Would a small hole in your expansion tank cause your temperature to rise? Not a hole that's causing you to lose much coolant, but just a small hole or crack. Hope the Yankees lose today!

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1984 300D, 228k, Light Ivory, Java MB-Tex

So, any opening will defeat the system's correct operation.

'I'll leave the "OverHeat" answer to Experts.

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'84 300SD sold
124.128

My guess would be no.

"Correct operation" means that the pressure is what keeps the coolant from starting to boil prematurely, but you hopefully aren't at that stage yet. If you want the red mark to not be way earlier than 120c, then you need the proper coolant mix and a pressurized system.

Its a pressurized system. Any leak would raise the boil temp. If not, then why would they use a pressurized cap?

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900

Actually, a leak will cause a reduction in boiling temperature.

A leak would cause the boiling point to decrease. Coolant systems are pressurized so that they can operate at higher temperatures and not boil.

Answering your question, running your system at a lower pressure (ie pin hole leak) will not raise the operating temp of your system, but will raise the likelihood that your coolant may boil (at a lower temperature).

I remember camping in Colorado at 10,000 feet and noticing that boiling water in my coffee was not really that hot.

Somewhere on the net you should be able to find the boiling point of water at various altitudes to show my point.

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1993 W124 300D 2.5L Turbo, OM602.962
2000 Chevrolet Cavalier, 2.4L DOHC
2002 Ford Explorer, 4.0L SOHC
2005 Toyota Prius, 1.5L

http://www.fuelly.com/sig-us/40601.png

IIRC properly mixed coolant will boil at 106c instead of 120c if not under pressure.

There's a chance that with no pressure building in the system that you get "localized boiling" which can cause air pockets to develop in the cylinder head or blaock and look like it is running at a higher than normal temperature when those impede the flow of coolant elsewhere. Even if the gauge reads under 100C it is possible that parts of the engine, where coolant flow is less than optimal, can boil (that's what I mean by localized boiling). That can be made worse over the years from scale and rust in the cooling jackets. So, in answer to the question, yes, in some circumstances running the system with no pressure can lead to higher operating temperatures.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

oh yeah. i had it backward. dangit

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900