I have heard that the article in the STAR about cooling was VERY informative...unfortunately, I am not a member (maybe someone here will post the article). I checked my manual and my Haynes manual, and there is nothing in there about when to be concerned about the reading on the temp gauge.
It's been hot here in Texas (I think it got to 105 on Sunday). My 93 300E with 107K miles shows the temps around 85 - 87 when the outside temperature is around 75 - 95. This is with stop-and-go driving around town with the A/C on. If I sit at a light for a LONG time, then it creeps up to 90.
Trouble is, when it's REAL hot (over 100 outside), as long as I'm driving, everything remains normal, but if I make a stop, shut off the engine, and come back a few minutes later, the needle climbs to 105, and by then, I can't get things to cool down by driving, unless I shut off the A/C. Sometimes I see a bit of coolant on the ground from the overflow.
By the time I get home and shut off the engine, I'll open the hood, and can pretty much bet that the expansion tank will overflow (and it has a couple of times).
The water pump was replaced half a year ago, and the radiator, about 3 months ago (thanks FastLane). Got the cooling system flushed and replaced with a mix of water and MB coolant by a trusted tech (don't know the concentration). I just replaced the thermostat and coolant tank cap last week for (cheap) insurance. No oil in the coolant, and no coolant in the oil.
My ML320 doesn't seem to mind the heat, and it's always reading around 85, no matter the outside conditions.
Am I seeing the first signs of an impending head gasket failure, or am I just a little too nervous?
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car