Temperature won't rise? Good or Bad?
 

My last post regarded a broken bolt that needed to be extracted at the water pump housing....trying to change many cooling system components. One of them was the actual thermostat.

1987 300SDL OM603

I put in a new thermostat that was rated at 80C, Behr. Ever since then, the temperature just don't get above 80C. It's normally hot here in LA so I decided to change the cooling stuff before it gets hot, but we've been experiencing a gloom. During this gloom, the car stays really cold at about 70-75F outside temp. However, today on June 14 2009, It actually stayed at 85-90 because the sun came out and the day was about 85 at its highest. Then after the Laker game (woohoo!) I drive home in the cooler afternoon and the temperature stays at about 70 for 4.5 miles. My MPG suffered a little bit and I'm worried about the components in the engine that suffer from cold.

Should I put my old [rusty] thermostat in or just "break in" this new thermostat? Change the water? Below 80C doesn't sound quite right.

[Picture = Old Tstat]

If it stays at 80C all the time then thats OK. Nothing to worry about.

as long is above 40 C and not above 90 C you should be fine...

See?
 

Here's a visual of where I see it mostly (red line)

You can pull the Thermostat out and test it.
You will need to suspend it in a pot of water (do not let it rest on the bottom of the pot). Heat the water an monitor the temp with a meat or candy thermometer in the water (do not let the thermometer rest on the bottom of the pot).
I am not sure of the specific spec on a Mercedes Thermometer but if you watch it it should start to open at close to the 80 degrees C. If it is opening sooner at a lower temp you can exchange it for a new with the info one based on your test.

You could also test the Old one to see when it functions and compare them.

The candy thermometers do not cost much (Walmart, Kmart, Target) are often glass and you need to make sure the have the temperature range you need.
The cheap all metal meat thermometer work but are not preciscely accurate.
Automotive stores have thermometers used for AC and I believe they also have a hot temp range to.

I tested the Old Thermostat and it seems fine. However my current one is new. Not to mention I payed $250 to get a bolt extracted [while trying to replace my thermostat]. So needless to say, I'm hesitant about opening up my thermostat housing unless I ABSOLUTELY have to...

I don't know what lower temperatures can do to an engine...I bought this thermostat from Performance products and it was rated at 80C...wonder if there is an 85C?

For now I don't notice significant changes, but at least I don't have to worry about overheating right? I'm that much farther from cracking a #14 head right? Eh? I did all these cooling system stuff based on avoiding the cracked 14...

Did you check the number on the stat? It sounds like it might be a seventy degree stat.

If you just had it out the bolts in the stat housing should be a piece of cake to get out.

I wouldn't run it that way permanently.

Where on the stat is the rating?

Answer
 

80C is normal operating temperature.

It's stamped on the lip of the thermostat

It would be nice to assume that new parts will work perfectly; unfortunately the truth is new parts sometimes do not work properly and they also sometimes get into the wrong box.

My 86 300sdl runs at 80 almost all of the time. Now that it is 95 degrees here in North Louisiana I am seeing it come up a little over 80. My 14 head is hanging in there nicely. If I saw temps at 90 or above all of the time, I would think radiator problems, but it sounds to me like your car is running just like mine. I think a good test is to see what the actual temp of the coolant is while your gauge shows a little under 80. Use one of those fancy thermometers that you point at the upper radiator hose. Maybe the gauge is a little off.

That's cool! But the thing is it stays now at below 80C. It won't hit 80C until I floor it up a hill (which is on the way home). Nothing wrong with under 80C right (75C and below)?

Hmmm
 

That sounds like:
* A bit of trash stuck in the thermostat holding it partially open.
* A defective new thermostat.
* A defective temperature sensor?

For the cost of another unit, I would replace it.

FYI:
I always replace the thermostat housing bolts with new, and tap out the bolt holes.

I had a similar problem when I picked up my car. It would never get above 80 and took a while to get up to "your red line". After doing some reading here and finding out that it is actually better to err on the hot side (as long as it is not 100+) I decided to open things up and have a look.

I found my thermostat was eviscerated. Someone had ripped the guts out and just used kept the outer piece to use because of the seal. Running cold all the time is NOT a good thing.

Later I found out my fan clutch had failed at some point and the morons put a bolt through it to help the cooling. Long story... And caused more problems (for me) in the long run. The failed fan clutch is probably the reason for "removing the thermostat". They just were not smart enough to figure it out.

Hopefully whoever put the bolts in your new thermostat housing used anti-seize. Steel bolts + Aluminum housing = corrosion. If anti-seize was used and they were "torqued properly", removing it (or any bolt in aluminum) should not be a problem. Most of those problems arise from rushed jobs with improper tools, improper materials and the old "calibrated arm syndrome" (IE: no torque wrench).

Today, I was outside looking around under the hood and found another botched repair job. The hose barb running out of the top of the radiator was leaking. Someone at some point managed to mess this part up as well and installed an improper hose clamp, return line, etc... Another mess I had to deal with.

I guess the bottom line is this. When every you decide to take time to "delve into" your cooling system on one of these cars.... Take your time and do it right. Look at the entire system closely. God only knows what you will find later if you don't.:rolleyes: