My 300E running too cool!

[pmizell]

Hello gentlemen,

This one has me baffled. My M103 engine is running waaay too cool (or at least appears to be) at around 76/77 deg C fully warmed up. Sometimes it will run at this temperature with the ambient at 81F or 93F -- no difference, sometimes not, but if engine temp does rise, it'll barely touch 80c. Gas mileage is way down due to this condition.

Occasionally I'll catch the needle "dancing" just a little.

Things I've checked:

1. thermostat - changed it out, no difference.

2. coolant temperature sensor - my '91 M103 (103.983) has this sensor on the head itself (two pole blue). This was recently changed due to an aux fan problem, so I doubt this is the cause. I've double checked the connections -- no problem there.

What should I check next? Does the coolant temperature sensor serve as the engine temp sensor? Are these two different sensors?

The "dancing" needle leads me to believe that whatever sensor/relay/etc that controls it is bad.

Please offer any insight you may have.

TIA!!

~Paul

_______________________
'91 300E, 212K miles

Check your Engine temp sensor again. The connector post has the tendency to come loose and start rocking inside the sensor. This is probably the reason you catch the needle "dancing " sometimes. You also mentioned a sensor was replaced due to an aux fan problem. I believe that is a different sensor and while replacing it, engine temp sensor which is a single pole (black) and located in the same area got disturbed in some way and it went bad. Check it out.

[haasman]

pmizell Amazing I have had the same problem and was going to post today .... except now it seems I am getting more normal readings. Must be a bad sensor.

[pmizell]

I've double checked the connections, no problem found. My newer M103 does not have an "engine temp sensor" -- MB calls it an ACC temperature sensor, and the parts page incorrectly states that it's located on the intake manifold when in fact it's on the rear of the head.

This sensor turns aux fans on high when 105c is reached, and cuts out ACC altogether when 115c is reached. Here's a pic:



The older 103's had a 3 pole green terminal that ran thru chassis #-A-079037 and had the same function as my 2 pole blue sensor. It is called an "engine temp sensor", so I'm assuming this newer 2 pole blue is my "engine temp sensor". My chassis # is B-399221.

Any other thoughts/solutions are appreciated!

~Paul

_______________________
'91 300E, 212K miles

[G-Benz]

I see someone else has discovered my signature avatar...DRAT...I need a NEW one!

[JimSmith]

The instrument panel gage sensor cannot cause mileage to drop by failing to register the correct engine temp, can it? I did not think this sensor "spoke" to the engine controls.

My 1988 300E runs just below to just above the 80 degree Celsius mark, and never goes higher. While driving the car home from Utah to Connecticut I got 29.8 mpg or so and figured the temp was normal for the car. Did your car's normal operating temperature just change and get lower?

My 1986 190E 2.3-16 has consumed two thermostats, with the symptom being it would warm up to its normal temp, about 82 degrees C, and then drop to about 70 or lower. Each lasted about 85,000 miles. And, at that temperature, mileage did suffer.

Based on my experience your temps seem pretty close to normal, but the real test is "did it change recently and abruptly?"

Good luck, Jim

[Cap'n Carageous]

Just a thought, but can a rich fuel mixture condition cause an engine to run cooler? I know it works the other way around. When we used to lean out our carbs and ran timing advanced for racing we almost always saw higher engine temps. Maybe your problem is too much gas!!

I believe M103 uses this temp sender for the temp gauge here is pic

[pmizell]

brcrosson:

Thanks for the reply. Do you have a part number for that sensor? I do have this single pole sensor right next to the blue two pole on the head.

Thanks!

~Paul

______________
'91 300E, 212mi

[Arthur Dalton]

The single wire sensor is the gauge sensor and can be checked
with an ohm meter.
Here are the readings -C/Ohms.
60-110
80-67
100- 38

The gauge should read these temps at these resistance values

[pmizell]

Time to put my multimeter to use!

Thanks a bunch!

~Paul

_________________
'91 300E, 212K miles

[pmizell]

Well the gauge sensor checked out OK, but I think I've identified the problem. The low temp readings I'm getting seem to be related to an intermittent high idle problem I'm having recently -- when idle is normal, so are temp readings, when not temp reaadings are low.

The high idle seems to be related to the throttle linkage/microswitch. When idle is high (1200-1300 in neutral) and I depress the microswitch roller, idle returns to normal. So I either need to adjust the linkage or replace the microswitch.

Thanks for all the replies everyone!

~Paul

_____________________
'91 300E, 212K miles

[Ron Johnstone]

I had a fluctuating gauge reading on my 300E and solved it by adding a loop of thin solder to the temp. sensor post to make a very solid physical connection to the press-on connector. It cured it! There was just too much mechanical slop in the connection.

pmizell

part number for single pole temp sender is
005 542 10 17 purchase one from fastlane last week, hopes this helps