612 runs too cold

When I'm driving on the freeway the engine temperature drops to about 150F. Driving in traffic it's 190F, at which point the fans come on. We do not understand why the temperature drops to 150 on the freeway. We have checked the thermostat, which is set for 176F. The gauges are accurate and match the actual temperature we can measure right at the thermostat. Because it's running so cold it gets P0128 error code. We, including our mechanics, have no idea what it could be. And on the freeway, it's going to be 150F, no matter whether the outside temperature is 32F or 70F.

 

Thank you for answering. We took it out, dropped it in boiling water. It opened, no surprise. We lowered the water temperature slowly and between 180F and 170F it slowly closed, completely. It had stamped on it 80C, which might be 176F. These are long time Ferrari mechanics, but mostly work on older cars, like 550 Maranellos. I've seen a 250LM in their shop. To complicate this more, the problem showed up in January, so they replaced the thermostat and the problem remains.

And we verified the engine temperature with a probe.

 

Oil fluctuates by about 10 degrees more than the water does.

 

Radiator cap? I don't understand your comment. Is there some way that a radiator cap could cause an engine to run cold?

 

Thank you. If it holds pressure does that mean it's OK? Anytime we have been working on the cooling system the mechanic has had to very carefully release the pressure at the cap.