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My 1983 Mercedes 380SL seems to be running hotter than I like. It settles down at right at 100 C by the instrument panel temp gauge after running at highway speed for 10 minutes or so.

When I first got the car, it had a bum O2 sensor that made it run rich, but on the highway it'd run about 90 C. After I fixed the O2 sensor, it started running hotter. I replaced the thermostat (and the water pump, as a result), with the one called out in the factory service manual. That specification is for a thermostat that starts opening at 84 C and is fully open at 99 C.

I had the cooling system power-flushed and refilled a few days ago. Aside from that, about the only thing left to try is servicing the radiator. That'll be all kinds of no fun, since when I tried removing it last year while I had the water pump out, it wouldn't budge...

There's no oil in the coolant, or vice versa, and no white smoke out the tailpipe. I'm pretty sure everything's intact. It's just running hot. The question is: is that too hot? Or just right? Or hot but acceptable?

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Nope that sounds about perfect. Remember there is a spring on the radiator cap that keeps the engine coolant pressure to around 13 PSI for most manufacturers. Water doesn't boil until around 242°F (117° C) at that pressure. The manufacturers want the water to stay as a liquid because that helps the radiator work better. (Liquid to vapor transfers heat MUCH better than vapor to vapor in a heat exchanger.) I wouldn't worry until the heat got up to 112 to 115°C numbers.. I would think for your car, 84°C to 107°C pretty normal.

One thing if you see that the temperature NEVER EVER gets above 100°C that could be a big problem. It means your radiator cap is probably not sealing well, and should be replaced.

And as a safety reminder, because the coolant cap is pressurized you never want to open the cap when the engine is hot. If the temperature of the coolant is greater than 100°C (212°F) and you open the cap, all the coolant in the car will immediately flash from liquid to steam. That is very very dangerous.

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  • I replaced the radiator cap when I replaced the thermostat. (It's on the coolant overflow tank, not the radiator itself, but same basic idea.) The cap is rated at 1.2 bar pressure. I haven't seen it get much above 100 C, but it has gotten to 105 while running once and up to about 110 when started after being shut off for a short time (long enough to get in and out of the grocery store). In the latter case, it dropped to 100 within 20 seconds after startup. I do need to look at the thermostat housing with an IR thermometer to make sure the gauge is reading right.
    – Jay Maynard K5ZC
    Commented May 8, 2016 at 1:17
  • To be pedantic, "all" the coolant won't flash to steam, because it takes about five times more heat to boil dry a quantity of water, than to heat the same water from 0 to 100 C. The danger is mainly because some flash boiling inside the cylinder block (which is likely to be the highest temperature) will violently pump most of the liquid water out of the radiator cap.
    – alephzero
    Commented May 8, 2016 at 5:59
  • This is probably a better link: The water changes from a liquid to a gas with extreme speed, increasing dramatically in volume. A steam explosion sprays steam and boiling-hot water and the hot medium that heated it in all directions (if not otherwise confined, ...), creating a danger of scalding and burning.
    – zipzit
    Commented May 8, 2016 at 7:40
  • I've had my head under the bonnet when a header cap was removed above 100degC, it was a small car (Pug 106) and foly huck I learned not to do that again! Also worth noting that heat transfer from radiator to air gets more efficient as the temperature difference goes up - the hotter your rad / the cooler the air, the more heat gets transferred.
    – John U
    Commented May 9, 2016 at 16:26
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Make sure your temp sensors are correct. It's not uncommon masking temp out of range w/incorrect sensor. Still battling over heating on 83 500sl

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