Y2kimmel

Funny you should ask - I replaced mine two weeks ago (1995 E420) for exactly the same reason.

Here's what you need to know:

- you need to drain the cooling system down about 6 liters to do it. I inserted a piece of pvc tubing in through the coolant header tank and down into the bottom hose that goes to the bottom of the radiator. It was a breeze to siphon the coolant out, and it precludes removing the bottom engine cover.

- take your VIN and engine serial number and get a new thermostat from the dealer. Depending on your VIN/engine number, there's an "early" type or a "late" type of thermostat. Early looks like a regular thermostat, and you remove the cover/hose connection to get at it. Late is built right into the cover and hose connection, it's all one unit. Mine was "late" but less than 100 after the change over of production.

- get a new hose elbow. Mine was in good shape, but for $7 (or so) I just don't want to bother not replacing it.

- removing the thermostat housing is "relatively" easy. After draining the coolant, remove the hose. It comes off once you remove the clamps and loosen it from the aluminum that it's been attached to for a decade. DON'T scratch the fittings!!! I was replacing the housing, so I let it get damaged, but I was careful with the one that was staying in place.

- two of the three t-stat bolts (my kit came with new ones and a new o-ring seal for the t-stat itself) are easy to remove. The one at the back is a b***h and there's no other word for it.

- To reach the back bolt, undo the allen-head bolt that holds the dipstick tube bracket in place. Move the dipstick tube as far up and as far away from the t-stat as you can (not very far, though, as you'll see). In a perfect world, you'd be removing the spark-plug wiring loom from top front of the intake manifold and pulling the dipstick tube right out. If you damage the o-ring at the bottom of the tube, you might have to do that anyway.

- now that you've created a tiny working space around the back bolt, use a 1/4 inch drive 10mm standard socket and a 1/4 flex handle (there's no room for a ratchet head, but the flex handle head will just fit between the dipstick tube and the top of the socket on the head of the bolt) to loosen the bolt so you can unscrew it.

- I unscrewed it with a universal joint plugged into the socket (a one-piece u-joint socket would be best but I didn't have one).

- once you have it out, replace the t-stat (the easy part) and put it back together. Assembly is the reverse of disassembly, and the dipstick tube will still be in the way of the back bolt.

The last bit of fun is installing the short rubber elbow. Once the plumbing is bolted back together, the rubber tube has to be lubed (with coolant) and slipped into place. It only took me about 1/2 hour of twisting and swearing, but it finally popped into place.

Refilling the coolant is not a problem - mine was "full" at about 3 liters put back, and after the engine warmed up I got another liter in, and after a short drive on some hills, the light came on and I got the last of it in. It takes a while to fully "burp".

All in all it's not a bad job, and without the dipstick tube issue, it literally could be done in 1/2 hour. Mine took nearly three hours as I was trying to figure out how to do it, and which tools fit and which didn't. Having done it I wouldn't hesitate to do it again.

I hope that you find this useful!

Cheers
JJ