My 87 300SDL (190K miles) has recently developed a concerning noise which is clearly coming from inside the timing chain enclosure on the front of the engine.

The noise sets in only when the engine is at full operating temperature, and is a loud rapping noise that generally occurs during hard acceleration (though some times it's louder than others). I have owned this car for 8 years, and have never heard anything like it before. I know exactly where it's coming from since once the noise sets it, I can pull the car over and make it reoccur by revving the engine by hand with throttle linkeage while the hood is open.

I initially suspected the timing chain tensioner, though I have replaced it and the noise has not gone away. I now suspect that it must be one of the chain guides, but I don't know whether they have some kind of hydraulic actuator or not.

Has anyone had a similar experience they can share?

I'm assuming the repair required here will be to remove the timing chain cover on the front of the engine, which requires the fan hub and radiator to come out too -- I assume 6-8 hours of labor, plus replacement of whatever parts have failed inside. This is a 20 year old car, and I'm struggling with whether I can justify the cost.

Do you know the history of the vacuum pump? You can attempt to isolate the sound with the use of a stethoscope. If the vacuum pump is the strongest source of the sound, you would do well to replace the pump immediately. In fact, if you've got another vehicle to drive in the meantime, I would not drive the Mercedes until the source of the sound can be identified and corrected. Should it be a failing vacuum pump - and if the pump is of an early design, there would be a high chance that extensive damage to the engine at large will result from total failure of the pump. There is much about these vacuum pumps to be discovered here, on this forum. Take a bit to do a search and become acquainted with your engine if you are not already up to speed.

There are a ton of highly knowledgable folks here; always quick with the help you may need. Welcome.

First thing would to NOT drive nor start the car in its condition so you can isolate the issue.

When I had that issue, it was my vac pump taking a dump.

Consider pulling the valve cover and taking a peek inside to what you see. Maybe your upper chain guide is broken. Maybe you need a tensioner. Maybe the tensioner arm is going bad.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Unfortunately, it's definitely not the vacuum pump.

The factory book says 10 hours if I recall, to pull the front cover. I would guess its probably a chain guide, I believe theres one low down on the Rt side of the engine, is that the general area the sound is coming from?

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!