Quick Help needed! Replacing Thermostat 85 300D
 

I found out why my alternator belts were slipping. The thermostat seal is leaking and coolant must have been getting onto the belts.

I have new belts installed and am ready to go except for this

Do I have to drain the coolant in order to take off the thermostat housing? It seems quite high up, but I don't want to spill - I am working in my garage!

Just waiting for call to see if dealer has a gasket. Otherwise, maybe some permatex would work? Tried tightening bolts and bubbles of AF oozed out of joint!

Danger Danger be careful those bolts can break off with little provocation. Then you will be swapping the whole thermostat/water pump housing.

I would definitely get the right gasket and 3 new bolts from dealer.

You will need to let off some of the coolant but not all of it.

Then be careful to properly burp it, you will have introduced a large air bubble in the head.

The easiest way to lower the coolant level is with the 17mm plug on the turbo side of the engine. You can get at it through the wheel well.

For the tstat, tap around the bolts and use penetrating oil When you reinstall use fresh bolts and chase the threads with a tap.

OK Thanks - Good Tips!

I decided to get a new thermostat kit from dealer. $38! But at least I can pick it op here. Napa and others may have had one, but not worth the effort to shop around. Most of them freeze as soon as I say Mercedes!

I didn't order the bolts. Mine may be OK (not been in there that long since engine rebuild) but if not I imagine they are standard metric machine screws? Maybe I should use stainless?

Thermostat will be here tomorrow. Gives me a chance to start dismantling and draining.

If you use either stainless or steel bolts I would recommend the use of some anti-seize when reassembling. Makes life a lot easier if you ever need to do this again.

I used regular old ace hardware bolts. IIRC they are M6x1, which is common.

I checked and I think found the block drain. But seeing I am doing this in garage, I doubt I could get the AF to go into a container from there - too many things to splash on! I found the rad drain plug. Haven't loosened it yet. Looks in good shape and doesn't seem to have been used before. I think I will try using it, just because its a better point to drain into a pan from.

Started engine and squealing noise I had before has gone! I got to reacquaint myself with the belt adjustments! Need quite a selection of wrenches and sockets to get at the bolts!

Ratcheting wrenches are soooo nice

yeah... if spillage avoidance is your goal DO NOT PULL THE BLOCK DRAIN! only pull that if you want EVERY DROP OF COOLANT OUT OF THE MOTOR! I'd just open the radiator drain, and drain the radiator itself, not the entire block. (unless of course, you are wanting to perform a full engine flush)

Yes, that's the plan.

I noticed AF on the short hose just below the tstat housing. I hope it got there from the leak above! If not, it looks like that other housing will have to come off.

EPC shows a screw in plug on the part of the thermostat housing that bolts to the block. Is that a good place to vent when re-filling?

that short hose is ALWAYS grimey... I'd plan on pulling the entire housing off, and changing that short hose as well. I use 1-5/8" fuel tank hose... it's STRONG, and thick and the right size for the application.

I didn't receive my thermostat kit today (dealer screwed up) :furious3: , but i prepared for the installation.

I drained rad using drain plug (came out easy and it is in good shape). Then I took off tsta housing and quite bit more AF came out. Was able to catch most of it.

I noticed that there was some pitting on the face of the part of housing that is still bolted to block. That and fact tstat seal is deformed were likely reasons for leaks.

I cleaned up the mating faces with wax & grease remover and the filled pitting with JBWeld. They are very small pits, but hopefully JB-Weld will stay in place.

In the location of one pit, the rubber seal was deformed. Almost like something was in between the rubber and the housing face. (pic below) Anyway, hope to get it all back together tomorrow and burp the baby.
https://dl.dropboxusercontent.com/u/...3tstatseal.jpg

If you degreased it first, JB weld will be fine. That is exactly the type of repair it's intended for-I have patched large radiator blowouts with it before (2"x4")

I let the JBWeld set for 24hrs, sanded face smooth and now have tsta in and everything back together. I was about to fill with coolant, but couldn't remember if I had tightened the rad drain plug. I gave it a small twist. Crack!! OK, I know I was warned. I ordered two. :o

I removed the temperature switch on the tstat housing. Just seems to me like a good place to vent air as I refill. Will see how that goes.

:( Interestingly, the Factory Service Manual (FSM) (20-010) specifies a tightening torque for the radiator drain plug for models 123, and 126 as 1.5-2 Nm, and "This torque can be set by means of a washer or coin."

Oh well, they're not very expensive, and it was probably due for replacement, anyhow. :) If you don't have one, you can get a FSM on CD from Mercedes. They were selling for $20 a few years ago; I don't know what the current price is.

Edit: There is a link to the FSM in the Condensed Sticky Thread.