Can radiator cool down a car too much?

The thermostat controls the temperature in the engine. It opens at a set temp, 195 degrees for example and allows coolant to flow through the radiator. If it's stuck open it would allow coolant to circulate through the radiator all the time not allowing the car to heat up properly. The complaint usually is the heater isn’t warm enough. If the thermostat sticks closed (obviously not in this case) the engine would overheat.

Too big of a radiator won’t cause the car not to warm up, however to small of a radiator could cause it to overheat.

The thinner radiator might make it easier for the cold air to flow through the engine compartment.

(The radiator itself wont cool an already cool engine at all if the thermostat is intact.)

In cold environments it is often necessary to use a grille cover to limit cold air flow to the engine.

Grille covers are especially necessary with small diesel engines not producing enough excess heat.

A typical DIY solution is a piece of cardboard.

The temperature drops when you start driving because you have more air coming into the radiator (the car is moving at higher speed). The same thing happens to my Acura Legend ('87). It'll be coolest when driving on the freeway, but if I'm stopped at a light for a while, the temperature may rises (on a hot day). Both my fans (radiator and condenser) work.

Why was the rad. replaced? If because it was faulty it might have taken out the thermostat as it went. As others have said above the radiator itself can't overcool with a working thermostat.

An oversized radiator most certainly will overcool the block. I installed a five-core diesel radiator on my 350 small-block engine, and had to go back to the original because motor could not heat past 150°F (65°C). It would climb to 195°F, thermostat would open and immediately drop temperature by 20 degrees. Verified with an IR thermometer.

The problem is, with so much more surface area being cooled by air and the larger amount of coolant in the radiator, it is so cold by the time it gets to the motor, never giving the engine a chance to stay at a proper operating temperature.

I run an enormous race radiator with high flow spal fans, an oil cooler and an oil cooler fan and it keeps the coolant and oil at exactly 180-185F around town or on the highway, regardless of ambient temperature, even if I run the fans unnecessarily. Why doesn't it overcool? Thermostats.

It 100 percent doesn't matter how much radiator you run so long as your thermostat is working. That's how a car's cooling system works- the coolant circulates in the engine until it gets too hot and then a little is sent to the radiator and replaced with cooler coolant from the radiator. When the coolant comes down to below a certain point, the thermostat closes again. The water pump is always running and the radiator should usually (especially in cold weather) have plenty of excess cooling capacity. This is intentionally how the system is supposed to work.

I'm having similar issue. Recently replaced 30 yr old stock radiator with a performance unit. With onset of winter, the engine initially comes to normal operating temp, but does not consistently hold it, sometimes dropping below normal. When this occurs, I also note that the system is not building pressure, and the radiator cap is not warm. The answer was very quick & easy- I blocked half of the radiator with cardboard.

The thermostat isolates the radiator when engine temperature is below the preset value. Overcooling is due to thermostat failure (staying open) and not because of an "oversized" radiator.