loud valve tap noise
 

My 78 300SD started making a lifter tap noise when under light load only. I suspected an injector was failing at first. The noise has progressed and now makes noise all the time. I tried to locate the cylinder making noise by cracking injectors (a long shot on valve noise...but it was quick and easy). No luck. It is definitely a mechanical noise because it does it when engine braking and shutting off the key, eliminating the chance it is a combustion noise. I checked my valve clearance, and the re-adjusted them all. The camshaft lobes look OK to me, no broken valve springs, it all looks OK. I'm convinced the noise is in the upper end. It is loud enough to attact the attention of people I pass on the street, and pretty obnoxious in the car. I really don't want to start tearing it apart to diagnose this noise. Has anyone experienced something similar that might be able to offer some advice here? Any help appreciated.
Joe

udate
 

I took the valve cover off and ran the engine. There is no change if I put pressure on the followers with a screwdriver as it's running, so it doesn't act like a valve clearance problem. There is a tensioner/slider on the back side of the chain (I'm sure many of you know that), If I put pressure on that slider, moving it tighter to the chain i feel a bump in it that corresponds to the tapping noise. Also as soon as I shut it off, there is slop in that slider. Should that thing hold tensioning pressure after the engine is off? It doesn't appear sloppy with ther engine indling. Lastly, now that I've fooled around in there, the noise is gone except when I throttle the engine, which you can only do with the valve cover back on.
Joe

Can you tell us what the chain touches before and after the item you are describing ? By ' backside' of the chain... what do you mean ?

I mean it rides on the outer side of the chain, as opposed to the cam/crank gear side. It is the last thing to touch the chain before it gets to the cam gear.
Joe

And your engine is a non turbo ?
Ok,, you know which way the chain travels... (?)... so this is on the passenger side of the engine ?

No, it's a 5cyl turbo.
The chain travels clockwise when looking at it from the front, and the tensioner is on the passenger side of the engine.
What made you think it was a non-turbo?
Joe

I don't memorize all the start times for turbo, etc... I know the wagons did not have them until about 1980...
and I need to know for sure because there are some differences in them.. and I have a manual for each...

Do you think you are describing the " ratcheting tensioner"... ?

I don't think it is a racheting type. When the engine is off it depresses pretty easily, and makes a gurgling noise when it goes in.

The ratcheting part of that tensioner is a " long range" slack take up arrangement... the day to day additional slack is taken up by the oil pressure reaching the tensioner... so your description fills the bill...
I suggest you take this situation very seriously... most chain blowups are not the chain breaking... but worn out tensioner or rail friction parts.... or gummed up ratcheting tensioner....
Why it is making that noise I can't visualize at this time..

Thanks for your help. If the tensioner uses two mechanisms to tension(mechanical and hydraulic) then I suppose it is OK if it retracts a bit with the engine off. It also explains the gurgling noise. I can see the sliding face of the tensioner and it doesn't appear to have excessive wear at all. Does the chain ever develop a tight spot in it? That might account for the "bump" I feel in the slider with the engine running.
As I said earlier, everything that I can think of checking looks good on this. I was hoping for the "magic bullet" effect today, where I get a bunch of responses saying "check the _______!! they make that noise all the time!!" But after not finding anything with a search I should have known better.
Leathermang, thanks again for all your help. I'll double check the chain and put it back together. My 1100 mi/week commute will surely make the problem obvious soon enough (unfortunately at a higher price tag, usually). I'm done fooling around with this. I've got too many hours in it now...
Joe

I have many chains that have developed a ' tight spot' .... but only after I left them out in the weather for months with no lubrication... a chain inside a protected space like our MB engines should not do that unless they are in DIRE shape.
However, for the record, my impression of the speed at which your sound is happening does not correspond to a chain having one noise making section in it... these chains are very long and I am sure they are made so that the same spots on the chain to not match up to the same sprockets on any item in the engine...
Can you put a stethescope on that tensioner ? If the sound is coming from inside it... would be pretty easy to replace it.