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Thread: Lower-temp thermostat for M119?

  1. #1
    Junior Member
    Join Date
    Aug 2018

    Lower-temp thermostat for M119?

    Hi all

    Apologies if this has been covered before. Am writing to see what temperature thermostat you all use in your 500E/E500. I have 75 degree thermostats in my 6.3 and 250C, which keeps the temps right around 175 degrees. I believe my E500 came with a 87 degree thermostat which is higher than I would like. Does anyone run a 75 or 80 degree thermostat? I am also installing a new radiator and fan clutch, plus all hoses, so I might as well change the thermostat. Car runs between 90-100 which is hotter than I would like.

    Thanks, as always.


  2. #2
    Site Honcho
    Ich bekenne mich
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    gerryvz's Avatar
    Join Date
    Nov 2008
    Annapolis, MD

    Re: Lower-temp thermostat for M119?

    Earlier MB engines such as the M110, M130, and M100 were designed to run with different (cooler) operating/cooling temperatures than later engines such as the M117 and M119, and thus used thermostats that were "cooler" by design.

    This cooler thermostat/cooler running design included the earlier, iron-block versions of the M116 and M117 engines (3.5 and 4.5-liter displacement).

    I mentioned a cooler running (71C) version of the thermostat that is designed to be used in the alloy-block M117 and the M119, in this post. It is readily available in the US through various vendors.

    My recommendation is to use the original-spec version of the thermostat for your car. This is based on geography and expected running conditions for the vehicle, and the "stock" operating temp is programmed into the computers of cars that have them (perhaps not applicable to pre-computer cars).

    In short, I would be very very careful about running a cooler-than-stock thermostat in a car that wasn't expressly designed for it. For M119 cars, which do have both ignition and injection computers, this is especially true. I would consult an expert like our member Klink to help with the decision to do this or not.


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  4. #3
    postwhore posterchild Klink's Avatar
    Join Date
    Feb 2014
    Abstract Plains, USA

    Re: Lower-temp thermostat for M119?

    I like running the thermostat that the engineers specified for the car in question. A fine point that is very often missed when people are discussing thermostats between different engines, thermostats that often differ by 5 to 10 C is that when used in their correct applications, these almost all result in fairly similar operating temperatures. A common enough example exists with the M110, though there are numerous others. It is well known that the M110 thermostat which is also used in a number of other engines has an 87C “start of opening“ temperature. Note that this thermostat is in the hot outlet area of the system, the area from where hot coolant flows TO the radiator. People have pointed out that in comparison, some diesels of the same vintage run an 80 thermostat of very similar construction. What they fail to notice is that on those diesels, this 80 thermostat is in the water inlet area. They are in the area where cold water flows from the radiator to the engine. This is one example of at least four that I know of where people compare "apples to oranges" when discussing the relative merits of either temperature. Do note that all M119 engines that I know of, and that’s probably all of them, have a thermostat that is in the cool water inlet portion of the system.

    Do note the location of the thermostats whenever you are thinking about relative temperatures, etc.

    Overall, the people that designed your car knew at what temperature relative to ambient they wanted the engine to run. They chose the cooling system components accordingly. Interestingly enough, manipulating thermostat temperature on newer engines often leads to comical results. Most of the engine management going into production from about 98 on have temperature versus time and load comparison maps and they will fault code for too cool of a thermostat, or a thermostat that has been “modified“ Lots of check engine light fault codes for a number of years now have been caused by lazy, broken and/or leaking thermostats.
    Emissions, fuel-efficiency and even to some degree engine life all really hate a slow warm up period. A number of models for years now have had thermostats regulated by the engine management system.

    Imagine a conventional wax thermostat with a say, 115 degree C start of opening temperature. Put a heating wire into the expanding wax portion, and there you go. They dummy up the instruments so the customers don't freak. Here's an approximation of what actually happpens: When the management is keeping the engine at a hot and scary emission and efficiency enhancing 115 when idling in traffic, the gauge shows a nice confidence building 88. When it is lowering the temp to 72 during a low load high speed cruise for lower NOX emissions, the gauge shows a nice stable 84. The gauge goes to undamped live when a malfunction or implausibility is detected, along with a warning of cooling system defect, and/or a check engine light. Truth is, they could probably completely dispense with the coolant temperature gage, as it is still mostly there just to satisfy tradition, just as oil pressure gauges were for a good decade or three after which they no longer served any genuine purpose.

    DON'T remove that pressure cap from a newer car sitting there at hot idle showing 85 on the gauge! You are about to spray REALLY hot coolant all over the place, including possibly your face...
    Last edited by Klink; 09-20-2018 at 12:15 AM.
    Putting the fun in dysfunction...

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  6. #4
    Junior Member
    Join Date
    Aug 2018

    Re: Lower-temp thermostat for M119?

    Thanks, guys - interesting information. I think the primary unnerving factor to me is the variability in the engine temperature - my other Mercedes (6.3, 250C, 300D-Turbo) all run around the same temperature unless it is really hot outside or in prolonged traffic, whereas the E500 varies quite a bit highway vs city, regardless of ambient temp. Hopefully a flush and fan clutch (fan clutch not engaging correctly) will help that.


  7. #5
    E500E Guru
    Join Date
    Feb 2011
    DFW Texas

    Re: Lower-temp thermostat for M119?


    I've found on my car- the temp is very stable *IF* I am utilizing the climate control system(aka w/ the a/c on). The a/c system has a high pressure switch which kicks in during low speeds to keep the freon pressure down to a reasonable level which also keeps it cooling. That has a net effect of keeping the car running 90 c. For me, that seems rock-solid till I get above 100-105 F outside. This last year, I had to replace that pressure switch- oil consumption dropped and the temp went from bumping 95-105c, back down to the normal 85-90c.



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