I have recently purchased a 2006 Mercedes Benz ML500 W164 with approx 215K KM/130K miles on it.

A few days ago I was showing it to a friend who after a drive inspected the engine and mentioned he thought it was a little hot in there when I opened up the engine bay.

I've never had any sort of engine overheating warning and today I was keeping an eye on the temp gauge and it didn't really go over 84-85 degrees celcius.

However, yesterday after I got an oil change done with 0w40 synthetic Mobil 1 oil and today I also noticed it just seems extremely hot in the engine bay like the metal components in on either side which lay a little lower and are slightly brown in color from being so hot in particular when I sprayed those with a hose today released quite a large amount of steam and sizzle noise when they came in contact with water. Also in front of the plastic cover those metal components and the large rubber hose get quite hot as well. I know water starts to steam around 80C but boils at 100C so that metal down there had to have been at least 110C in temperature to turn water to steam like it did so quickly.

When I decided to hose it down with cold water quite a substantial amount of steam came from the engine.

Should I be worried?? I know engines are supposed to run hot and this is a big V8 but should they be producing this much steam when contacting water? I am a bit paranoid now as I purchased the car as a family vehicle and now sort of hesitant to drive it and feel more comfortable driving my chevy malibu.

Edit: After some research I believe that really hot metal component on either side of the engine that sits lower is called a "catalytic converter". And it was probably the hottest (released the most steam when sprayed with a hose).

2 Answers 2


Both exhaust headers and catalytic converters are meant to get really hot under normal operation, well in excess of the boiling point of water.

Though I don't have the specific thermostat rating for this model, 85 °C coolant temperature strikes me as quite unremarkable; neither too hot nor too cold.


One way to break, and then need to replace, expensive components like catalytic converters and exhaust manifolds is to dump cold water on them when they are hot... The cats can run between 800 to 1000 degrees C on the inside , so even with the insulation that is fitted around them they do get very hot and they are meant to.

Prices for cats range from about $800 upwards and some are seriously expensive... Which is why there have been posts about removing cats...

  • Not only the exhaust components, but what about heads and other metal parts on the engine itself warping ... very not good thing to do. Jun 19, 2020 at 20:50
  • @Pᴀᴜʟsᴛᴇʀ2 but the cats have a ceramic matrix which can shatter through thermal shock, water chucked on heads less so...
    – Solar Mike
    Jun 19, 2020 at 20:53
  • do you think it'll be fine if I just did it once? I was worried and believed to smell something burning. No engine lights came up so I assume it is fine. However, I do also live up in Canada and this past winter temperatures went down to around -44C (much much colder than that of the cold water which was probably around 20C from the hose that was left in the sun. \
    – sawreals
    Jun 19, 2020 at 21:13
  • @SolarMike - While heads don't shatter, warping one (or two) is as expensive a proposition as destroying the ceramic matrix in a cat, if not moreso ... now ... consider doing both at the same time. Might as well throw the car away and get a different one. Jun 19, 2020 at 21:21
  • @paulster don't believe I hosed heads. The other metal component that I sprayed a bit was the ignition coil housing. Which does not appear to have warped at all after I just took a look.
    – sawreals
    Jun 19, 2020 at 21:46

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