Many have no idea where to find the power steering reservoir on the M120. It is genuinely hidden in plain sight! Also questions that pops up are which fluid do you use for the Mercedes W140? I will answer both.
Open the hood and look down on the engine. Then take your finger and point to the first star emblem you can see.
The star is part of a plastic lid. Remove this, it is held in front by two tabs and then you slide it towards you.
Now point at the container with the cap on it. Congratulations you have found the power steering reservoir.
Open the cap and measure the . I am pretty sure the level will be low. This has to be the most neglected fluid on cars. This is the main reason the steering box gets sloppy, due to the fluid running low, wrong type of fluid and just pure neglect.
The fluid you should use is Febi Lenkgetriebeöl 08972 or an oil with MB 263.3 approval. Alternatively you can get it directly from Mercedes. Do not use ATF for the love of God. AFT works, but will wear the steering box prematurely.
The W140 How To.. articles are too and far between, especially on this site. Gladly on my part (sadly for you guys…) there has been very little wrong with this car and there’s not much maintenance either since the S600 has not been used as a daily driver. The W140 appears to me as a very over engineered car with an extremely durable design. As mentioned in the review, it is quite heavy due to all the extra sound dampening and extra equipment for comfort. I have never been in such a quiet car ever. Rolls Royce is probably the only comparable here. But as any other car, it too require regular maintenance such as oil changes. So how do you change the oil and filter for the engine in the M120 V12 engine. Sounds a bit scary right?
This Mercedes model was one of the last one without the common OBD2 interface, and it is not required to a use any electronic diagnostics tool to reset some car computer during the oil change. What you need though is a lot of oil, 10 Liters. Remember to get an oil filter too, please get a good German known brand since they will last longer, not disintegrate and not ruin your engine. The price is anyway insignificant at this point. You don’t want to cheap out here, trust me.
Another tip is to get 2 new air filter elements since you will be messing with the air filter box anyway. And since they are cheap and should be replaced regularly it is a good time to change them now.
What type of oil then? Well it kinda depends on the conditions used, warm climate vs cold, harsh driving vs. balanced etc. I am using the car mainly in summer in a moderately tempered climate and use the car quite balanced. So no need for high performance or very cold weather conditions. However the engine is quite large and is quite slow to get up to operating temperature, so a thin cold start oil is probably beneficial to avoid unnecessary wear and tear when the engine is running cold the first 5 minutes.
I have used Castrol’s recommendation chart and it recommended the 0w-40 Edge fully synthetic oil. This is not an ad for them, but easily available here. I am also a fan of Mobil1, where I have used 0W-40 SHC Synthese Technology.
You will also need a special oil filter cap tool which can be acquired quite cheap from any car parts supplier. The cap is tightened to 25Nm and is nearly impossible to remove by hand. It’s also made of plastic so it can break, a new cap is not cheap. The trick here is however to get the correct one since there are hundreds of different ones. I spent quite some time to figure out the correct one without actually looking in the car and measuring. Firstly I couldn’t travel physically back then due to the lock-down and secondly I didn’t want to fiddle with the car just to find out the specs for the tool. (The oil filter cap is not visible by just opening the hood and requires removal of the air intake box). The oil filter removal tool is 74mm width with 14 slots.
Parts and tools
Oil filter (oil filter comes with new oil filter cap o-ring and a copper washer for the oil drain plug)
11L of Oil, your choice, e.g. 0w-40 fully synchetic
Oil filter cap removal tool. 74mm 14 slots
The oil removal process can be done with a oil suction pump inserted from the top of the dipstick tube and then removing the drain pan plug after, this makes less of a mess and is done in most workshops. However then you need an extra tool, so this guide will only use the drain plug method.
The engine should have normal operating temperature while changing the oil, since the oil will drain out easier, so take the car out for a drive before or plan to do the oil change straight after a trip.
Start by lifting the car up in the front and place it on jack stands in order to access the oil drain plug. Some of the w140 have plastic panels hiding the drain plug, so make sure to remove these. However my car does not have them and the drain plug is easily accessible. Before draining the oil from below, the oil filter must come out to allow better drainage. After opening the hood it can be very confusing locating the engine oil filter housing, and in fact you cannot see it unless you remove the left side air filter box. It’s well hidden below it.
Thankfully it is easy removing the air filter housing. It’s attached by two bolts and a couple of clips to the intake housing. You also need to disconnect one airflow sensor cable. Take care not to loose the intake hose clips into the engine bay which can be hard to recover, since they can come loose by themselves after loosening the air box. Loosing such a clip will make dirty air potentially enter the engine since it can then bypass the air filter.
After the left side air filter box is removed, you can now access the oil filter housing and remove the cap with the special filter housing tool. Use a joint on the socket to make the operation easier since access is poor down there. Pack a lot of paper and keep a tray on hand to avoid spilling oil when taking out the oil filter. Clean up all of the spilled oil, since it’s hot down there and we don’t want smoking and fires while the engine is running. Also use a small suction pump or syringe to get the little left over oil which lies in the house after the old filter is removed.
Then it’s time to open the bottom oil drain plug. You will need a large oil drip pan which can have at least 11 Liters. Beware the engine oil is hot and will come out with great force when removing the drain plug. Use plastic gloves to protect the skin and try to not loose the bolt into the pan when removing it. Expect about 10-10.5 liters of oil to drain out. It’s nearly impossible to drain it completely of 11 liters by just removing the plug as with most cars.
When the oil has stopped draining after 10-15 minutes. Change the copper washer on the drain plug that comes with the oil filter kit, and the reinsert the drain plug. Do not over tighten since it will ruin the threads in your drain pan. Tighten the drain plug bolt to 30 Nm. It have happened a few times over the history that people have forgotten to insert the drain plug bolt and poured in the new oil which just lands on the floor. Do not stress and make this mistake!
Then replace the O-ring of the oil filter cap with the one that comes with the oil filter kit. Many recommends giving the O-ring with a coat of engine oil before assembly. I have no idea if it’s any useful, but I did it. It gives at least some less friction when installing the cap. On most cars it’s good to fill the oil filter hosing with oil so the first few rotations of the engine will have faster access to the new oil, but on this car it is not possible at all. Pouring new oil into the housing will just drain straight into the block. Also it is not possible to install the oil filter without it being attached to the cap itself, so there is no chance here.
The oil filter is installed by popping the oil filter to the cap and then inserted into the filter housing. Tighten the cap to only 25 Nm and it’s very important to not over tighten here since it’s made from plastic and easily strip threads. And a new cap is very expensive!
Also inspect the bottom of the oil drip pan, look for sludge and which can indicate overdue oil change interval and metal shavings which can indicate excessive wear in bearings and pistons. In my case it was no such indications and all is good.
Double check drain plug and oil filter cap is secured. Before poring in the new oil you need to know how much to add so you’re not over filling the engine. Adding 10L should be enough, but measure before you pour the last liter in.
dd fresh oil using a funnel to avoid spilling oil all over the engine. After you have filled the quantity you need, measure with the dipstick and inspect it’s not above maximum level. Some of the oil will need to get into the filter and you probably have to add slightly more after the engine have run a few minutes.
When starting the the engine, make sure that you get good oil pressure quite fast, and inspect for leaks in the drain pan and around the oil filter cap. After the engine has run for a few minutes, turn it off and measure the level with the dip stick again. Add oil if necessary, but do not over fill. If too much is added then you need to remove the excessive oil with a suction tube.
Reinstall the air filter box. Now It’s a good time to also change the two air filters on either side of the engine. No tools required here, it’s made like a drawer, just like how you store your underwear and socks at home.
Then take a short drive of 5-10 minutes, see that you have good oil pressure and that the engine feels normal. When coming back home, inspect for leaks around the oil drain plug. And remeasure the oil level with the dip stick, add a bit oil if needed. Then you are done for another 10.000-15.000km.