Recently they replaced my stock radiator with aftermarket one (at dealership, so I hope they knew what they were doing), and since then I feel that a car is not warming up enough.

Even if I warm it up completely at garage - as soon as I start driving temperature drops. Driving at high speed drops it even more, when temperature outside is cold it gets worse too. I don't have precise temperature gauge though (only 4 digital bars on gauge) so I can't be sure.

This new radiator is smaller/thinner. Can a radiator be too "good" and cool down a car too much? Thermostat should completely disconnect radiator from the circuit if a car is cold, isn't it?

(at dealership they say it all behaves as it should, car is not over-cooling)

Car in question is Honda Prelude'92.

  • Too many variables. The radiator is non-stock (why did you do this?), the dealer mechanics had their meathooks in your engine bay, who knows what they did? You don't even specify any performance changes. Maybe the gauge sending unit took a hit when the dealer did the work.
    – geoO
    Commented Jun 9, 2013 at 10:12

9 Answers 9


If the thermostat is operating properly the radiator will only come into play when the thermostat opens, when the engine is at normal operating temperature (around 190 degrees, give or take). Adding a gigantic radiator won't make a bit of difference because if the engine gets too cold, the thermostat will close, causing the engine to heat up again.

In the winter, the cabin heater will draw around 20% of the engine's heat in order to heat you. This will cause the car to heat up slightly slower, but once it's warm it will stay that way.

It's possible that the thermostat isn't fully closing when you are driving, but works ok at idle. You might consider getting a high quality thermostat and replacing the existing one. It's also possible that the coolant temp sensor is on it's way out..

On the highway, a car produces lots of heat (at a higher RPM), which indicates to me that the thermostat is opening more (or failing to close properly) at higher RPMs since you are seeing a temp DEcrease at speed. Again, a new high quality thermostat should fix this.

  • Looks like a car thinks it is cold and starts warming up the engine increasing idle rpms up to 1800rpm from default sub-1000 (this happens at traffic lights after driving for some time), so it is not something I can just ignore.
    – serg
    Commented Mar 17, 2011 at 20:38
  • 5
    You can test a thermostat by dropping it into boiling water to see if it opens.
    – chris
    Commented Mar 17, 2011 at 20:39
  • @serg the engine running cold would not cause the car to idle at 1800 rpm. That's too high, that sounds like an unrelated problem. Commented Mar 17, 2011 at 23:53
  • 3
    I agree that it sounds like a thermostat. However, I have run into a situation where I had too much cooling... It was 40 degrees below, I was driving on the highway, and the 1.9L engine just couldn't keep up with providing enough heat to the cabin. So I turned on the propane furnace. :-) It was a VW camper van, water cooled. Commented Mar 22, 2011 at 11:17
  • @Larry Depends on the car. On one of mine the idle speed and engine timing are linked to coolant temp. It'll idle as high as 2500 rpm initially on sub-zero days. On a typical spring/fall day it starts out at around 2000. Commented Jun 30, 2011 at 16:15

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 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.


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.


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.


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.


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.

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