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Registered 2006 SLK200
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Discussion Starter #1
Hi there guys, I noticed that my temperature gauge goes down to 75 degrees Celsius when driving, when it should stay constant at 90 degrees. I have read quit a lot of forums here and the guys say its a thermostat problem, it gets stuck in the open position.
I do understand that so I decided to tackle a DIY repair. I have watched a video on how to do it, the only problem I am facing is that I don't want to lift the car up and remove the under covers to drain the water. I was thinking if it possible to just empty the water reservoir with a suction pump so that the water level is under the spot where the fitting is to the thermostat. Do any of you guys know if I do empty just the water reservoir will the level drop below the level of the thermostat so when I remove the black plastic pipe the holds the thermostat no water will pore out.
I have added a picture of the engine compartment with a red line showing where the level is below the thermostat and also a picture of the thermostat which is to be replaced. Any help on this will be much appreciated.
 

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Registered 2014 SLK250
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1,001 Posts
just undo any hose that is below the waterline and let a suitable amount of water out!
The only trick is to calculate or catch the drained water so that you know how much antifreeze to add when you top it all up after the repair.
 

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Registered 2006 SLK200
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Discussion Starter #3
just undo any hose that is below the waterline and let a suitable amount of water out!
The only trick is to calculate or catch the drained water so that you know how much antifreeze to add when you top it all up after the repair.



Thanks for your advice Hal, but I don't want to mess any water by removing pipes, remember when you remove a pipe water will flow from two holes and there is no room to catch the water. Do you think emptying the reservoir will do the trick?
 

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Registered 2014 SLK250
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Well, not really - the reservoir is there to take excess when it is hot and add it when cold again. You have to take the water from the business side of the radiator pressure cap.
I cannot remember exactly, but I thought there is a tap on the side of the block for draining water.
Anyway, why not just drain the lot, and flush it out at the same time and start again?
 

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Registered 2006 SLK200
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Discussion Starter #5
Well, not really - the reservoir is there to take excess when it is hot and add it when cold again. You have to take the water from the business side of the radiator pressure cap.
I cannot remember exactly, but I thought there is a tap on the side of the block for draining water.
Anyway, why not just drain the lot, and flush it out at the same time and start again?
Like I said I don't want to go under the car removing the covers and then draining. Here is a video of the job, I just want to change the thermostat without going under the car.If the reservoir is connected to the water in the engine surly when the level in the reservoir falls so will the level in the engine?
If you have a glass u tube open on both sides and you pore water in one side the level of the water will be equal on both sides, surly the coolant water in a car works on the same principal?


https://www.youtube.com/watch?feature=player_detailpage&v=Oyobl5jBCI0
 

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Registered 2014 SLK250
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A bit like saying I want all the facilities in life without paying income tax.

You will either have to take the covers off or find a place to partially drain. Full stop.

The car coolant system is not like a u tube in any shape or form. Pressure comes into it. Again, the reservoir is there to accept surplus water from positive pressure and return it during negative pressure. By all means drain it but you will get a rush of water when you take off the t'stat housing.
 

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Recently I tackled this same job on my car. It looks like thermostat replacements are quite common with this vehicle. I think it would be best to bite the bullet and remove the underside cover. There are about 6 or 7 screws holding it on. This will give you access to the radiator drain, and then you can cleanly drain off most of the coolant. Another issue occurred to me after I had started the job. I'm not the most sure-handed mechanic and more than once I dropped the tools I was using. One time was a socket, the other time a bit that was inserted into a socket. They landed on the floor, so were easy to retrieve. With the cover in place they would have landed on it, and might necessitate removing it just to retrieve them.

Even with most of the water drained, some water still came out along with the thermostat, so I would advise placing absorbent cloths or paper towels all around the thermostat housing.
 

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Registered 2006 SLK200
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Discussion Starter #8
Thanks for your replies guys. This morning I decided to let the car idle for half an hour to get the temperature up past the 90 degree mark to see if the thermostat will UN-STICK and close up more. Well after the engine reached the high temperature I went for a drive and happy to say that the temperature remained at about 90 degrees. I will be monitoring the temperature for the next few day to see if in fact I need to replace the thermostat. :grin: I think the problem came about because two months ago the weather was unusually hot and I got stuck in traffic, that is when I think the thermostat got stuck in the open position and did not return to the close position until I let the car reach that temperature this morning, which UN-STUCK the Thermostat again. Well I think I got my wish, no going under the car for now. :Banane23:


By the way anybody know at what temperature does the radiator fan turn on?
 

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Registered 2014 SLK250
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All depends on what market you are in and what thermostat is used - around 95 - 100 degrees though. Check with your dealer.
 

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Registered 2006 SLK200
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631 Posts
Discussion Starter #10
All depends on what market you are in and what thermostat is used - around 95 - 100 degrees though. Check with your dealer.

Just checked on another forum and there they were talking about the fan going on when the A/C is on, so I tried putting the A/C on full and true enough the fan was turning whilst the engine was still cold. Thanks any way. Check out my new update on the thermostat.
 

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Registered 2006 SLK200
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Discussion Starter #11
I just went out for another test drive and sadly it's doing the same thing, when driving on the highway the digital temperature gauge drops to 75 degrees from 90 degrees. What I don't understand is that it drops in a matter of 30 seconds. Is it possible that the water temperature can drop that fast if it is the thermostat that is stuck or could it be the sensor itself which is faulty? When I came home I pulled the sensor plug out and pushed it back in a couple of times just to make sure that the contacts are perhaps not making a proper contact. I will go for a test drive later to see it that helped. Any thoughts on the solution?
 

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Registered 2014 SLK250
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Boy, are you making hard work of this!

yes the fan goes on when the aircon is on - you would go on too if you were pumping that stuff around. You did not ask when the fan goes on, you asked at what temp will it go on. There are several factors governing when the fan comes on. Have a look at how many radiators there are in front of the fan will maybe give you and idea why…

So temp is 75 degrees. What harm is this doing? Is the needle in the red or the white? It is in the white. No panic. Yet. What is the heat from the heater like? Still hot? Probably a sensor playing up. Cooler than normal? Either a sensor or the stat. Change the sensor first. Are you changing altitudes in driving? If yes, then leave it. Normal. Is there air in the water system? If yes, bleed it, if no maybe sensor maybe stat. Has the fan come on for some other reason? If yes, leave it. Normal. Does the needle go back to 90 eventually?
 

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Registered 2006 SLK200
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Discussion Starter #13
Boy, are you making hard work of this!

yes the fan goes on when the aircon is on - you would go on too if you were pumping that stuff around. You did not ask when the fan goes on, you asked at what temp will it go on. There are several factors governing when the fan comes on. Have a look at how many radiators there are in front of the fan will maybe give you and idea why…

So temp is 75 degrees. What harm is this doing? Is the needle in the red or the white? It is in the white. No panic. Yet. What is the heat from the heater like? Still hot? Probably a sensor playing up. Cooler than normal? Either a sensor or the stat. Change the sensor first. Are you changing altitudes in driving? If yes, then leave it. Normal. Is there air in the water system? If yes, bleed it, if no maybe sensor maybe stat. Has the fan come on for some other reason? If yes, leave it. Normal. Does the needle go back to 90 eventually?
Hal, I asked about the temperature before I read the other forums, I did not remember that the A/C also has a radiator to cool the gas. I was just worried the fan did not work that is why I asked at what temperature.
Regarding the coolant temperature, I read on these forums that more fuel is used and not good for the engine if it is running below the temperature it was designed to run at 90 degrees. Seems a waste of energy to me, I would thing the less heat the less fuel consumption.
There is no needle it is a digital gauge, must still check the heater but I would think at 75 degrees the air would still be quite hot but I am sure the temperature on the gauge will go down even further, I remember a while back when I had the heater on it went down further, can't remember to what though. As regards altitude, does 100 meters count much? It has the same problem at sea level. Don't know if there is air in the system, bought the car in January but it has full history from the MB dealers. How would one know if there is air in the system? I don't know if or when the fan comes on when I am driving, I have only noticed it come on when the car was parked and I turned on the air/con. The temperature gauge always goes back to 90 degrees when the car is stopped and idling for a while. As I said yesterday I idled the car for 30 minutes and then the went for a drive, It stayed there for the whole drive. Maybe it's the sensor, when it reaches 90-95 degrees it behaves good for longer. Still would like to know if the coolant water can go from 90 degrees to 70 degrees in 30 seconds when driving at 60 kilometres per hour in 18 degrees weather with a thermostat stuck in the open position. If it can not then the sensor or temperature gauge is not working properly. If someone can answer me that question then I will know if it is the thermostat or not, may even be a combination of all the things discussed.
 

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Registered 2006 SLK200
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631 Posts
Discussion Starter #14
Hal, if I do decide to change the thermostat I will pump out the water from the reservoir, I noticed there is a thick pipe at the bottom of the reservoir which goes to the bottom of the radiator. There is a thinner pipe that joins the pipe at the top near the sensor, I presume it is where the air goes in when draining the system besides other reasons I have not thought about. I still think I can get enough water out from the reservoir by sticking the pipe in down the pipe to the bottom of the radiator. I still think the water in the engine will sink down as I pump out the water in the reservoir with the cap off allowing the water to move down on the same level in the engine and the reservoir, allowing the air to enter the engine side through the thin pipe from the reservoir. I am sure I can get much more water out from the reservoir than the reservoir can hold so I can do the job because as the water in the reservoir drops so will the water in the engine, that is why there is a minimum mark in the reservoir to keep the water level full in the engine side. Am I right or wrong, am I missing something I don't know?
 

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Hi B,

All things are pointing to a thermostat.
When driving at 60 in 18C temps the temperature will drop quite quickly.
When stationary with no air flowing through the radiator the temp will rise up to the trigger point for the cooling fan to activate.
This temp would be around 105C - that is about the temp the cooling fan switches on my '55.

The level mark you are referring to is on a resevoir that is not directly connected to the main coolant circulation. It is there to take overflow caused by temperature expansion and return it to circulation when cooling. This is via non return valves so that pressurisation is maintained in the main system.

Changing the thermostat would be easier and cleaner on a hoist - if you were local you could come over to my factory and use mine.
 

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Registered 2006 SLK200
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Discussion Starter #16
Hi B,

All things are pointing to a thermostat.
When driving at 60 in 18C temps the temperature will drop quite quickly.
When stationary with no air flowing through the radiator the temp will rise up to the trigger point for the cooling fan to activate.
This temp would be around 105C - that is about the temp the cooling fan switches on my '55.

The level mark you are referring to is on a resevoir that is not directly connected to the main coolant circulation. It is there to take overflow caused by temperature expansion and return it to circulation when cooling. This is via non return valves so that pressurisation is maintained in the main system.

Changing the thermostat would be easier and cleaner on a hoist - if you were local you could come over to my factory and use mine.
Thanks for you input and offer to loan your hoist Tricolour, your answer to my question that it is possible for the temperature to drop quickly is convincing me that it more likely it is the thermostat. There is a local mechanic shop about 300 meters away from me whom is also kind enough to let me use his hoist on a Saturday morning when it is not too busy, if I decide to drain and change the thermostat.
I can quite understand now from what you said that there are non return valve between the engine circulation and the reservoir preventing the water to return back into the reservoir if I try to empty the reservoir. I have a diagram of the coolant water system however it is not of my slk 200 but of a six cylinder, Perhaps you could inform me where these valves are on the diagram, the only ones I see are the ones by the heater core. Do you know if that thin pipe from the reservoir to the thick pipe next to the cover of the engine and also where the coolant temperature sensor is located drains the water out from there back into the reservoir when the engine is turned off? If it does will I be able to remove the temperature sensor without draining any water? If the car is cold and I remove the reservoir cap does the pressure in the whole coolant water system also drop to nil? Just another question, is there a way to check the coolant sensor without removing it but by unplugging the wires and measuring the ohms of the sensor, if yes what ohms must it read to know the sensor is not faulty?
 

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Registered 2014 SLK250
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Now you are going all over the place.
what you have on your car is not the same as a V8 and in your last post you are introducing a V6 circuit. Different again.
I do not know why you do not just drain the wretched thing and use it as a means to give the engine a flush. It is the easiste thing in the world - unless water is so scarce where you are. Besides, engines should be flushed every three years. You are due your third this year - so, combine all the jobs into one.
The irony of all this is that you claim to run cool. A reason for running cool can be t'stat sticking open. A reason for a stat sticking open can be crud in the throat. A reason for crud running around is a dirty coolant. Method to clean the coolant is, guess what? A flush.
As it happens, if you do have the type of tank with a thick hose to the bottom of the system, you just might be able to do what you want - depending on condition of valve. If it only had two thinner hoses, then you can't.
 

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Premium Member 2005 SLK55 AMG; 2005 SLK350
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You should try it and post back your results I'm sure others would like to know if it's possible to drain it down far enough the way you propose. One thing is clear: nobody here knows!
 

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Beppi, any thermostat which is operating questionably should be replaced. It is an inexpensive part which can cause very expensive engine damage should it stick closed, which it could also do if it is sticking open. Just replace the thing before it leaves you on the side of the road with an overheated engine.

Coolant on these cars is supposed to last 15 years before needing replacement, so I can understand why you don't want to simply flush it out and start over again. I would suggest that you contact a dealer mechanic and ask him whether your simple solution of emptying the catch tank would suffice to reduce coolant loss. They usually know the simplest way to do a thermostat replacement, because it is a very common device to eventually fail; probably isn't even that expensive to let the dealer do it, unless there are more complications to the job than we anticipate.
 

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Registered 2006 SLK200
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Discussion Starter #20
Now you are going all over the place.
what you have on your car is not the same as a V8 and in your last post you are introducing a V6 circuit. Different again.
I do not know why you do not just drain the wretched thing and use it as a means to give the engine a flush. It is the easiste thing in the world - unless water is so scarce where you are. Besides, engines should be flushed every three years. You are due your third this year - so, combine all the jobs into one.
The irony of all this is that you claim to run cool. A reason for running cool can be t'stat sticking open. A reason for a stat sticking open can be crud in the throat. A reason for crud running around is a dirty coolant. Method to clean the coolant is, guess what? A flush.
As it happens, if you do have the type of tank with a thick hose to the bottom of the system, you just might be able to do what you want - depending on condition of valve. If it only had two thinner hoses, then you can't.
Hal, Believe me the coolant in my car is clean and transparent green, the manual says to flash every 15 years, mine is only 9 years old. The problem is I don't know if I am running cool I don't know what the true temperature is. First of all I am not quit sure what temperature my car (SLK 200 / 2005 model) must run at, is it 90 degrees or 80 degrees. I have been reading different MB forums and some say 80 and some say 90. Second, whatever temperature it is how do I know if the temperature gauge is in fact reading the right temperature. How would I know if the sensor is working properly. The only way to know if the sensor is working properly is first to know what the water temperature in the car is, then to measure the ohms on the sensor. If the ohms reading in relation to the water temperature is correct then you know that the sensor is working properly, and if the sensor is working properly the reading on the temperature gauge must be the same as temperature you knew when you measured the ohms. So how can one measure the temperature of the coolant water when the car has been idling for 10 minutes so one can do the test? It is easy to test the sensor when the car is cold if you know the temperature of the air, if the air temperature is 20 degrees the water temperature should be also.
Lets assume the sensor is faulty by 10 degrees, it reads 80 on the gauge when in fact the water temperature is 90 degrees, changing the thermostat would be wrong if it a 90 degree thermostat. I don't want to just rush into doing something I have not got my mind around, that would just be a waist of time and money. The job is half done when you know what you are doing.
Here is a list of ohms reading for a MB sensor not sure if they are for my car's sensor but I tested my sensor and it read 3080 ohms @ an air temperature of about 18 degrees and a cold car and 410 ohms @ what the temperature said was 80 degrees, @ 85 degrees it was 350 ohms. So the question is how do I measure the temperature of the water when the car is hot so I can verify the reading on the temperature gauge. Once I can establish that both temperature are the same and I know at what temperature the car must run at then I can arrive to a conclusion to change the thermostat or not, that is if the car is running too hot or too cold. If I need to change the sensor only it only needs about two litre of coolant to be removed as the sensor is near the top of the engine. If the thermostat needs replacing a bit more. So if I can get away with just pumping out the coolant from the reservoir why not, I can DIY in my own garage at my own time without having to lift the car, besides I have not the right equipment to safely lift the car and I don't want to hold the car up with tripods, where do you put them on SLK without damaging things.
 

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