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1985 300d
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9 Posts
Discussion Starter · #1 ·
hello everyone, it is currently 57f here in Dallas and my 300d wont get to operating temp. i assume operating temp is 80c ish.

it gets to 60 and some change but that's it. drove for 30 plus min.

my thought is thermostat. am i on the right track?

please let me know because i don't want to drive it with the engine cold everywhere i go.

Thanks!!
 

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2001 Volvo V40
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2,955 Posts
You are on the right track.

Thermostat opens at 80 degrees C and is fully open at 95 degrees C, normal operating temperature should be between those values.
 

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1991 300zx 2+2 @ 129k Miles.
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194 Posts
You are on the right track.

Thermostat opens at 80 degrees C and is fully open at 95 degrees C, normal operating temperature should be between those values.
Literally, it should be above 80c running? Mine hits 80 at its max, but generally hovers just below 80c. This is great info....time to crawl around under the car again.


Also, could this explain why my vents don't blow HOT air?

EDIT*

Also, is there a listing of brands of coolants that are compatible with this engine that are not "Mercede$ Brand"?
 

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2001 Volvo V40
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2,955 Posts
It should be above 80 degrees C. If not, it is usually the thermostat, but it could also be the temperature sender.

Vents not blowing hot air is an entirely different ball game.
 

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1984 300D
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5,513 Posts
Heater not blowing hot could be several things.
The Monovalve diverts Coolant circulation to the Heater Core or not.
The signal to do the above has to come from the Climate Control.
Vacuum has to swing the Door to divert the Blower Air Flow in the proper direction.

A good way to see if your Monovalve is working is to turn on your Defroster. If no warm Air comes out of there it is either the Monovalve itself or the Monovalve is not getting Voltage from the Climat Control.

If you get hot Air at the Defroster Vents but not hot Air at the Heater Vents you have a Vacuum Issue or the Climate Control is not sending the Vacuum to the Vacuum Servo/s to move the Door to divert the Air Flow to the Heater Vents.
 

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1982 300D Turbo, 126xxx miles
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196 Posts
It could be your temperature sender. If it hasn't been changed recently, you probably should change it as they're known to go bad after a while. I changed mine this past summer, it would never get above 40c.
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
Yes, thermostat.
 

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Registered
1991 300zx 2+2 @ 129k Miles.
Joined
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194 Posts
Heater not blowing hot could be several things.
The Monovalve diverts Coolant circulation to the Heater Core or not.
The signal to do the above has to come from the Climate Control.
Vacuum has to swing the Door to divert the Blower Air Flow in the proper direction.

A good way to see if your Monovalve is working is to turn on your Defroster. If no warm Air comes out of there it is either the Monovalve itself or the Monovalve is not getting Voltage from the Climat Control.

If you get hot Air at the Defroster Vents but not hot Air at the Heater Vents you have a Vacuum Issue or the Climate Control is not sending the Vacuum to the Vacuum Servo/s to move the Door to divert the Air Flow to the Heater Vents.
What I notice with the heater is that WHEN I am idling at one of the few redlights in my daily hour and half commute, is that the car blasts hot air when sitting, but while driving, the air quickly becomes cool. This leads me to believe that I have a vacuum issue. I know I have a slow leak somewhere, which makes it harder to diagnose. My door locks work great locking. Unlocking usually works slowly, and after a few hours, the other 3 doors will simply not unlock UNTIL the vehicle is started.
 

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Registered
1983 240D, 1974 240D
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501 Posts
It could be your temperature sender. If it hasn't been changed recently, you probably should change it as they're known to go bad after a while. I changed mine this past summer, it would never get above 40c.
+1, after replacing the thermostat. I just had this problem on mine, would never exceed 45-50c, finally tracked it down to the temp sender, and discovered that the car was really running at 100c all the time!

Quick check of the gauge in the dash too is to ground the sensor lead wire on the block, should send the gauge to max temp. If it doesn't it may not be a mechanical problem at all and could be a gauge or wiring problem.
 
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