Help me Understand: Themostats
 

Here's what I know, or think I know, about thermostats.

It stays closed when cold to a certain temperature then opens allowing hot water to flow out to the radiator and pushes the cooled water into the engine, and when the cooler water reaches the thermostat, it closes partially, or completely, depending on the temperatures, to allow the cooler water to warm up to the opening temp, and repeating the process.

In a FAILURE mode, If closed, It will NOt allow the cooler water the enter the engine, and prevent the now, boiling water to leave the engine, resulting in an overheating situation.

Herein lies my uncertainty:
If the failure is open, Then the entire water volume increases in temperature, to overheat, or when in motion or with increased airflow over-cool.
When the thermostat is stuck open, it disrupts the 'pulse' of cooled water and can't overcome the rate at which the engine is producing heat.

I don't think there is a pulse as you describe. The thermostat achieves a steady state mostly open position at the intended operating temp of the engine (set point of the thermostat). If the engine becomes too hot then the thermostat will open even more to try to cool it down. In my experience bad thermostats fail open so the engine does not warm up enough or fast enough. If the engine is overheating it usually a clogged radiator, but could be other things such as bad head gasket or bad fan.

Surprise!
 

The Hydro/Thermo Engineers in Stuttgart have designed the Cooling System/Passages/Thermostat Housings in an "Maze" configuration such
that the Thermos have Three distinct Operational Phases.
1.Closed
2.Variable/Bypass
3.WOT

(With what we've learned about German Engineering in the last Hundred Years
the Systems are designed to default/Fail "Open".

While the typical failure is stuck open, they can fail stuck closed. It isn't as common but it can happen and shouldn't be discounted in an overheating situation.

Mine (on a 1989 SL560) failed in the open position, leading to a slow warm up and rarely attaining full normal operating temperature. Not operationally a huge problem except that combustion and oil both work better at full temp, and on a cool morning it took longer than it should for the heater to have anything it could share with me. Don't know details, but as was said above, given German thinking I would expect that failure producing overheating is designed to be less likely.

Bottom line--It's not an expensive part, and should be checked if you suspect it for any reason.

race cars don't use thermostats,instead a restrictor to slow water thru radiator.

I'd disagree with the "uncertainty" part.

When open, the entire water volume is potentially exposed to the radiator. This differs from closed/partially closed when most of the water is restricted to the engine. So you have the same heat input (only source is the engine) and more heat output (greater exposure to non-heated or cooled parts) and it shouldn't overheat. Warm up times will be greater, and full temp may not ever be achieved since the engine may not be able to produce heat to overcome the rate at which it's being dissipated (to a certain temperature).

If a "closed" failure the coolant won't flow to the radiator and it will overheat.

A slight complication is the "bypass" feature on M-B thermostats. You could use a different word, e.g. "blend" or "mix".

When cold, the bypass is "fully open" (note: this is the FSM terminology) and the water "passes by" the radiator and stays in the engine. This accelerates warm up. There is no blending or mixing of 'chilled' coolant that has been exposed to the radiator.

When warm the bypass and main valves are "more or less open". This lets some coolant into the radiator for cooling but some coolant still "passes by" the radiator and remains in the engine to maintain higher temps. You get a blend of chilled and un-chilled coolant.

When hot, the bypass is closed and all of the coolant flows to the radiator for max cooling. Nothing "passes by" the radiator and all coolant is exposed to chilling by the radiator.

This is why M-B says removing (an otherwise functioning) bypass thermostat won't fix an overheating condition since it lets uncooled coolant go directly back into the engine.

Link the FSM description of the bypass sent via PM.

It's going to be replaced.

thermostat for the SEL is $61 $6 for the gasket. a week away.

The thermostat on my 1991 300CE failed in the closed position leading to overheating. Taking a look at the thermostat the brass bridge had broken in two.