normal engine temperature? - m103 - Mercedes-Benz Forum
 
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#1 (permalink) Old 02-09-2009, 08:05 PM
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normal engine temperature? - m103

I changed my thermostat this past weekend because the old one had failed. At least it failed in the open position, which in my mind is better than the alternative. But it has meant that lately the heat hasn't been as strong, and I'm sure the gas mileage suffered from the low engine temps - I was getting about 20 mpg mixed driving. I haven't had a chance to track any change in mpg since.
The new thermostat is rated for 87 degrees C.
Now the engine temp comes up quickly as it should (whereas before it wouldn't get above 60 degrees C), and sits right at what I would guess is 85. If I'm driving in the city and then sit at a long light the needle will creep up to 90 (ambient outdoor temp today was something like 40 degrees F or 4 degrees C).
I just want to confirm that this is normal. Or is it a little high?
I seem to recall that before the last thermostat failed it hovered around 80. But maybe that's because it was on its way out ...
When I went to add coolant back to the system I also noticed that the rubber gasket on the reservoir cap was crumbling. I intend to replace it soon so that I don't get little bits into the cooling system. Could it be that the cap is leaking pressure, and that in turn would cause the temperature to swing up a little?
Thanks for your feedback.

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1988 MB 300E
1988 Saab 900s -- RIP
1987 Volvo 245 -- RIP
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#2 (permalink) Old 02-09-2009, 08:21 PM
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Originally Posted by merc-one View Post
I changed my thermostat this past weekend because the old one had failed. At least it failed in the open position, which in my mind is better than the alternative. But it has meant that lately the heat hasn't been as strong, and I'm sure the gas mileage suffered from the low engine temps - I was getting about 20 mpg mixed driving. I haven't had a chance to track any change in mpg since.
The new thermostat is rated for 87 degrees C.
Now the engine temp comes up quickly as it should (whereas before it wouldn't get above 60 degrees C), and sits right at what I would guess is 85. If I'm driving in the city and then sit at a long light the needle will creep up to 90 (ambient outdoor temp today was something like 40 degrees F or 4 degrees C).
I just want to confirm that this is normal. Or is it a little high?
I seem to recall that before the last thermostat failed it hovered around 80. But maybe that's because it was on its way out ...
When I went to add coolant back to the system I also noticed that the rubber gasket on the reservoir cap was crumbling. I intend to replace it soon so that I don't get little bits into the cooling system. Could it be that the cap is leaking pressure, and that in turn would cause the temperature to swing up a little?
Thanks for your feedback.
That's normal. I wished my car would settle around 87-90 C as I think I could get the CIS to lean-up the mixture a little more for better mileage. I'm not taking a chance. Both the thermostat and the CIS coolant temperature sensor are getting changed out.

WRT the expansion tank cap... change it and make it a regular item to change! My coolant temps would shoot up over the summer at stoplights and I realized that this was from boiling coolant in the upper radiator (the coolant overflow tank would be dripping when I parked) because the system couldn't hold pressure. Once the cap was changed everything went back to normal.

I now keep a spare cap in my boot. $10. Cheaper than a tow!
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#3 (permalink) Old 02-10-2009, 07:58 PM
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The temp readings that you mention sound absolutely normal to me. My 88s all run right around 80 - 90 C during most operating conditions. In summer the temp hits 100 C and then the electric fan comes on, just as it should.

As far as the surge tank pressure cap, I wouldn't sweat that at all. In the temps we're seeing right now there's no way that the system will boil coolant unless the cooling system is FUBAR.

On my TE #2, the white one, I've been running with the surge tank cap loose for almost a year without any temp issues or coolant boilover. I did this as a temporary measure since I think the heater core has a tiny leak. I get a slight sweet smell from the vents when I fully install the surge tank cap. I am really NOT looking forward to pulling the dash and all the other crap to get to that heater core but I guess I need to get going. How time flies.
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#4 (permalink) Old 02-10-2009, 08:40 PM
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It's good to hear the consensus. It makes sense to me that the normal operating temp should be just below what the thermostat is rated for...

There's one thing (besides the spare wheel of course) that I keep a spare of in my trunk - voltage regulator with brushes. You just never know.

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1987 Volvo 245 -- RIP
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#5 (permalink) Old 05-06-2010, 02:22 PM
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80-90 degrees is what my 1991 300CE runs except with the A/C it may climb to 100 degrees and then the fans cycle on and off*. I've taken temperaure readings at the thermostat housing and got a temp about 5-7 degrees less than the guage was showing so I really don't think it is running that hot. I just get a little nervous whe the guage starts to climb. 100 degrees celcius is equal to 212 degrees fahrenheit so 100 degrees celcius is not a big deal. I do agree that anything over that for an extended period of time is not good. 120 degrees C is equal to 248 F and that definely is cause for alarm.

*On the subject of Auxilliay fans... My fans seem to have a delayed reaction in connection with the A/C running. The compressor will kick in, then the high pressure/refigerant temp switch will close a few seconds later and the fans come on. After the compressor shuts off, the switch stays closed for a few seconds longer, then opens and the fans shut off. Is this what most of you encounter, or are the fans instantanious with the compressor? The switch doesn't seem to function like I think it should. I'm wondering if it needs to be replaced but I don't want to do anything with this old R-12 since it works fine otherwise if the switch is functioning as intended.
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