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The Bentley manual states that the air flap should be at full cold air input at temperatures above 59 degrees F (15C) and full hot air input below 41F (5C).
The manual suggests removing the air box to make these tests. What you might do is open the box and remove the air filter. If it is cold out, the flap should be in the position to let in all hot air. Then take a hair dryer and aim the hot air at the thermostat and see if it closes, It will be slow acting.
Another way to test is to see if the thermostat closes the hot air and lets in cold after the car is fully warmed up. The problem with this is the thermostat may have time to open the hot air while you are trying to open up the box.
Yes, that spring on the thermostat is very stiff.
Because it is not often below freezing here in western Oregon, I removed the thermostat and flap and blocked off the hot air inlet to my air box in my '88 240. It starts just fine even on cooler mornings. We had to replace the AMM in it and I didn't want to have it go out again from overheating. (I can't be sure what made the original AMM go out though.)
You question reminds me tha that I need to check the air box thermostat in my '93 940. This car has the Regina system so there is no AMM. However, if the thermostat is stuck on the hot air position, it will affect the gas mileage, I believe and maybe make it run poorly when very hot. I need to do some research on this. Thanks.
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