Now that I've completed the service I am wanting to know just what the adjustment of the valves accomplishes. I don't know what goes on in the cylinders or how that adjustment actually affects the intake and exhaust dynamics. Can somebody throw me a bone or at least point me towards the info.?

Engines are designed with specific relationships between the amount of time the valves are open, the relationship between when the valves are open, the relationship between the valve timing and the piston travel, the ignition source, the fuel source, the distance the valves open.... etc,,, so in setting the valves to a specific gap when cold you hopefully are keeping the engine performing like it was designed to perform. However, the most important function is that of making sure the valves spend the proper time seated against the head.... because a large portion of the cooling necessary to keep them from burning is due to that contact with the head. As a valve wears against the valve seat the distance between the valve end and the cam (on overhead cam engines) decreases, at some point giving the possibility of the valve not completely seating against the head. That is the short version,,, but should get the ball rolling.. Greg

Adjusting the valves allows them to continue to close properly. Clearance between the round part of the cam and the valve stem will diminish over time, because the valves wear into the seat, allowing the stem to rise slightly. As this rising persists and the stem stays in contact with the cam 360 degrees, the valves never totally seat (close off). A valve that never closes completely loses compression through it and will also run hotter, because in the closed position against the seat, heat dissipates to the head. As the compression drops the result is a poor running engine, difficult to start, and eventually the valves can litterally burn up.

So...you did good by adjusting your valves. Your engine will thank you with continued good performance.

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold

Thanks, guys. I'll take a look at the exploded view in the Haynes and try to put this all together in my head.

How often should the valves be adjusted? Mine were last adjusted by the previous owner, about 15,000 miles ago. The engine idles slightly rough, and one mechanic has advised to adjust the valves to fix this.

Beware of mechanics that are not diesel experts! They'll kill your car or your wallet.

Factory recommendation is adjust the valves every 15K miles. This forum can walk you through it if you choose to DIY.

For rough idle do a Search in this Diesel Forum on "Rack Pin". It is a spring loaded pin that helps control the Injection Pump Rack during idle. VERY SIMPLE to adjust and will not cost the many $ a garage will charge to monkey with every facet of the IP in their hunt for the bottom of your wallet.

When I got my '75 300D. I removed the valve cover several times to adjust the valves. I never did encounter one that needed adjusting! I just gave up on the process and drove it another 140k after that without ever adjusting the valves. I must admit that it was not smart to do so and an an engineer I should have known better, but what I wnat to know is: Is it uncommon that valves stay in spec or was it possible that I was not performing the task correctly?

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

Because a diesel engine produces so much pressure, it is apparently very nearly impossible for the valves to remain static. They just move. They close up on the cam shaft and this means that they are not making full contact with the seats; loss of compression.

This is stuff I am just learning myself. Repeating it helps me drive it into my gourd for hopeful long term retention.