Since moving to Maryland in late October, I've noticed that with the cooler temps, it has been taking my 560SEC an extremely long time to reach operating temperature, as indicated on the coolant temperature gauge in the instrument cluster. Even after driving 10-20 miles, the coolant temp gauge was only registering 75-80C, not even quite getting up to the 80C line. This is in 40-degree ambient temperatures. There was plenty of hot coolant and the interior of the car was heated up as normal with ACC.

This reluctance to get up to a proper indicated 80-95C operating temperature (depending on load and ambient temperatures) is a classic sign of thermostat failure -- the thermostat is stuck in the "open" position.

Thus, I resolved to change the thermostat. First of all, I checked my records for the 560SEC. They showed that the thermostat had last been changed in October of 2009, at 180,000 miles -- that was 60,000 miles ago. It was using a Behr thermostat. So, with eight-plus years and 60,000+ miles on the thermostat, it was definitely time for a change.

I pulled a fresh, new Behr replacement thermostat from my parts stock, and went to work on the job. The replacement rubber o-ring is included in the Behr thermostat kit.

NOTE: Wahler also makes a replacement thermostat of good quality. However, the metal is "bonded" (pressed) together, whereas the Behr unit uses a slightly different design, that in my opinion, is a bit more reliable. I have heard a couple of experienced mechanics say that the Behr design has proven to be a bit more reliable in practice, because the bonding can be a weak spot.

Here is a visual comparison between the Wahler and Behr thermostats for the M117 and [early] M119:

Behr unit

Wahler unit

To successfully complete this job, you will need the following tools:

  • 10mm socket, 1/4" drive
  • Flexible and/or short extension, 1/4" drive
  • 10mm deep socket, 1/4" or 3/8" drive
  • small/stubby Philips-head screwdriver
  • medium-length flat-blade screwdriver
  • small hammer or rubber mallet
  • tray or drain pan for coolant

The following part numbers are required for this job:

  • 2 gallons MB coolant or Zerex G-05 mixed 50-50 with water
  • Behr part #TX2680D or Wahler part #1162000315
  • or equivalent MB thermostat 116 200 03 15

Getting started, this is what the area looks like when you pop the hood, per the US-spec M117 in 5.6 liter form. This is the area where you will be concentrating your efforts.

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And, a couple of views of the thermostat housing, below the upper radiator hose. Note that the lower radiator hoses connects directly to the thermostat housing.
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The first step is to release the pressure at the coolant expansion tank, by loosening (twisting) the aluminum cap at the tank. ONLY DO THIS WITH A COLD ENGINE THAT HAS NOT BEEN DRIVEN IN AT LEAST THREE OR FOUR HOURS. You do not want to release the expansion tank cap when the engine/coolant are hot and/or under pressure. WAIT UNTIL THE CAR IS COLD.

Next up, put a proper drain pan under the radiator on the passenger side, and unscrew the light-blue "pet cock" (radiator drain plug) in the bottom of the radiator using a small Philips head screwdriver. Quickly place the pan under the end of the radiator to catch the draining coolant. You should see approximately 1.5-2 gallons of coolant drain from the radiator and expansion tank into the pan. Quite a lot -- a good opportunity to replace this drained coolant with new coolant, especially if your coolant is more than two years old.
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Here is what the stubby and tiny screwdrivers I used look like. These are excellent for getting into the tight confines of the engine compartment and under the radiator of my lowered 560SEC. The small/tiny Philips screwdriver is great for loosening hose clamps in tiny spaces.
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Next up, while the coolant is still draining from the radiator, you need to remove the lower radiator hose from the thermostat housing. Using your stubby/tiny Philips head screwdriver, loosen the ring clamp and carefully slide it down the hose and out of the way. Then using your hand, slowly work the end of the lower radiator hose back and forth and off of the thermostat housing flange.

Be sure to visually inspect the end and body of the lower radiator hose for damage, cracking, or other age and temperature related wear. If it is in bad condition, make a note that you need to replace it ASAP and order a new one. If it is in good condition, as my hose was (although it will need replacement in the coming years), then move it aside as the coolant drains from the top of the hose and the thermostat housing into your drain pan still under the car.

Here are a couple of views of the lower radiator hose being loosened and removed from the thermostat housing.
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Moving the upper end of the hose out of the way after removing from the thermostat housing flange.
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Next up, you need to prepare to remove the three 10mm bolts that hold the thermostat housing to the water pump. Two of them are easily reached with a 10mm socket on a 1/4" or 3/8" ratchet. The third bolt is behind and below, and takes more effort. It is best to use a deep 10mm socket for this, or your regular 10mm socket on a short 1/4" extension to reach behind and remove it. Eventually, you'll get it using a combination that you have to discover. You can see I tried a couple of combinations of extensions and flexies, but eventually settled on a 10mm deep socket on a 3/8" ratchet to get at the last/back bolt holding the thermostat housing to the water pump. It will continue to drain as you remove the three bolts.
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Once the three bolts are removed, you need to CAREFULLY prise (pry) the thermostat housing off of the water pump. I did this with a skinny flat-bladed medium screwdriver, and a couple of light taps with a small ball-peen hammer. This separated the top of the thermostat housing away from the water pump enough to break the seal, and then I could remove it the rest of the way by working it back and forth with my hand.

See the three bolts and the thermostat housing, removed, below.
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