The valves should be adjusted at regular intervals every 15.000-20.000km to be at peak performance and save on fuel economy. These old engines for the W123s don’t have automatic valve adjusting like later models and needs therefore regular adjustment. The valve clearance has to be exactly to specifications to ensure you get the peak performance out of the engine. Especially for the diesels where you have relatively few horse powers to play around with from the beginning (the biggest 3.0L non turbo only has 88hp). The valve adjusting procedure is actually quite simple and can be done in an afternoon, but it needs some patience to get it just right when it’s your first time. This is why many mechanics shops do not necessarily get it right all the time due to their time pressure, so you should just do it yourself: Save money and have the peace of mind knowing that the job is done right!
The only special tools needed are two valve adjusting wrenches and a feeler gouge. A very good kit of these tools can be bought over at mercedessource.com, I highly recommend to check their site and kits out, they are highly prtofessionals!
Get a new valve cover gasket, the old ones are always leaking after some years and this is a good time to change the inexpensive gasket.
If the valve cover bolts have missing wave washers, obtain spare ones as well.
The valve adjustment needs to be done when the engine is cold and have a temperature of around 20°C. The valve clearance specifications are set at this temperature, remember your physics that metal expands when warm and you will get a tighter gap between the valves, oppositely the gap will be too small when the engine is very cold. So you can’t just do this after driving home from work. The easiest thing is just to leave the car over night before doing it. The valve adjustment can be done when the engine is warm with different valve gap specifications, but I wouldn’t recommend it since you might burn yourself and the temperature of the engine would slowly get lower during the adjustment process so you have to be very quick about the adjustment. We both know that is not going to happen so just do it when the engine is cold…
For accessing the valves, the valve cover has to be removed. For better access other components attached to the valve cover have to be removed before. Remove the air filter housing which is attached with 3 bolts. Since this particular 300TD has a manual 5-speed transmission, there are no vacuum lines and a complex throttle linkage attached to the top of the valve cover, so the removal is more straight forward than with an automatic. If you have the automatic, then take a picture so you remember where all the linkages and lines went.
After removing the throttle linkage attached to the cover, start unbolting the valve cover itself. It is attached it four 13mm bolts.
Pull up on the cover and then you can see the valve springs and the upper components of the engine. Sometimes it is a little tricky to pull the cover off so just wiggle it side to side until it is off.
Time to prepare the valve adjustment. Make absolutely sure that the engine is cold, it should have the ambient temperature at about 20°C when adjusting the valves. For the 617 engine WITHOUT a turbo (also called non-turbo versions indicated by only the D or TD if you have stationwagon, if you have a turbo it says Turbo-Diesel in the designation), the intake valves needs clearing of 0,10mm and the exhaust valves needs a clearing of 0.30mm. There are 10 valves in total, 2 for each of the 5 cylinders, to determine which valves are intake or exhaust valves, just look at the corresponding manifolds. I highly recommend to draw a simple valve chart and cross out which valves you have adjusted to leave all confusions out. I hate doing things twice..
For accessing the right position on the valves, you have to turn the camshaft. For turning the camshaft, you have to basically turn the whole engine. How do you turn the engine over? And can you do this by hand? Sure you are strong enough, remember back in the days they started the cars by hand with turning the crank shaft. So that leaves you two options:
- The first option is turn it by hand on the crankshaft pulley bolt, I think this is the easiest and fastest way to do it. But it requires some use of muscles and taking care of not scratching the hands on the fan or the radiator, because here the space is quite tight. ALWAYS turn the crank clockwise when looking from the front of the engine, never even think about turning it the other way!!!
- The other option is to reconnect the starter motor so you can turn the crank slowly with the ignition, this procedure needs you to crawl under the car and play with the wiring on the starter motor. Remember you don’t want to start the engine so the starter has to engage slowly.
When adjusting the valve clearances, the cam lobes needs to be pointing straight upwards from the valve you are working on. This is why you need to turn the crank to get the right position of the camshaft. Use the feeler between the cam lobes and the rocker arm, there should be a slight drag when you move the feeler in and out, just between squeezing and no resistance.
The are two bolts on top of each valve. The top one is the adjusting nut and the lower is the tightening nut, quite counter intuitive! Use the valve adjusting wrenches to adjust the valves by loosening the bottom nut and the actual adjusting is done with the upper nut while keeping the lower nut still. In an old engine they might have become very hard, so it’s very important to have the proper tools with super tight clearances otherwise you might ruin the nuts. Turn the upper nut clockwise to make the gap bigger and anti clockwise to make the gap smaller. It takes some tries to get it right the first time because the adjustment is very sensitive so just be patient and try a few times until it gets right. It is easier to have a companion that continuously checks the clearance when you do the adjustment to get it right. I have only done it alone though so it’s not impossible. When you are alone, you have to always recheck your own work with the feeler after tightening and it takes a couple attempts to get it right.
After each valve, turn the engine and do the next one, since the lobes are not in order one after another, note down which valve you just adjusted so you don’t have to spend all night turning the engine over a million times.
When all 10 valves are adjusted, replace the valve cover gasket to prevent leaks. Don’t over tighten the valve cover bolts, let the rubber gasket do the sealing and not brute force. Put all the other parts together and have a test drive, and most likely you will feel an increase in performance and acceleration. That is usually the case if all valves were too tight before the adjustment and is a indication of neglected valve adjustment maintenance.