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Befuddled ... engine temp ... after new Tstat

I am Befuddled:

My '91 300E coolant temp has been running at or just below 80c after ten minutes and more of driving at ambient temp of 60-70f. So I replaced the Tstat thinking this was the problem.

The car still hardly shows anything above 80c. Now I am wondering is it the Tstat or possibly a bad sending unit?

Questions:

-How do I test the sending unit to the temp gauge?
-Is sticking a thermometer into the coolant reservoir a good test of the true engine coolant temperature?

I checked the coolant and it does need more anti-freeze, it only goes down to about +5 degrees. Not a problem though here in N. California.

Any suggestions? Thanks as always.

Did you use a dealer or OEM t-stat?

I used a dealer (MBZ) Tstat.

Re: Befuddled ... engine temp ... after new Tstat

Quote:

Originally posted by haasman
I am Befuddled:

My '91 300E coolant temp has been running at or just below 80c after ten minutes and more of driving at ambient temp of 60-70f. So I replaced the Tstat thinking this was the problem.

The car still hardly shows anything above 80c. Now I am wondering is it the Tstat or possibly a bad sending unit?

Questions:

-How do I test the sending unit to the temp gauge?

Ohmeter at sending unit should read :
60C-110 ohm
80C-67 ohm
100C-38 ohm

-Is sticking a thermometer into the coolant reservoir a good test of the true engine coolant temperature?

Not really

Check that you have installed the right thermo.
Should be 87C for that car. That is start to open spec.

I checked the coolant and it does need more anti-freeze, it only goes down to about +5 degrees. Not a problem though here in N. California.

The least you should have is about 45/55 [af/h20].
A 50/50 mix will read -37C [ when hot]. -34F.

* The trivia question of the day is - When does C and F read the same ?
"Minus 40 degrees"

Any suggestions? Thanks as always.

Somebody already mentioned this ...

on a different tread but, did you check the water pressure cap? ;)
aka: coolant cap

Thanks for the replies.

My guess is that the sending unit is bad. Am I correct in believing that the sending unit has two connectors on it. It is between the valve cover and the air cleaner?

Haasman:

Yes, the coolant temp sensor is located on the head (two pole blue), second from the rear on your '91 300E. If you have a multimeter with a resistance function you can test it before replacing it. ($31)

~Paul

_________________
'91 300E, 212K miles