Hello Forum Goers,
I have a 2010 GLK350 with the m272 engine, and I'm having some strange issues after a recent thermostat replacement. The original thermostat was stuck open which prompted me to replace it. (My replacement thermostat is a motorad brand) After replacement everything was looking good, and then the temp gauge started falling again about 10 minutes into the test drive. I parked it overnight expecting to come out in the morning to the same issue, but it was working perfectly fine. Completed a 1 hour drive without anything remotely concerning happening. (Both highway and intown) Then I took it out this morning and noticed the temp gauge falling again once I got into town, never dropping below 80c on the gauge but not sitting where it should be (very close to the 80c line). So on my drive home today I plugged in the scan tool and watched 3 main things, engine temp, specified value for engine temp, and Duty cycle of three disk thermostat. While watching I noticed that the specified value would change from 80c to 99c and when it changed the gauge would go down or up. I thought at first this might correlate to the AC being on (as it originally changed around the time I turned the AC on, changing to 80c) but as I messed around it stopped changing and stabilized at 99c for the rest of the 15 minutes of my drive. At this point i'm very confused on what the issue is, could it be a faulty thermostat, a bad seal, air pocket stuck in the system? I did try to burp or bleed the system of air but I didn't see much air exiting from the reservoir. For some added context, weather this is important or not, I replaced the water pump last summer.

I appreciate any suggestions that you all have, and thanks in advance.

 

The "specified value" is the target temperature. If the actual temp is tracking the specified value then that's how it's supposed to work. Unless it's lost its mind and the specified temp is wrong...

 

Yeah, I figured that was how that worked, but the jumping around a lot is what confuses me. Maybe that's how it is supposed to work and my old thermostat was just that broken.

 

Actually, the trend lately has been to make the gauge read a steady temperature until the actual temperature changes enough to be outside the normal expected range.

Can you tell if the actual temp lags or overshoots the target? That would account for the fluctuating readings.

Never heard any praise for the Motorad brand, other than price...

 

The actual temp doesnt match the gauge but does match the specified value. When the specified value is at 99c, the real temp is at 99 or 100c. When the value is 80c, its at 79 or 80c. So I think its matching the thermostat or cars computer, but it still seems like a large gap that it bounces between with very little reason between it.

 

I take that back! I see a vendor with a picture of the Motorad part and the body has the brand "Wahler" cast into it. If that is correct, then it should be as good as OE.

 

So the target temp, actual temp and indicated temp all seem to agree? Then you need to figure out what affects the ECM to determine the target temp.

 

The actual vs indicated temp on the gauge has a complicated logic. I think @TimC300 has the logic map for the C300. If he doesn’t chime in, you may want to PM him.