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Hello, I just drained the coolant on my 2014 c250 and refilled it with zerex g40. I had dealer top off coolant and pressure test and everything was good. However I’m getting these codes and the coolant temp starts at 40 and then gets to 80 then like 5-10mins later it drops down to 40. What’s the issue, not sure if it’s the thermostat or coolant reservoir or something different, any insight would be appreciated.
You may have a bad coolant temperature sensor. But before you dive into replacing the thermostat (the sensor is in the thermostat housing on M271) do these things.
1) After the engine has been running for 10 minutes or so and should be warmed up, stay clear of the fan and grab the upper radiator hose. It should be hot. If it’s not, the coolant system may not have been completely bled of air.
2) Check that the thermostat connection is on and not surrounded by grease or general under hood grime. Clean it and reconnect if it is.
If these checks are okay, it’s probably a bad coolant temperature sensor, but it is a bit unusual that it would be okay then fail immediately after renewing the fluid. I suggest waiting a few days to see if you get any additional suggestions before buying the part.
The temperature gauge on W204 is operated by the ECU which reads the temperature sensor then applies a bit of convoluted logic, so there could be a problem there, making diagnosis a bit more complicated. If your scanner can read real time data, it would be interesting to see what the ECU sees when the gauge is fluctuating.
Because you just changed the coolant, and the dealer had to top it off, I’m thinking you have air in the system.
Dealer pressure tested it and only reason I took it to them was because I was being cheap and only bought a gallon of Zerex when the car needs 1.2 gallons.
Dealer pressure tested it and only reason I took it to them was because I was being cheap and only bought a gallon of Zerex when the car needs 1.2 gallons.
1. How much did that 0.2 gallons cost at the dealership?
2. Pressure testing only checks for leaks, not air pockets.
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