Just sharing my misadventure.
My W124 was running cold (temperature never exceeded 70degC). Checked at workshop and found the thermostat element missing - only left the housing.

Changed a new BEHR thermostat, now temperature slightly higher but still less than 80 degC on a hot day. Found out that a 71 degC rated thermostat was installed.

Bought a new 87 degC thermostat and changed it myself. Problem started. I could hear loud "tak tak tak" sound from the thermostat housing when engine hit 80 degC, and when driving off from cold the temperature would climb to 105degC before the aux fan cut in and temperature would drop and after a few times the temperature would be steady at 78degC even after long drive from JB to KL. Checked coolant expansion tank and level left half only.

(sambungan)

Coolant expansion tank would lose level after each drive when I topped it up but level somehow would maintain at half and not dropping further.

The scary part of seeing the temperature gauge climb to 100-105 degC to activate the aux fan, and dropping back to sub-80 degC continued. Ditto the topping up of distilled water to coolant expansion tank. By then, I realised coolant was lost via overflow/overpressure release hose at expansion tank.

Did leak check at workshop, not conclusive. Shit, head gasket leaking? Dammit. Stopped topping up the expansion tank and lived with half-tank of coolant.. still it feels lousy seeing the needle climb to 105 degC.

--- to be sambung-ed

Thanks for sharing...

Here are 10 steps for your consideration to isolate the fault...


COOLANT LEAKS

There are a number of things that can go wrong with your car's cooling system, causing the engine to run warmer than it should. The following is a list of things that are easily checked at home by the do-it-yourselfer, and can always add to your knowledge of cars in general.

Step 1

Understand that overheating problems may be caused by a low coolant level, the radiator being plugged, the thermostat may be stuck, or other related common cooling system problems.

Step 2

Check the coolant level at the radiator overflow/plastic coolant reservoir tank to see if you're low on coolant.

Step 3
Open the radiator cap and look inside the radiator when the engine is cold.

Step 4
Fill the radiator with a 50/50 mix of antifreeze and water if it's empty or low, and close the cap.

Step 5
Look at both the lower and upper radiator hoses located at the top and bottom of the radiator on the back side and held by hose clamps. Make sure the hoses are securely clamped to the radiator and aren't leaking.

Step 6
Touch both the upper and lower radiator hoses when the engine is warm and the car turned off. They should both be somewhat warm. If one is cold, you may have a thermostat that's stuck closed.

Step 7
Use the palm of your hand to feel the outside of the radiator from top to bottom when the engine is warm and the car turned off. It should be warm to the touch evenly throughout. If there's a cold section, you may have a radiator that's internally blocked.

Step 8
Check under the car, inspect the radiator and look around the engine compartment for telltale signs of a coolant leak: Coolant is usually greenish, slippery and sweet-smelling.

Step 9
If there is always a puddle of coolant under your car it could be due to water pump failure, or a cracked coolant reservoir.

Step 10
Visit your mechanic if you know there's a leak but can't find it. A mechanic can detect a slow or small coolant leak by pressurizing the cooling system.

----------------
Cheers/Mabel

Nothing great was ever achieved without enthusiasm
Take time to read. It is the fountain of wisdom
Membership Number: One Nine Seven Seven

sambungan-

Yesterday, decided to undo my last modification by installing back the 71 degC BEHR thermostat. Voila, no more symptoms of overheating and lost coolant.

Tested the 87 degC original Waller (spelling) thermostat by soaking in hot water in a pot. Realised it does not open even when the water was boiling.

Ha, now I know the "tak tak" sound was emitted when the temperature was high and it tried to force the thermostat to open. Once it opens up (at > 100degC), it remains at full open position and this explains why the temperature drops like a rock after it hits a high. The temperature rise was not caused by air lock in radiator as I thought.

When the thermostat suddenly opens up at > 100degC, it allowed full bore of water through the breather tube that is fixed to expansion tank and overpressures the expansion tank and the pressure is released via the overflow hose.

This mistake has cost me:
1) Six bottles of original Merc coolant
2) petrol used to keep the car idling during troubleshooting sessions
3) Endless sleepless nights worrying about whether I have damaged the head gasket and why I kept losing coolant and almost overheating the engine.

4) A useless 87 degC original thermostat which I'm trying to get a replacement from the shop.

...I see...thanks for sharing MB200E...

Glad you managed to overcome the fault..

----------------
Cheers/Mabel

Nothing great was ever achieved without enthusiasm
Take time to read. It is the fountain of wisdom
Membership Number: One Nine Seven Seven

MB200E, glad you berjaya but success never comes free.

Who says you can get a free lunch?

The original Merc thermostat made by Wahler

The 71 degC thermostat by BEHR

Comparison. L=Wahler, R=BEHR. They look different. One is fail open and the other fail close?

My family members still do not understand why I make myself sibuk by owning this old car. They think I should just get an Altis or Camry and be free of the misery of changing this and that and getting my hands dirty

Truth be told, I didn't know cars have coolant thermostat until this incident. And I am getting worried of turning into petrol head: this explains why I found it compelling to take photographs of the thermostats during the swopping over.