louder under acceleration until warm.