How to change engine oil and filter in OM617 engine

This is one of the first things you will perform when learning to DIY your car. Changing engine oil is easy and satisfaction is high when you know you performed preventative maintanence. Regular oil changes on the 617 engine will literary make it run forever and ever and ever and ever and ev….. until diesel is forbidden.

If you drive in harsh conditions and only doing short trips, you should change oil more often than 10.000km, if you are living in the flat lands and only doing long drives on highways, then every 15.000km might be enough for you. I recommend every 10.000 especially dealing with these older cars where engine wear is more prominent, also easier to remember than the other numbers.


Buy some engine oil, you will need 6.5 liters for the non turbo 617 engine. Determine which condition you need to drive your car in, hot or cold weather. I will do a short introduction to oil “weights” which are the numbers you see indicated on the oil can. Also decide if you want to use fully or part-synthetic oil. While modern cars normally use fully synthetic oil, these older Benz are actually made for using part-synthetic oil. However this is up for preference, I won’t discuss which one is better. On my part it seems that the fully synthetic oil is easier leaking out of gaskets than the part-synthetic one, but I cannot confirm this. I have used both types for the 617 diesel.

Oil Viscosities.

The numbers tells about the viscosity of the oil at different operating temperatures. The higher the number, the higher the viscosity. Low viscosity gives less friction, but don’t give as much lubrication. Usually you will see a number like this: 5-40

The first number indicates the viscosity when the engine is cold, while the second number indicates the viscosity when the engine is warm. To ease startup the engine oils are engineered to give less friction when the engine is cold and it will start to thicken when the engine comes to operating temperature giving better lubrication.

The handbook in your car will tell the recommended viscosities at different ambient temperatures. The higher the viscosity on the the second number will make the oil more expensive, race cars have up to 50 and 60. If you choose to have 40 it is more than good enough. Living in cold Norway where temperatures can drop below -25°C easily I used 5 viscosity on the first number which made startups easier on the battery.

So to sum up the preparation you will need:

  • Engine oil
  • Engine oil filter and filter gasket
  • Oil collector
  • Paper towels
  • Basic tools

Oh and don’t wear your fanciest white pants and shirts for this job!


Have paper nearby in case of spills, plastic gloves are smart unless you want to look like a real mechanic while going to your desk job the day after (actually for the rest of the week). If you are agile you don’t even have to jack the car up for draining the oil out, the W123s have a good ground clearance and you have room enough for a low oil collector under the pan. Have a 13mm socket ready. The engine should be warm while doing the oil change so more of the crud will come easier out.

Start by removing the oil filter cover. Wait with removing the oil filter itself unless you want oil all over your engine.

oil filter cover
Remove oil filter cover, wait with removing the oil filter itself.

Unscrew the oil pan bolt. Keep in mind that the oil will come shooting out at you and it will be warm, be careful not burning yourself. Also hold on to that bolt so it don’t fall into the oil collector. Let the oil drain for 15-30 minutes until it completely stops.

oil pan plug om617
Place the oil collector under the oil pan and remove the plug.

Remove the old oil filter, try not to spill oil all over the engine bay, protect with old rags. Watch if more oil comes out of the oil pan, I usually wait another 10 minutes.

oil filter housing om617
Empty oil filter housing.
oil filter om617
New oil filter and gasket.

Reinsert the oil pan plug now before doing anything else so you won’t forget it! I laugh every time when hearing about stories where people have filled up new oil and it just runs straight through onto the ground again because they forgot the oil pan plug. Take care not to over tighten the bolt, use only 40nm of torque, there is a copper washer there to seal, replace if it’s lost. Also clean the underside of the pan for oil. So you can more easily spot leaks later.

Insert new oil filter. Replace the oil filter lid with a new gasket that follows the new filter, put a thin film of the engine oil on the rubber gasket. Tighten the the filter lid. It is not necessary to tighten this much at all, just enough so the rubber seal sits nicely.

oil filter om617
Install new oil filter in the oil filter housing

Open the oil filler cap and pour 5 liters of new engine oil into the engine. It’s good to use a funnel unless you are a pro at pouring into small holes. Wait a couple of minutes for the oil to collect in the oil sump. Then start the engine and let it run for 1 minute. Check the oil level, pour up one liter more, check the oil level again. If not full, pour up the last half liter while checking the dipstick not to overfill. If there was some old oil left in the engine the maximum capacity of new oil i less than 6.5 liters, there is also residue oil in the oil cooler.

Clean up your mess, pour the old waste oil into the empty cans from the new oil you just poured into the car. And check under the oil pan for leaks again. If no leaks take a short trip to see if engine is running smooth. Then after the little trip check the oil pan for leaks again and check the dipstick for oil level, if not completely full, pour a little more of the new oil into the engine until full.

That saved you lots of money on workshop bills. Workshops tend to overfill a bit too which is not recommended. Your level should be under max, not over it.


How to Adjust the Valves on a Diesel 617-engine

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, 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.

valve adjustment kit for diesel engine
Special tools, valve adjusting wrenches and feeler gauges.

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…

Mercedes 617 engine in W123
The 617 engine, just to make sure
Adjustment Procedure

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.

For accessing the valves, the valve cover has to be removed and for better access other components attached to the valve cover have to be removed too. Since my 300TD has a manual 5-speed transmission, there is no vacuum lines and a complex throttle linkage attached to the valve cover, so the removal is more straight forward. I begin with removing the air filter housing, it's only secured with 3 bolts.
Air filter housing removed

After removing the throttle linkage attached to the cover, start unbolting the valve cover itself. It is attached it four 13mm bolts.

valve cover bolts 617 engine
valve cover bolts, not the small washers, which are special wave washers, in 99% of the cases they are missing. Obtain spare ones before doing this job

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:

  1. 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!!!
  2. 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.

valve adjusting 617 engine
Valve adjustment, use the feeler when the cam lobes are pointing upwards from the valve.

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.