Disclaimer: taking this advice, as has been warned in a previous thread, can result in personal injury if not done safely. Do so at your own risk.


Use a mechanic's stethoscope if you have one and a broom handle or some such thing if you don't have a stethoscope, and place one end against the vac pump housing while the engine is running. Place the other end to your ear --- BEING CAREFUL not to get the stick in the fan or the serpentine belt, both of which are in very close proximity. If you do this without getting tied up in the belts and taking an ear off (I have done it many times; it's a fairly old trick) you will hear the vac pump hitting very clearly if it's indeed the culprit.

Mine was broken into pieces and the remainder was being slapped around in the housing and destroyed the lobes on the injection timing gear that were supposed to operate the pump.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

Thanks I'll be sure to try them out

Does anyone think it could be piston slap?

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1975 240D W115 version 3 (Australian car )

unlikely piston slap in a Benz

get the tempo figured out
if the noise repeats at RPM speed, it's low end / crank related
if the noise is at 1/2 RPM, it's top end / cam / valve train related
if it's 1/4 RPM it's a particular valve

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Cheers!
Scott McPhee

1987 300D

That's actually a good Maxim to remember Scott. Thanks for posting.

I am trying to find a "random" noise in my car too. It comes from the engine bay, a loud multiple time "clack", only between about 1200 to 1700 rpm. Seems to mostly happen when I am accelerating around a corner, occasionally when leaving a stop light. Never at idle or cruising on the highway.

Sometimes I can get it to repeat if I hold the engine at between the 1200 to 1700 rpm range.

Vacuum pump has maybe 10k on it, fan clutch the same. All other components are oem. No play in the water pump shaft, some play in the new fan clutch though.

I've only been able to recreate the sound in the garage a few times while the hood is up and I'm manipulating the throttle linkage.

I need to figure out a way to diagnose this...

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1993 W124 300D 2.5L Turbo, OM602.962
2000 Chevrolet Cavalier, 2.4L DOHC
2002 Ford Explorer, 4.0L SOHC
2005 Toyota Prius, 1.5L

http://www.fuelly.com/sig-us/40601.png

Hello, did you ever figure out what this noise was? I get that same clack or tick at the same RPM ranges, never at idle and washes out with all the other noise at high speed. Mine is constant hot or cold.

[QUOTE=scottmcphee;2743237]unlikely piston slap in a Benz

Why no slap in a Benz? Do they just break? I am chasing a noise that seems like slap, but can't find it. Before I throw parts at I want to make sure the engine is good and not a money pit.

I just tracked down a ticking like that. It turned out there were two sources:

1. The vacuum pump check valve. I changed the vacuum pump and that went away.

2. The alternator "fan" on the front was hitting the body of the alternator. I changed the alternator and it went away.

Another possibility that is related to the timing chain is the timing chain tensioner. If it is shot the timing chain can slap around. I tried changing mine to make the ticking go away, but it didn't work.

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside