Differences in thermostats
 

While in eastern Massachusetts we enjoyed 70 deg. F weather, I did one of the items on my Mercedes to do list ....I took out the Wahler (auto zone) which is marked 80 deg.C, and keeps the coolant temp. at a steady 81-82 degrees, according to the dash gauge. I changed to the Behr, which is also marked 80 deg. C, and did this to "winterize" the car (oddly on a 70 F day in January!), because this thermostat keeps the coolant temp. at a nice toasty 93 to 95 degrees C (better for heat in the cabin)!

I just thought I'd mention it....probably covered in other posts. In case you think your car runs too hot, change the thermostat to a Wahler. If you have a wahler, and want better winter heat, change to a behr.

Mark

If the thermostat is marked for 80* and it keeps the engine 93-95* then it's a bad t-stat or poorly built by the manufacturer.

Another consideration is that since this is mechanical device, drawing conclusion from only one sample of each may be a reach.

I'm waiting on my fifth thermostat, 174 F from Autozone which is supposed to be OE temp. I've gone through three thermostats from Fastlane that were all supposed to open at 80 C, none of which even started to crack till almost 90 and causing my car to run right at 100 C. I got a 180 F degree from Autozone and it also didn't open when it was supposed to. By the way, the radiator and water pump are new within two months and I also did a citric acid flush and made sure to get all the air out of the system.

hmmm... I put in a new one and my car runs at 90C. Should I be thinking about puting in another new one?

me too
 

its, funny, because just yesterday i did a flush and a stat change an while not running hot , my old sta kept it at justa hair over 80c but the new one is closer to (90-100c) i thought this was odd, maybe i just need to wait for the air to get out?

I learned a trick about 20 years ago on a BMW 2002 I had which had an external tstat ... have used it on every tstat I have installed since then - I and several other BMWCCA members went through dozens prior to finding ones that actually operated properly - someone came up with this idea and it appeared to work then.

I drill 3-4 tiny holes around the perimeter - the portion of the tstat that is thin (the part that moves) .. while I don't KNOW if this had made a difference since then in the 10-15 tstats I have replaced or simply removed and then reinstalled, I have not had a tstat that didn't appear to operate properly since then.

How tiny are we talking? I took one and drilled just one the size that Diesel Giant shows on his page and it kept the car from getting above 60 C. I was toying with the idea of drilling tiny holes in one if the one in the mail doesn't fix the problem.

This was the third in a row that did this, two from Fastlane, one from Performance....two were behr (germany) and a third one (bought to try a different brand) was made in France (I forgot the brand)....at $15 to $20 each, and being in the Summer when I did NOT want the car to run hot, this was very annoying.

Finally someone here had the same experience and suggested that I get a Wahler from Auto Zone, and the thing runs slightly above 80...just perfect. So now, I use one of the Behrs for winter.

Mark

Sailor,

I had not seen the Diesel Giant trick, but I just reviewed the photos - holes that I have drilled in are far smaller - maybe 1/4-1/2 that size - his look awfully large ... I also just spread the holes at about 90 degree points.

While I can't prove it, I believe the extra flow of water as it heats up assists the heating effect on the tstat itself - perhaps ensure more positive opening?

i would not drill any holes in my stat.

they are designed by real engineers. benz has been using this style of stat for over forty years that i know of personally. many other european cars use the same style of stat. they work properly left alone.

drilling holes in them is not a good idea, imho.

tom w

The original T stat which comes with the OM616 is 78C, keeps the engine at 90C flat, MB then decided to give out 72C T stats which keeps the engine at 80C flat, 82C under extreme heat conditions.

Good inputs - however, consider that there are many, many folks who report having tstats that don't work right. They can be "designed" by real engineers (but, in this case, the design work was done about 80 years ago, I think ... recently simply a lowest cost production issue), but still respond well to custom mods that could not be done to meet the price points in large scale production.

This variety of tstat has stood the test of time pretty well - think about them being virtually unchanged for decades - when everything else on the engine has changed substantively. What if tstats had evolved the same way - imagine a sensor based electric system that measured water in 3-5different places, also using inputs from other thermal sensors and position sensors to anticipate loads so that as you start up a hill, the tstat opens in advance of the load -maintaining temp +/- 1 degree ALL THE TIME. If it is gonna be electric, why not make the coolant pump electric, too (I think this is already in the works with move to higher voltage systems) - and then the pump could work with the sensor system and the tstat to completely control thermal issues without belt drive - it could even operate off the battery when the belt does through itself .... pretty cool!

The "extra flow" should have the opposite effect on a bypass system. The added flow would be from the radiator.

Now for the $64,000 question. Is an 80 degree thermostat supposed to maintain a coolant temperature of 80 degrees? Or does the thermostat temperature rating describe the temperature at which the thermostat begins to actuate? At what temperature does an 80 degree thermostat completely actuate?

Yeah Yeah!!

an 80 degree stat is designed to maintain 80 degrees. it will begin to open at some lower temp and will be completely open at 80.

tom w

go right ahead and do whatever you want.

but i would not if it were mine. it is not rocket science, but the system is designed to work all together and it works very well when all components are in good working order.

folks who have problems have one or more elements that is not working properly. modifying the stat will not help, imho.

tom w

The service manual for the 617.95 engine states that an 80 degree thermostat should begin to actuate at 80 degrees and not fully actuate (main valve fully open and bypass valve fully closed) until the temp (at the thermostat) reaches 94 degrees. So it would seem that the thermostat that maintains the coolant temp at 80 degrees is the one not performing to specification.

Relying on the coolant temp gauge to evaluate thermostat performance is further complicated by the fact that the thermostat and temperature sensor are not co-located, at least not on the subject engine. When the thermostat is fully actuated, it is "seeing" coolant that has just exited the radiator, which is arguably cooler than the coolant to which the temp sensor is exposed.

In short, having an expectation that the indication on the temp gauge should equal the thermostat rating is not well grounded.

well, i wont argue with the factory shop manual.

but i will say in my experience which spans over thirty years, an 80 degree stat has resulted in a normal operating temp of very near 80 degrees unless the engine is working under severe conditions in which case the temp will go higher.

i apologize for any misinformation about when it opens fully that i apparently disseminated.

tom w

I'll just throw in my experiences. This comprises around 75K on a '90 300E, 140K on my '92 300E, 10K on a '93 E320 and currently 6K on a '97S320.

All engines had '87 degree stats. The '90 and '92 were M103's temperature gauge was pretty much always one needle width above the 80 degree mark.
I don't remember the '93 particularly, but think it was the same. Definitely not at what would be 87 degrees if it were marked.

When I got the '97 and had put several thousand miles on it, I noticed that it was running right at the 80 degree mark. I swapped thermostats between '92 and '97 and sure enough the '97 ran hotter. Stuck or sticky thermostat. Swapped back and then replaced with Dealer 87 degree unit. It now runs at what would appear to be proportionally higher level on the gauge.

So I conclude that as long the indicated temperature is somewhere in range above the 80 mark and below the 100 mark and is consistent day to day, then I'm happy.

Steve

your experience seems to indicate that in the cars you have owned the t stat sees higher temps than the guage sender.

on the cars that i am more experienced with, the 123s and 115s , they must be closer. they have 80 degree stats and run around 80.

tom w

I was on K6JFR's website looking at his section on Water Wetter. He describes assuming that his water to coolant ration should have been correct given that the car had been serviced at the dealer. He found it to be incorrect.

I have the same scenario. Car had it's 90K dealer service at 99K and coolant was changed. Think I will buy the little Prestone ratio tester and check it out.
Maybe the coolant to water ratio is too high, which would cause the engine to run slightly hotter than I was used to in the 124's.

Steve

Steve

warm
 

Which stat will make the motor run warmer...my 617 seems to run much better at an elevated temperature..and which makes it run coolest

Engineers may design them but I am not convinced that the actual product is a very exact device. I have seen t'stats all over the place versus what they are stamped. Right now I am running one stamped 90dC in my 240D (winter thermostat) and it will get to 100dC in traffic which is fine for keeping the inside of the car toasty when sitting at a traffic light.

i have always used german made stats.

i have found them to be remarkably accurate.

if you are reaching 100 at idle i would suspect a problem.

tom w

I just bought a new MB OEM thermostat from the dealer for my newly acquired 300CD Turbo. The old one was stuck open and the car never got above 50C. The new one runs it at 90-95C constantly.

The radiator is brand new and since I know the cooling system is in good enough order to keep the car running at or below 50C all day long I can say conclusively that this new thermostat is keeping it running at these temps by design in my opinion.

I didn't look too closely at it for a makers mark or temp reading but the car does not overheat...it never goes above 95C and seems to run fine at that temp, so I am leaving it alone...FWIW I think there are some that keep them at 80C but I do think the MB factory model intends them to run 90-95C now for some reason.

Thermostat operation.....
 

I just changed my thermostat to one that I purchased from my MB dealership, because I wanted exactly what was designed for the application......so I have an official MB thermostat, marked for 80 C.....

The thermostat is designed to open at 80 C to regulate the water entering the block for 80 C.....your temperature gages are reading a sensor in the cylinder head at approximately 90 C.....this is a normal 10 C temperature rise from water entering the block at 80 C and exiting at 90 C....

For those of you without an MB service manual.....there is a little arrow stamped on the edge of the MB thermostat.....this arrow points up.....

The thermostat that I removed was an AutoRad made in West Germany...had a slightly different construction and did not have an arrow.....

SB

it sounds like you got a higher stat. maybe an 87, i think maybe that is a common one.

tom w

It had no arrow on it that I remember seeing...but I think those are only on the ones with the little vent holes and this one didn't have that either. If it doesn't run any warmer in the summer than it does now I don't plan on doing anything more with it...if, for some reason it runs warmer I will pull it and look at it again to make sure I didn't miss anything during the installation.

Exactly. When the thermostat main valve is open, the temperature on the gauge should always be higher than the temperature at the themostat.

Tstat determines the MINIMUM termperature of the engine, given circumstances conducive (i.e. enough thermal transfer) - right?

Temp while at idle in traffic is determined by the ability of the thermal transfer system - coolant, two types of fans, radiator, etc - to deal with the extra heat - as they kick in the temp will never go back down below the set point. That said, I have had engines cool below the tstat set point on extremely long grades when the cooling effect was simply too great ... but, this was not excessive.

Maybe we should do a poll more formally, asking where everyone's needle points in normal operations - i.e. when tstat is determining minimum temp. Do it by specific engine type, and look for the most common position.

Both my previous 300CD and my current 350 SDL maintained temp so that needle pointed about one width above the 80C mark on the gauge.

that is where mine run. i cant remember what my 124 bodies did, it has been a good while since i had them.

when i first got the 350sdl it ran around 60. that is not good. i think it contributed to the sludging of the engine.

tom w

i am so confused on what thermostat to buy, at the moment my car doesnt have one, it hasn't had one for 4 months because the ones ive put in make the car over heat or run at 120

Buy a Wahler from Auto Zone...it runs coolest...just above 80C

i bet you put them in backwards.

tom w

well i just ordered one from dieselgiant. does anyone know off hand what the ones he sells are rated at? just curious.

also, other than the tstat working or not working correctly, not being the right temp rating for the right car, or having being modified....i have a 93 ford e150 van, it runs "hot" when on tour. but its really not. even when not loaded down and just using it as a daily driver...it reads hot. its because the temp sending unit is not accurate anymore. so even though the engine/tstat/coolant is correct and running cool and smooth, the gauge is showing hotter. the location of this sending unit, cost and the fact that its a van, being hard to work on, i have decided not to change it. have had it for four years, numerous tours across the states and have had no issues with it. just a thought.

Lots of Factors
 

There are a lot of factors affecting the quality and performance of products including tsats as we all know.

In theory, it should work as designed... but thats in theory and may not always work as designed. Thats why not same products perform the same. Variables include workmanship, material, installation, other components in the cooling system, etc.

As said, Dieseldiant has a pictorial on drilling holes to help adjsut the temp. I believe that one big factor (covered in a discussion in this forum) affecting cooling performance is the oem radiator vs aftermarket. Based on the performance of my 84D (aftermarket rad) vs my 85D (oem rad), I beleive that the oem rad does not cool as well. The cooling performance of the rad has a direct bearing to the engine temp. I don't think this means that the oem rad is no good. Its just the the non-oem maybe more efficient (more surface area?).

Of course the stat is a key element. The oem stat is designed to work with the other oem components in the engine.

my 2 cents

I'm not sure that's possible......
 

Although I did not make the attempt, from what I saw when I changed my thermostat about 10 days ago......I don't think it can be inserted backwards.....do not think that the thermostat will clear the housing if inserted backwards.....

SB

BUT....don't forget the arrow....:D

I heard a rumor that a new, clean radiator transfers heat better than an old, gunked-up radiator.

Well, after watching the temp gauge on my newly acquired 300CD wander around after replacing the thermostat with a brand new MB one I decided to replace the temp sender.

Same thing happened with the new sender...first start of the day the car would run warmer than I like - going up as high as 100C before dropping down, then when I would pull up to a light the temp would drop below 80C...so I went to NAPA and bought a WAHLER thermostat and replaced the MB one...now the temp seems to be rock steady at just above 80C on the gauge

I have concluded that the MB T-Stat was defective...I call these "lazy" thermostats because they open late and close too slowly...anywho...it seems that I will be going back to the MB parts desk with this one and getting a refund as this Wahler is working fine.

Wow, lots of people chasing a problem that is not really a problem.

#1: I don't believe the temp gauge is really precise. As long as mine reads somewhere in between 85 and 105 C I do not worry about it.

#2: I cannot think of anything wrong with the car running at 95C all the time. In fact that is a good operating temp. It's not good to consistently run the motor cool, and it's obviously bad to run it hot.

A properly operating thermostat will keep the operating temperature relatively steady. I've had many cars, any many Mercedes and when you see the temp gauge moving around under normal operating conditions something isn't right. The only time the temperature should fall below normal is if you happen to switch the heat on while the car is at or near idle...otherwise it really should operate within a couple of degrees of the design temp.

The MB stat I removed would spike over 100C and drop to under 80 during routine running...that's just wrong. The new Wahler behaves normally, the car warms up and the temp stays steady. These gauges are pretty accurate if they're working right, and this one is.

A car which runs at 90C+ is above its design temp. When new, these cars ran at just above 80C...a car running at 95C-105C means either the gauge is wrong or the engine is running warm. 80-85C is not cool...it's normal for these.

Very true. I just often don't believe that the car is running at the temperature displayed on the gauge. My car is 25 years old and has a quarter of a million miles on it. There is a long list of things that are no longer within design specs. When mine says 90C, it might truly be running at 80, or 85, or 95. The only way to tell for sure is to use an infrared heat gun. I find that it's splitting hairs for me to worry about the gauge reading as long as it's relatively normal (i.e. not 60C and not 115C.)

As far as your wild fluctuations in temperature, agreed, that was most definitely not normal!

Nothing offical, I've heard the 80 C stats keep 90 degrees C. I have replaced two 80 Behr's both read 90 at gauge. For all it's worth.

oh yes they can be put in backwards!!!!

don't ask how i know!

tom w

I just don't buy the excuse that the temperature gauges are not accurate. It's easy to say, but I've never seen one that really was. All the gassers I've ever had sat right on the thermostat setpoint regardless of operating conditions. The Mercedes diesel seems to be a completely different beast that varies it temperature depending on load -- mine sits around 80C +/- 5C until I get up to freeway speeds, then it rises, and if I'm cruising up a hill, especially at high altitude, I can get close to 100C.

I also hear the thermal load for a diesel is higher, but I cannot understand how. A diesel is a more efficient engine, and in theory should shed less heat to the radiator as more heat is used doing work -- it has a cooler exhuast and better mileage than a gasser.

So, what is the purpose of the arrow on the thermostat anyway? I used to think those were on the models which had the little bleeder valves in them but I noticed there is an arrow on the MB one I just removed even though it has no bleeder...why does the arrow need to point up? It seems like the thermostat is symmetrical and there is nothing special about it when it points up.

The SD has a brand new OE thermostat........Wahler.

It runs at 82-85C. when it's below 2000 rpm with a light load.

It runs at 90C. when at 2800 rpm at highway speeds.

It runs at 95-98C. when at highway speeds at maximum power (held for more than 30 seconds). It very slowly returns toward 90C. after 10-15 minutes.

I've checked the cylinder head and confirm that the head runs about 7C. cooler than the gauge.


Therefore, don't draw any firm conclusions on the capability of a 617 to maintain a perfect temperature under all conditions.

The 606 may be different.

Brian,

I may be missing the point, but your SD does not sound as if it is controlling temp appropriately. While I can understand some increase in temp under heavy load, with appropriate air flow at speed the transfer should offset the increased heat load.

Something isn't right ... or, am I missing the point?

Well, it has a brand new water pump, a brand new thermostat, it behaves in an identical manner whether the ambient temp is 0F. or 95F...........so, if it's not behaving properly, I'm willing to listen to all rational ideas..........

You can't condemn the radiator if the performance is identical at 0F. It's the only item in the system that is "aged".