W203 Coolant Thermostat(s)? Sanity Check!

[Paul Goff]

I have done a search on this subject and have gleaned a lot of useful info but before I apply spanners thought it best to consult the all knowing ones on here ;-)

I have owned my 2002 C220 CDi for 6 months/4K miles now, and in recent weeks the fuel consumption has worsened by some 10%, particularly so when the ambient temperature is below about 6 degrees!

I drove about 120 miles in an ambient of 4C on Friday and took particular note of the coolant temp on the display, it never got as far as 80C. Back in the Summer it would get to just over 90.

While searching for info I found a very useful schematic for the cooling system on the car and a lot of great discussion.

Yesterday I went out for an investigative drive and expected to find that the MAIN thermostat had failed "open" but this does not appear to be the case. I stopped and felt the main radiator about a dozen times in total and even though I managed to get the display to read about 90 the rad was never more than about 35!

The auxiliary radiator/thermostat/pipes were a very different matter though, they fairly closely reflected the displayed temp every time I had a grope. At a displayed 50 the aux rad was warm but comfortable, at 65 hot but holdable and at 80+, OUCH At this point the main rad was still just tepid.

Apologies for the long windedness, here are the questions.

I understand that the main stat should operate at 92 C(?) I would say that yesterday it did not open at all because the coolant never got hot enough for it to do so? If it was stuck open, surely the rad temp, as measured by my sweaty paw would reflect the displayed temp?

I believe that the aux stat has failed open? What temp should it operate at? But is this circuit, along with the transmission cooling circuit actually capable of providing enough cooling to hold the engine temp down so far when it's cold out? I drive very gently nearly all of the time!

Thanks for reading this far ;-)

Lily Lightfoot.

 

[mercedes13156]

Is the AUX thermostat you talk of sitting in front the radiator on a very slim pipe? I'm not sure it's on the water side. It might be for the oil cooler or the gearbox cooler. I'm not 100% sure though. I've had a couple of C Classes and they've only ever had one thermostat that needs changing. It's at the top right of the engine on the top hose to the radiator. Doing that certainly fixed the odd temperatures every time.

 

[Paul Goff]

Hi 13156,

Thanks for your reply, that's the one, behind the horns on my car.

According to my schematic it is on the water side and mainly serves to cool the water that runs through the fuel cooler, but just MIGHT also take some heat from the transmission cooler. It does take off from the return feed from the trans cooler but the only item deliberately serviced by it's circuit is the fuel cooler.

Lily Lightfoot.

 

[EmilysDad]

....
I understand that the main stat should operate at 92 C(?) I would say that yesterday it did not open at all because the coolant never got hot enough for it to do so? If it was stuck open, surely the rad temp, as measured by my sweaty paw would reflect the displayed temp? ....

In general ..... as I don't know your car ..... a stat that's stuck open will cause the engine temp not to rise. It will normally be closed when the engine is cold to prevent circulation through the rad. As the engine gets to 92, it'll start to open.

I reckon your stat is stuck open. Start your car from cold, place your hand on the top rad hose, it should stay cold till the stat opens & then quickly get hot. When a stat is stuck open, the top hose will slowly get warm

 

[Lord_Lucan]

I can't comment on the aux stat but if it's not reaching 80c on the display then that to me indicates the main stat is open. I changed one on my previous w203 and it was not an expensive "fix"....something in the region of <£30 and afterwards it would reach temp within 5-6 mins of driving at a gentle pace and faster if driven faster.

 

[umblecumbuz]

If after driving gently for over 100 miles in cold conditions, your display did not even reach normal operating temperature, then the thermostat is the first suspect. Definitely change it before doing anything else, and monitor the results. It's a straightforward and relatively inexpensive job. Mercedes 'stats are known for playing up. Your display should show a rock steady reading once the car has covered a few miles, so if it's fluctuating, again the 'stat is the primary suspect.

Bear in mind that consumption generally increases with colder weather, but over-cooling would also account for worsening fuel consumption. The radiator, particularly after a period of extended steady running, will often be at a lower temperature than the main engine block - its job is to dissipate heat, not store it, but it is the 'stat that governs the flow of excess heat to the radiator. If the 'stat isn't doing its job, the radiator temperature will be unpredictable.

 

[Paul Goff]

Thanks for your thoughts everyone

Just back from a foray into the big city (Exeter!) so engine now too hot to do anything constructive. I will have a fondle of the top hose tomorrow.

Lily Lightfoot.

 

[Lord_Lucan]

Thanks for your thoughts everyone

Just back from a foray into the big city (Exeter!) so engine now too hot to do anything constructive. I will have a fondle of the top hose tomorrow.

Lily Lightfoot.

Did it reach temp on the gauge? My gut feeling on this is the same as the others...the stats are known to fail.

 

[Droverunner]

Paul/Lily,

I think you have been sidetracked with thoughts of an aux stat/rad you've found. Simple fact is the main stats are prone to get tired/fail on these engines and seem to be a routine replacement at "X" years/miles. When we bought our CLK with the same engine as yours it behaved in the same way re temperatures. I checked with the guys on here who all said... "Change the stat (main)". I did and it sorted the problem and fuel consumption immediately.

So I'd 100% follow the advice above... to change the stat at the top right of the engine as you face it.

Our old one was working to an extent but it never fully closed. When I took it off it looked fine but could be blown through when cold... the new one sealed completely at the same temp.

See these threads...http://forums.mercedesclub.org.uk/index.php?threads/clk-220cdi-usual-running-temperature.163967/

http://forums.mercedesclub.org.uk/i...ant-thermostat-swap-tips.164120/#post-1557524

 

[Lord_Lucan]

Top tip: if you plan to reuse your coolant you can jack up (and support with stands!) the nearside of the car and that way you'll only have to remove the coolant from the reservoir which is fair less than the amount in the whole system.

I found it easier to remove my oil filter housing to get the socket on one of the bolts but if you do, please cover the opening so as to minimise the risk of dropping anything down there!

 

[Paul Goff]

Thanks again everybody,

I have just spent an hour with the car ticking over on the driveway and the displayed coolant temperature has snailed it's way up to 70 C! If anything the top hose feels cooler than that. So main thermostat it is then

With that in mind, which of the replacement market manufacturer brands (Mahle etc etc) should I go for?

Lily Lightfoot

 

[EmilysDad]

.....

With that in mind, which of the replacement market manufacturer brands (Mahle etc etc) should I go for?

Lily Lightfoot

Depends on how easily accessed your stat is. If it's easy to change, you can always go cheap & see how it lasts. If it's a b'stard of a job, then go OE.

Ask LostKiwi

 

[Droverunner]

Always a personal choice cost vs availability vs quality. I fitted a Gates stat as they are a brand I've used for critical car parts for decades but it was still a good price on Ebay. I think the difference between the Gates and a no-brand cheapy was only about £13 so hardly worth the saving.

 

[Lord_Lucan]

I went with Mahle on the previous car and it was fine in the 16k miles I covered after fitting it and before it was sold. Fuel economy improved vastly on mine but it was only reaching 60c before so hardly that surprising. I'm sure there's a good thread in the DIY/tutorial area which will help you lots....it's all easily accessible at the top of the engine bay in the OM646 220 engine if memory serves

 

[LostKiwi]

On my Smart (where changing a thermostat involves loss of much skin and the invention of many new swearwords) I will only use genuine after using a cheapo and having to repeat the job just a month later (the skin hadn't grown back properly yet!).
IF I were to use a non-genuine Gates would probably be on my list of 'acceptable' options.

 

[Paul Goff]

Decision made, Mercedes Benz, Exeter, £39.60+VAT inc a new seal!

Most works on a car are a b'stard as far as I'm concerned, may as well give myself the best chance of success ;-)

 

[EmilysDad]

Decision made, Mercedes Benz, Exeter, £39.60+VAT inc a new seal!

Most works on a car are a b'stard as far as I'm concerned, may as well give myself the best chance of success ;-)

The stat on one of the Astras I had could be done & dusted in 15 mins including time to brew up & drink it

 

[Paul Goff]

And a good reason for NOT getting parts from my local friendly Mercedes dealer is..................
....................... parking, or the lack there of!

Spent 10 minutes driving the compound and being warned off particular spaces by prowling staff, 10 minutes driving around the industrial estate, ended up 15 minutes walk away! Rang the parts dept and they told me to come back and park on some hatched markings near their door. 5 minutes later, thermostat in hand, we were on our way home.

Twas ever thus at main dealers though, don't know why I thought it would be different at a Mercedes one

Certainly something to incorporate in my decision making re parts sourcing in future though

 

[Droverunner]

Yep that's how I feel. Recently I've given our cooling system and associated items a mid life overhaul with new stat, water pump, aux belt, alternator over run pulley, belt tensioner etc.

It is so quick to look at Ebay to see prices, brands, suppliers, delivery dates etc then have it arrive at the door over the next couple of days... compared with the hassle of phoning around, getting folks to look stuff up, wondering if they are looking at the correct part, visiting places all over town to collect... and in the main dealers having that stress at the counter of asking them the price of every part before knowing if it was reasonable value or not.... and looking a cheapskate if saying no it's too much I'm buying elsewhere.

 

[mercedes13156]

On my Smart (where changing a thermostat involves loss of much skin and the invention of many new swearwords)

I have a good one "Custard". An inconspicuous mix of two words when in the grandkids' presence.