300TE Thermostat change

Since purchasing my 300TE, I haven’t been happy with the operating temperature.   It hasn’t overheated, but it was running hotter on days in the mid to high 20s than I would like to see it.   I want to be able to rely on the car on days well into the 40C range.

My mechanic had suggested that the fan clutch be changed.   I had that done, and while there was an improvement, I still didn’t like what I was seeing.   I was hearing the fan clutch roaring while cruising on the motorway, and temperature was creeping up at motorway speeds.   Lowest temperature seemed around 80km/h.     I figured the next place to start was the 300TE thermostat.    They are wear items and I figured it made sense to make sure I had a new one in place before I started looking too hard at other aspects of the cooling system.    I thought that perhaps the 300TE thermostat was not opening all the way.

In addition, while I was at it, I wanted to replace the coolant level sender.    The light would sometimes come on, or start to glow dimly even though the level was fine.    I started by using a large syringe to drain out as much as the coolant as I could.    Better this than let it run out onto the floor.

coolant temperature sender

This is not a hard job, if you have the right tools.   I have a set of circlip pliers, which make changing this sensor very simple.   I spent an hour looking for them and can’t find them anywhere.   Tring to get this off with a normal set of needle nose pliers took almost as long.    I also managed to break one of the wires off the terminals when removing it.   I’m going to have to repair this before I can actually use the new sensor.  A job for another day.

300TE Thermostat

Back to the thermostat – I had a couple of new thermostats in my parts stash, so went with a 80C Wahler thermostat.   Wahler thermostats have the best reputation for actually opening at their rated temperature.   Some of the other brands open a few degrees hotter than their rated temperature.  I also find that R134A A/C performance starts to taper off when the coolant temperature is on the higher side.   Not sure why this is, perhaps the condenser’s proximity to the radiator?

The old 300TE Thermostat was also a Wahler, but it was an 87C.   This is the factory rating – I just prefer 80C for the Australian climate.    It broke when I was removing it, so I couldn’t test it to see if I was opening fully.

300TE ThermostatBefore I removed the thermostat, I snapped the photo above.  Not just for this site, but also to record the orientation.   The jiggle valve is supposed to be at the highest point in the housing to prevent air pockets.   As the M103 housing is almost horizontal, the photo was a quick way to ensure I put the new one in properly.

Unfortunately for me, the air bubble looked like the jiggle valve on the small phone screen, so I installed the new 300TE thermostat with valve in the wrong spot. I didn’t notice until the day after as I was about to write an article for this site, and saw the photo on the computer screen.

300TE ThermostatSince I didn’t know the thermostat was not installed properly, I went ahead with using the bleed valve on top of the thermostat housing to remove air bubbles and running the car up to operating temperature.   Despite the jiggle valve being in the wrong spot, the temperature didn’t move above 80C in the driveway, even with A/C on max.    Note the photo above has the thermostat the wrong way.  It is there to illustrate the wrong way to do things.

Today I went back and did the job again.   As the thermostat housing is fairly level, not sure it had a big impact, but I like to do things properly if I can.    Again, idling in the driveway, I wasn’t able to get the temperature to move over 80C.    I didn’t take the car for a drive, as I only have 60 days per year to drive the car, and it seemed a waste of a day in the logbook for a short test drive.

I want to take it out on the motorway and make sure its sitting at 80C at 110km/h.

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