Here's a seemingly simple problem - I noticed that the temperature gauge of my mother's 2004 Opel Meriva 1.7CDTI won't ever climb above minimum. I checked the OBD readings to see if it was a defective coolant temperature sensor but it showed that the car struggles to reach 70 °C even when driven hard, usual warm temperature is ~65 °C. A cold car's coolant temperature matches the outside temperature, so I assume the sensor is doing good.

First thought would be a faulty thermostat - coolant is always allowed to go through the radiator and does not let the engine reach proper temperature. So I changed it to a new one. And you know what that did? Nothing, same problem.

As far as I know the problem appeared gradually. At first it reached operating temperature fine, a while later the temperature wouldn't climb above 80 °C, then it would climb above 70 °C (minimum on the temperature scale) only in longer trips, and now it doesn't at all. I don't think anyone has been tampering with the cooling system, this problem progressed naturally.

One strange thing I noticed is that neither the old or new thermostat had an o-ring gasket, but it seemed to fit into it's place perfectly, so I assumed it to be normal, designed to fit in place with no gasket. Even though I've only seen thermostats with large o-rings sealing the edge before.

So, does anyone have any idea what's going on? Is coolant getting around the thermostat somehow? Could the temperature sensor fail in such a subtle way?

UPDATE:
-------

After 10 minutes of warming up the top radiator hose and the radiator itself feel slightly warm, so I assume some coolant is getting around that thermostat somehow.

I also tested the old thermostat to see if there was anything wrong with it at all. It's normally closed and submerging it into boiling water opens it up, so I assume it is fine.

Also there probably should have been a gasket for the thermostat housing. Very small amount of silicone sealant was used instead of it before, you could see pieces of the old silicone instant gasket on the old thermostat. I went the same way, but now am in doubt. I don't assume this missing gasket could somehow create a way for the coolant around thermostat?

[![This was missing](https://i.stack.imgur.com/KRjYh.jpg)](https://i.stack.imgur.com/KRjYh.jpg)

UPDATE 2:
---------

Put the (unbelievably hard to buy) gasket in, nothing has changed. I also noticed that the new thermostat has very slight gaps even when closed (seems larger in the image than actually is). It seems that they are a result of manufacturing imperfections, must be a poor quality part. I wouldn't believe that these would be sufficient to keep the engine temperature down below 70 °C by far, though. I am out of ideas here.

[![Slight gaps in the closed thermostat](https://i.stack.imgur.com/mC3AQ.jpg)](https://i.stack.imgur.com/mC3AQ.jpg)

UPDATE 3:
---------

Occasionally the electrical fan turns on, which is strange behavior considering the operating temperature is far from being reached. I doubt this is the cause, but it certainly does not help. Since the radiator warms up slightly before it should I'm thinking of covering it up to see if without proper airflow the car will reach higher temperatures. The only thing besides the thermostat supposed to be holding back the coolant is the thermostat housing, so it is my next suspect. Didn't notice anything unusual the last two times I took it off, but I'm really out of ideas here.