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Registered 1997 SLK230
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271 Posts
Discussion Starter #1
Of course I usually drive my SLK on nice sunny days with the top down but this morning even though we had a very heavy frost) and because I have my annual MOT coming up I thought I would take her for a spin. A quick blast to blow some cobwebs away and give her a bit of an Italian tune up! I don’t want to fail that wretched emissions test again for a high LAMBDA value through lack of use…
Cruising down the motorway the heating is fine but whenever stationary in traffic the heat falls away quickly and is soon barely warmer than the outside temperature. I have searched this and haven’t found anyone reporting it on an SLK but quite a few occurrences on other models – especially those with an electric pump in the system mounted by the Duovalve.
Anyone had (and fixed!) this or similar issue?
I fitted a new thermostat about two years ago as the original was stuck open and the engine was taking forever to warm up and giving rubbish fuel economy. It now heats up quickly and the temp gauge is stable at just over 80 degrees. My current thoughts are:
• There is still air in system somewhere from when I changed the stat. Seems unlikely given the water level is stable and the engine doesn’t under heat in winter or overheat in summer and the heating works well at faster engine speeds.
• The duo valve is causing some impediment to coolant flow at tick over. I guess this is the first and easiest thing to check but it seems to affect both passenger and driver sides equally. Also at tick over all hoses into and out of the Duovalve are hot and feel to be about the same temp.
• There’s some sort of partial blockage in the heater coolant circuit somewhere.
• The water pump just isn’t working as well as it should be.
• ?
Does anyone have any other suggestions – they would be gratefully received.
Cheers to all
Bob
 

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Super Moderator UK 2002 SLK320 Blue
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3,228 Posts

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Registered 1997 SLK230
Joined
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271 Posts
Discussion Starter #3
Yes I have been reading @Starvin's post with interest and admiration. Took another look at mine today and the duo valve definitely seems to be working fine. Whilst I had the front end up on ramps (to change the ARB bushes) and further elevated by the ramps built in hydraulic jacks I had another go at "burping" any air out of the coolant system. Looks like something came out as coolant level appeared to drop fractionally. There definitely seemed to be more heat at idle but time will tell. It was much warmer here today and it's also possible that I just got more of the coolant up to temp today. I will just have to see how it goes but I am considering buying (or making) a vacuum fill tool as I am due a full coolant change on both the R170 and the S211 this spring.

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Registered 1997 SLK230
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271 Posts
Discussion Starter #5
I tend to agree but having said that when I was testing yesterday the hoses at the duovalve got hot quite quickly even under tickover suggesting water is getting around the system OK although that could just be the action of thermo syphon. I have only just noticed this behaviour but it might have been like it since I bought the car 4 years ago or even longer. I will just have to see how it goes. If it's trapped air in the heater radiator that also makes sense as the amount of hot water actually in the radiator would increase at high revs as the action of the water pump compresses the trapped air. A vacuum fill would either confirm or eliminate that theory.

Has anyone made a hombrew vacuum fill?

I'm wondering if I could swap the drain valve at the bottom of the radiator for a good old fashioned brass one that you can attach a hose to. Then maybe I can modify a radiator cap so I can attach the pipe from my vacuum oil extractor and essentially suck in the coolant "bottom up" to minimise trapped air.


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Premium Member 2005 SLK55 AMG; 2005 SLK350
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5,152 Posts
My understanding (!) is that the coolant system will self-bleed after a few cold starts if you be sure to top it up after each run. The vacuum fill is more convenient because it gets it done in one pass, so it is preferable for a shop, but is not required for a DIY owner willing to take the time for several passes.
 

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Premium Member 2005 SLK350
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2,365 Posts
Of course I usually drive my SLK on nice sunny days with the top down but this morning even though we had a very heavy frost) and because I have my annual MOT coming up I thought I would take her for a spin. A quick blast to blow some cobwebs away and give her a bit of an Italian tune up! I don’t want to fail that wretched emissions test again for a high LAMBDA value through lack of use…
Cruising down the motorway the heating is fine but whenever stationary in traffic the heat falls away quickly and is soon barely warmer than the outside temperature. I have searched this and haven’t found anyone reporting it on an SLK but quite a few occurrences on other models – especially those with an electric pump in the system mounted by the Duovalve.
Anyone had (and fixed!) this or similar issue?
I fitted a new thermostat about two years ago as the original was stuck open and the engine was taking forever to warm up and giving rubbish fuel economy. It now heats up quickly and the temp gauge is stable at just over 80 degrees. My current thoughts are:
• There is still air in system somewhere from when I changed the stat. Seems unlikely given the water level is stable and the engine doesn’t under heat in winter or overheat in summer and the heating works well at faster engine speeds.
• The duo valve is causing some impediment to coolant flow at tick over. I guess this is the first and easiest thing to check but it seems to affect both passenger and driver sides equally. Also at tick over all hoses into and out of the Duovalve are hot and feel to be about the same temp.
• There’s some sort of partial blockage in the heater coolant circuit somewhere.
• The water pump just isn’t working as well as it should be.
• ?
Does anyone have any other suggestions – they would be gratefully received.
Cheers to all
Bob
Some of the early R170 had a coolant circulation pump mounted on the right side shock tower. Does you car have one? Is it working?
 

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Registered 1997 SLK230
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271 Posts
Discussion Starter #8
There isn't one now but maybe there was originally and a previous owner removed it. Many thanks for the thought. I'll have a look in WIS and EPC for my VIN and see if it shows anything.

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