'85 300SD: very loud valve noise!

should have mentioned that the mileage on the engine is around 250K.

That car you got... I think up to 88... I may be wrong here... needs valve adjustment every year. When is the last time the valves are adjusted?

Yea, I know the 85 needs regular valve adjustment. As I said, the valves were adjusted recently, maybe a half-year ago, probably haven't put over

4000 miles on it since. Why? Are noisy, clattering valves a symptom of out -of-adjustment valves? It was about as noisy before the last valve adjustment as afterward, and the mechanic said that the adjustment was "pretty close as is".

You might consider the possibility that the source of the noise isn't the valves, but an exhaust leak.

My '82 300CD was rather noisy when I bought it, so the first thing I did was have the valves adjusted. No change. A previous vehicle of mine, a Toyota pickup, developed the same sort of noise, and the cause turned out to be a warped exhaust manifold. The manifold had pulled away from the engine block, allowing pulses of exhaust gas to escape. The noise was more of a mechanical clicking sound than the sort of gaseous chuffing that one would expect.

Geoff

The sound you hear is due to excessive valve gap - the valves are being slapped open not pushed open. The correct gaps (cold engine) are about .004 inch intake and .014 inch exhaust. Some of your engine's are substantially larger possibly because the last adjustment didn't snug a lock nut against the adjustment nut and the adjustment was lost.

The "danger" is that the tapping will eventually fatigue the parts and some will eventually fail.

The tapping is due to excess gap - way beyond the .004 and .014" spec. The tap is really a slap as the rocker hits the valve's adjustment cap nut.

Probably the adjustment wasn't snugged up with the lock nut and came loose. The shop that did it should take a look for the slapping will eventually cause the parts to fatigue and break - eventually.

On second thought, the air cleaner's support bracket may have broken from metal fatigue and making a hell of a rattling sound. This is easily checked by pulling UP on the air cleaner - it should NOT lift up. The rearward leg of the support bracket breaks from the engine's sidewards oscillation; it's a poor design that's never been improved - so much for M-B's engineering excellence; my car is on its third bracket in 100K miles! I had the last new one reinforced by a welding shop before it was installed.

the fix on my 82SD air cleaner was getting a towel, folding it a few times & slamming the hood on it. it took away most of the noise. a new exhaust system fixed the rest!

2 towels & 70,000 miles later still working.

now at every fuel fill up use the towel to check the oil. shake the towel out

The speed that we can rotate an engine by hand is so slow that compression isn't a factor - at all!

I turn my engine by putting a ratchet onto the power steering pulley's nut and turning it that way. A ratchet ensures only turning in the correct rotation direction.

To adjust the valves one needs two thin 14 mm open end wrenches. The special offset wrenches are preferable but I've been able to get by using straight Sears Craftsman wrenches. The TOP or cap nut is the adjustment the lower nut is the lock nut.

I can send you detailed instructions.

Could it also be a worn small-end bearing? That's what happened to our engine. Sounded just like valve noise but very loud.