Correct radiator cap for a 280se 4.5? - Page 2 - PeachParts Mercedes-Benz Forum

Tomguy · #16 07-20-2016, 08:51 AM

Quote:

Originally Posted by Graham

The front fan is electric. It is actuated only if coolant goes above 100C (212F). You can test it by grounding the connector on the 100C temperature switch which is located at front center of engine above the water pump. I have never had that fan come on on my car under normal conditions.

The fan is supposed to be on 100% of the time the A/C is on as well.

If your car gets to, and stays at 212 without overheating, I wonder if you have the wrong thermostat installed? It doesn't sound like you're overheating, just that the thermostat - which is 100% in charge of your car's operating temp - is a 212 thermostat. The proper one is listed as 75C, which is 167F.

BWhitmore · #17 07-20-2016, 10:18 AM

The thermostat on any car is designed to allow the car to heat up properly, not to control the overall high temperature of the engine. The thermostat opens at a designated temp and then allows coolant to flow through the entire engine and cooling system in order to cool the engine. Before the thermostat opens the coolant circulates through the engine by a by-pass hose or tube. In normal operation once the thermostat has opened fully it has no control of the high temp of the engine. If the thermostat is stucked closed it will cause the engine to overheat quickly because the coolant is not circulating through the radiator. If stuck open and the engine overheats there are other problems.

Tomguy · #18 07-20-2016, 12:39 PM

Exactly, but since his car is getting to 212 and not moving from there at all, we can rule out issues with the cooling system, because it'd keep climbing until the coolant boiled. Therefore, the thermostat simply is not opening until the coolant reaches ~212 (give or take 5 degrees) based on the operating temp, the fact it isn't overheating, etc.

The thermostat, on a cooling system in PROPER order, is responsible for the max temp of the system. There are cases where this may not occur such as:

A cooling system with a bad radiator or dirty radiator, where it cannot cool the system; therefore the thermostat opens at 170 but the engine still gets hot because the radiator can't cool it. As a result, the temp will keep climbing, period.
The opposite, where the system has a bad clutch/mechanical fan, but a good electric fan. A lower temp thermostat that allows the engine to be say 170 on the highway but gets up to 212 in traffic until the electric fan kicks on.

Note from my 4.5 experience: Once you shut the engine down, the coolant stops flowing. The sensor is in the rear of the passenger head (if memory serves me right) where heat rises, so the sensor will rise in indicated temp as the coolant sits in the hotter block and heats up. The temp should rapidly drop as soon as you start it back up.

TroutMD · #19 07-20-2016, 01:56 PM

Thanks guys - It does sound like I have a thermostat issue then. I will also check if auxcilliary fan works properly, if I understand correctly it should starts spinning if I turn the A/C on when running idle?

Also, is the change of thermostat a fairly complex job? Thanks again.

Tomguy · #20 07-20-2016, 02:27 PM

There are 3 10mm bolts for the thermostat housing. It's an easy job... but after 40+ years, there's a good chance one or more of those bolts are rusted in place & may snap on the way out. Take care, spray PB or kroil or a similar penetrating lube (not WD40) on them 48, 24, 12, 6, and 3 hours before you start the job to try to make life easy. If it feels like it may snap, stop trying to loosen, and try to tighten instead for about 30 inch-lbs of torque, then try loosening again. Take your time, and you'll be glad you did.

meltedpanda · #21 07-20-2016, 03:02 PM

Gotta be the thermostat
are you sure the thermostat is opening, put in correctly right side up? Make sure you do not have an air pocket after R/R ?

BWhitmore · #22 07-20-2016, 04:13 PM

Be sure to make note of the position of the actual thermostat before removing. I have seen many thermostats installed upside down.

Graham · #23 07-20-2016, 04:32 PM

Quote:

Originally Posted by Tomguy

The fan is supposed to be on 100% of the time the A/C is on as well.

Is that the way the 280SE is? On other cars of that era, there is a 62C switch on the receiver/dryer that actuates the aux fan. I know I deactivated it when I switched to R134a on one car so fan would always come on.

Tomguy · #24 07-20-2016, 05:03 PM

Quote:

Originally Posted by Graham

Is that the way the 280SE is? On other cars of that era, there is a 62C switch on the receiver/dryer that actuates the aux fan. I know I deactivated it when I switched to R134a on one car so fan would always come on.

Always was on mine, unless a previous mechanic bypassed the switch you speak of & I was none the wiser.

Graham · #25 07-20-2016, 05:49 PM

Quote:

Originally Posted by BWhitmore

The thermostat on any car is designed to allow the car to heat up properly, not to control the overall high temperature of the engine.

I think that is correct. Once the tstat is open, the car will find an equilibrium temperature at which the heat transferred by the engine to the coolant will equal the heat transferred by the radiator to the air. The radiator's heat transfer capability is a function of air flow and temperature, so temperature can vary after the tstat opens. We all see this on our gauges, for example in stop/start conditions temperature goes up. When there is an imbalance, the aux fan comes on and tries to get the radiator to transfer a little more heat to the air!

BWhitmore · #26 07-21-2016, 12:33 AM

It is a common fallacy over the years that if a car is overheating that the problem must be the thermostat and it was completely removed in order to cure the overheating problem. A thermostat that was stuck closed would be a cause of overheating; a thermostat that was stuck open would not be the cause of overheating but in fact could be the cause of poor cold engine performance and not allow the heater to work properly until the engine was driven many miles to warm the engine.
The overheating problem was generally found elsewhere (improper ignition timing, bad water pump, blown head gasket, clogged or damaged radiator, clogged engine water passages -the list goes on & on).

Modern thermostats are designed that if they fail, they fail in the open position. Also on modern cars the thermostat plays a vital roll in emissions control, driveability, fuel mileage, engine performance and even engine wear. Changing the thermostat in a modern computer controlled/fuel injected engine to a thermostat with a different temperature setting can cause havoc with the computer/fuel injection and thus driveability issues. Many hours of research have resulted in modern thermostats that will actually improve performance, fuel mileage, and emissions if properly matched to a modified computer and/or fuel injection system.

TroutMD · #27 07-23-2016, 01:34 AM

I ordered the 75C thermostat and was reading up on the installation while I wait.

One M116/117 service bulletin I read stated that it is recommended to fully drain coolant, remove air cleaner (?) and disconnect battery/alternator prior to doing a thermostat change.

Is all of that really necessary?

BWhitmore · #28 07-23-2016, 06:34 AM

Disconnecting the battery is a safety thing, given that while changing the thermostat you are working in the vicinity of the fan, they do not want to see someone inadvertently crank the engine while your hands are near the fan. Removing the air filter will give you better access to the thermostat. Draining and replacing the coolant is a good maintenance item unless you have done so within the last year.

meltedpanda · #29 07-23-2016, 08:42 AM

whenever I mess around in the engine bay I disconnect battery. I cant tell you how many time i have laid a wrench on top of that battery , touched the cable and sparks flew

Coolant - only if it needs changed , and you will need everthing out of the way to get to that thermostat , including a long 5 or 6mm allen socket

gmog220d · #30 07-23-2016, 09:31 AM

The main reason for replacing the coolant is that the antifreeze looses its anti-corrosive properties over time. Suggested intervals vary, but I like to do it every two years. If you don't know when it was last done it's probably a good time to do it, while you have the system open for the thermostat replacement.