Diagnosing a bad thermostat on my Alfa Romeo Spider, again!

What remains a mystery is why this is the second thermostat going bad on me.

BHPian Jeroen recently shared this with other enthusiasts:

I was not happy with how the coolant temperature of the Spider was behaving. Yes, the new inlet and an additional piece of cardboard in front of the radiator did push up the temperature to about normal.

But it is not normal for me to resort to these measures. The only thing I could think of was the thermostat. Remember, when the engine is cold, the thermostat needs to be fully closed. If not coolant will start to circulate through the radiator and start to cool down.

The thermostat had been replaced not too long ago. Would it be broken again?

For good measure, I decided to check the calibration of my temperature gauge as well. I picked up a new thermostat and temperature sensor from my local friendly Alfa Romeo parts guy, Goos, and set to work.

Last time, I calibrated the gauge by dipping the sensor in boiling water and setting the gauge to 100-deg C. This time I wanted to be a bit more precise. I ordered myself a genuine electronic thermometer. It costs all but Euro 6, including packing and shipping. To put that in context: that is the price of Starbucks’ coffee here in the Netherlands. I hate being Chinese, but the flesh is weak, as they say.

My little “check temperature gauge/calibration set up”. I just take the wire of the sensor on the engine and attach it to the spare sensor. And put a ground wire to the car chassis.

With the sensor and the digital thermometer inside the water boiler, I switched on the water boiler. Sitting behind the wheel, I could watch the temperature gauge on the dash and the thermometer reading as well!

I checked with the temperature going up all the way to 100-deg C and back again. It is probably about 2-3-deg C off, but that is close enough. In all honesty, the needle of the gauge is already around 5-deg C “thick”.

So that seems to be working fine. I double-checked the resistance values of both sensors as well.

Next, I pulled the thermostat. Just two bolts and remove the hose. Sure enough, the thermostat was not closing properly. If you look closely, you can see a large gap.

Compare old and new

I put the new thermostat in and took the Spider for a test drive. The temperature went up to normal within 5 minutes of driving. It stayed there no matter what. Hard driving, idling, cabin heat on or not. So it is controlling the coolant temperature properly once again.

What remains a mystery is why this is the second thermostat going bad on me. In theory, you could reset the thermostat body and valve. But I can’t be bothered to risk that, as I would think it would become wonky soon after again. Maybe as an emergency repair, it would be ok.

The next Spider drive is next Saturday. There is an Alfa Romeo day at Alfa Specialist Koos. You will have seen us visiting him before. Very nice and very knowledgeable guy. Peter will be there all day to value cars for insurance purposes.

Jeroen

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