Arrie

A couple of days ago noticed my car engine temperature running at 80 degrees. It has been running at around the first mark above 80 degrees, which should be about 87-88 degrees.

Then, yesterday got the CEL. Connected my iCarsoft and it gives code explaining engine coolant temperature at below control range.

Looked at engine coolant temperature when driving. If I first let car idle the temperature climbs up to 81 degrees but when I drive long enough at highway speed it drops down to 72 degrees. Outside temperature yesterday was 67 degrees.

During this time the temperature gauge in the cluster shows 80 degrees when iCarsoft says 72 degrees. I get the normal 87 degrees in the cluster when iCarsoft shows 81 degrees.

iCarsoft shows how temperature constantly changes depending on driving speed and engine revolution.

I think my thermostat is stuck in too much open position and does not allow engine temperature raise up to normal level.

What do you all think about this?

Sunnyslope48

Surely could be the thermostat. It could possibly be the sender also. They both have a part to play in keeping the engine running correctly

MBNUT1

Sounds like mine when the thermostat went bad. I replaced it and coolant at the same time. Wasn't a big deal to do. Could be wrong (its been a while) but I thought I ended up getting a code also.

KEY08

I suspect the thermostat as well as they normally fail in the open position. The CEL will generally come and go until you replace the thermostat. This is exactly how my failed thermostat acted until I replaced it. My temp has always run just a hair above 90c.

Arrie

I bought a new thermostat today from my MB dealer for $195 including tax. Pretty pricey as normally you can buy a thermostat for about $30 but this one comes with the housing. Thermostat is built in the housing and cannot be bought separately. I’ll be changing it next weekend I hope.

iCarsoft actually gives a helpful hint for this. It explains how my issue is normally caused by an open stuck thermostat.

I made a 3 hour highway drive today and temperature went down to 70 degrees as a lowest. This was the iCarsoft reading. Car showed right below 80 degrees. CEL did not come back after I reset the code. It did retain the permanent code for it though but the light is off.

C280 Sport

I put one in from Amazon. Paid under $80 and I popped it in. My old one was stuck open so the car always ran cold lol.

kajtek1

The cluster gauges are dummy gauges, so they don't show few degrees changes till something big happen.
On W211 I had climatronic who would display temperature that ECU is seeing and it was quite different what cluster would show.
Now 72 C thermostat opening is close to normal and on cold days the engine will stay at opening temperature till you put heavy load on it.
Some engines have thermostat with heating element in it, so that might take more troubleshooting.
My routine test is observing heating time. Engine should come to operating temperature around 80C on 2 miles drive for gasser or 4 miles drive on diesel when you are above freezing temperatures.
When it takes much longer - you need new thermostat.

Arrie

My cluster gauge did not quite come to 80 C during 195 mile highway drive yesterday. It was 56 degrees outside.

Also the CEL came back on.

Oda112

Try replacing the coolant temp sensor in the back of the engine on the driver's side cylinder bank. I have a post about it somewhere here. Mine was doing something similar last winter, replacing the sensor cured it. Retails for under 20$ from MB.
Check the pump in front of the expansion tank, make sure it's not stuck open bypassing some of the coolant into the rad.

kajtek1

What temperature it normally keeps in steady driving?
Most of my diesels have thermostat opening at 78C and with steady driving it would keep that even with ambiedient at 100F

Arrie

By the cluster gauge my normal was at the first mark above 80 C, which should be 87 - 88 C. This it held very well regardless outside temperature. I do not know what this normal would have been measured with iCarsoft as I never measured it before the trouble started.

Replaced thermostat today and found the old one closed so I’m not sure how it failed to make engine cool as closed means hot engine.

But, after change now engine heats up fast as it always did and now the cluster temp gauge shows between 2nd and 3rd mark so it wants to run at about90 C. Interestingly iCarsoft shows the same temperature. Also, now iCarsoft shows up to 100 C and down to 84 C when changing engine load quickly but it soon returns to around that 90 C.

I think it is fixed now.

KEY08

Hovering a tick above 90c has been my experience. Sounds like you are good to go.

kajtek1

This is how it works in real life and only lack of understanding makes engineer putting "dummy gauges" on dashboard so uneducated drivers don't panic.
Did your thermostat had heating coil?
Proper test of old thermostat would be boiling it in pot of water.

Arrie

I don’t know if it has a heating coil but it does have a 2-wire connection on it. I thought the thermometer would be built into it?

The main thing is it works now and the engine definitely runs hotter now. I can feel a big difference when I set cabin temperature up for heating. With cool engine was not blowing much hot air but now it is again.