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#1
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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.
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RIP: 80 300SD RIP: 79 450SEL 2002 E430 4matic (212,000km) 2002 ML500 'sport' ____________________________ FACEBOOK: PANZER450 |
#2
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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.
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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine) 1988 F150 144,000 miles (leaks all the colors of the rainbow) Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra |
#3
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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".
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'84 300SD sold 124.128 |
#4
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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.
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90 300TE 4-M Turbo 103, T3/T04E 50 trim T04B cover .60 AR Stage 3 turbine .63 AR A2W I/C, 40 LB/HR MS2E, 60-2 Direct Coil Control 3" Exh, AEM W/B O2 Underdrive Alt. and P/S Pulleys, Vented Rear Discs, .034 Booster. 3.07 diffs 1st Gear Start 90 300CE 104.980 Milled & ported head, 10.3:1 compression 197° intake cam w/20° advancer Tuned CIS ECU 4° ignition advance PCS TCM2000, built 722.6 600W networked suction fan Sportline sway bars V8 rear subframe, Quaife ATB 3.06 diff |
#5
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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.
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Craig Bethune '97 SL500, 40th anniversary edition '04 Olds Bravada (SWMBO's) '06 Lexus ES330 '89 560SL (sold) SL--Anything else is just a Mercedes. (Kudos to whoever said it first) |
#6
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race cars don't use thermostats,instead a restrictor to slow water thru radiator.
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1999 w140, quit voting to old, and to old to fight, a god damned veteran |
#7
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Quote:
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. |
#8
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Quote:
thermostat for the SEL is $61 $6 for the gasket. a week away.
__________________
RIP: 80 300SD RIP: 79 450SEL 2002 E430 4matic (212,000km) 2002 ML500 'sport' ____________________________ FACEBOOK: PANZER450 |
#9
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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.
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