I drove out to Las Vegas on Tuesday and came back yesterday, and I noticed something a little funny with the coolant temperature.

On the way there going up a fairly short grade which I drive a lot the temps didn't pass (or even reach) the 100 C mark, though it usually gets to about 105 by the top of that grade. I thought maybe the car was having a good day.

On the way back I had to drive up a very long grade on the I-15 just out of Nevada. The car performed much better than usual - I didn't drop below 75 and I didn't have to move out of the fast lane :-). However, this still required me to floor it the entire way up, and my EGT showed about 1200 degrees the entire time (normal on grades like that) so I was completely boggled when, after four or five minutes of this, my coolant gauge was still reading below 100 C.

Is it possible? The outside air temp was 63, not too cool. Is my temperature gauge or the sensor maybe messed up? The gauge reads totally normal, just what you'd expect, under normal conditions; it's just that it doesn't seem to want to go beyond 95 C.

On a different note, on one of the long, empty downhill stretches of the desert I got my wagon up to 5200 RPM... somewhere between 105 and 112 MPH (damned 85 MPH speedo...). It ran and idled incredibly smoothly after that - I haven't started it yet today but I hope it stays that way!

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)

Same with my '76- now that it's cooler outside, the temp. guage barely gets off the peg. When I asked the forum, most replied that my thermostat must be stuck open. Haven't bothered to check it because I'm putting the car away for the winter in 2 weeks anyway.

Dave

'76 White 300D W115 "Pearl"

If your thermostat has been in there a long time, I'd change it.

Joe B.

My car had similar symptoms and after changing is back to normal.

Cheers,

Bill

As I said, it functions completely normally unless I push it very hard up a steep grade. It warms up normally, tops out a little over 80 C in normal driving, etc. Nothing is different except how it acts on grades (when the thermostat should be wide open anyway!). It isn't too cool in normal driving and has no problem warming up in the morning...

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)

Staying 95 C when you'd expect it to be 105-120 C? The thermostat should open up completely between 80 to 90 C anyway, so I don't see how the thermostat can have any effect on the upper range of temperatures. (Nor does it necessarily seem to be a problem that my car isn't overheating... I'm more worried it IS and my gauge or sensor isn't telling me about it.)

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)

I've had this problem before and I changed the thermostat and everything went back to normal. It kind of sucked cause I live in Jersey and the car was blowing NO HEAT

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Audi TT

Are you guys reading my posts or just the thread title? I wish I'd titled it something else. The car gets plenty hot, heater works fine, etc. etc. It just doesn't get TOO HOT, ie. over 100 C.

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)

We are reading your words.
Your car is running cooler.

100C - 105C is not normal to me.
Your average engine temperature should be 80C - 90C
You have issues with the sensor, gauge, thermostat or all of them.
Search the topic, the issue is very common.

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Not reading closely enough it seems. It is 80-90 C in normal driving. It is not running cooler in normal conditions. I'm talking about driving up a steep grade at full throttle for four to six minutes - it ALWAYS has gotten hotter in that situation and that, too, is normal. But it all of a sudden isn't doing that anymore, which I find kind of unusual...

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)

Perhaps a NEW radiator cap would help...
Best wishes always.....Happy Turky Buzzard day
spo out

Good suggestion... checked that already though, and it doesn't look like anything was evaporating out. This is quite puzzling. Happy thanksgiving to you too...

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)

Mine runs 80C on average driving, 4% grade or higher runs 95C.
Baker grade or Grape vine

The first thing anyone should do in a case like yours; replace the thermostat, this is common sense step one.
Cooling system flush will not hurt.
Replace radiator relief cap, cheap insurance.
Check water pump belt tension.
Check water pump flow.

My best guess:
Your thermostat is bad..
The thermostat has not been opening fully before; it has now failed in the full open position.

I think you guys are missing the point here. If he had a problem with his thermostat it would seem it was bad before but has now suddenly fixed itself. A full open thermostat would affect warmup and full open at high heat loads is normal. I've always understood it to be normal for the 617 to see temps up to 100 at hard acceleration on longer grades, I know mine does and has for the 14 years I've owned it. Cooler weather may help keep peak temps down but if you're not getting much of any temp increase then my bet would be on a bad sensor. The gauges usually work or don't work and the accuracy, while it may vary, is unlikely to suddenly change. Try replacing the sensor, it's relatively cheap and easy to do.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold