Neutral or in gear: why the varience in temp?

[Chris Simenstad]

I went down the marina to air out the boat and to run the engine after several weeks of rain, and I noticed that the engine temperature under load at cruising RPMs was about 165 degrees, but if I ran the engine in neutral, it ran at about 140 degrees. Why does having the engine in gear increase the temperature?

--Chris

[msmith10]

The harder the engine works the hotter it will get. It's not under any load when it's sitting in neutral.

[Chip Hindes]

Of course the engine works harder when it's in gear, but I'm assuming that wasn't really the question. I've wondered about this same thing since I first started reading about the A4 on this website.

The idea of the thermostat and bypass system is to maintain constant temperature regardless of cooling load.

On every vehicle I have ever owned, if the thermostat is fully open at idle, and the temperature climbs under load, then the cooling system is undersized, plugged, damaged or underperforming in some way.

If on the other hand, the correct engine operating temperature is 165, why doesn't the thermostat shut down and maintain that temp even at idle?

[Don Moyer]

Chris,

The engine will normally run hotter at cruising power settings because of the additional load being imposed on the engine. In most areas of the country, 160 to 170 degrees is a very normal temperature for late model Atomic 4's. Early model engines will sometimes maintain lower temperatures (as low as 110 degrees) if they have no thermostatic control. Otherwise, early model engines will also settle in around 160 to 170 degrees.

Don