I have a very minor leak between the radiator tank and coil. I unfortunately won't be able to replace the radiator until 3 days from now. I only lose about 2 inches per day and top it of everyday. I was wondering if it was better to drive with the coolant cap loose to avoid building pressure or is it better to have the cap on tight.

 

having the cap on increases the temp it takes for the coolant to boil. If you do try to drive without the cap on, you should end up with the coolant bubbling over by the time the car gets up to temperature.

 

Not sure about "should" (because I decline to submit to the idea that "normal" under freeway speed should exceed 212F) but certainly "could". Distilled water boils at 212F at sea level. Make 50:50 coolant mix and the boiling point rises to 223F or so. Add 15 PSI of pressure and that rises to 265F or so. Certainly drive with top on tight, and top up not just before the first use of the day, but also again before heading home from the office. Just take a $1 jug of distilled water with you. Your great peril is rate of loss increasing, and damage to head gaskets or more.

 

You never drive with a loose or removed cap, this will allow your coolant to boil at a lower temp and cause you more problems. Fill the coolant bottle to the lid every morning and before going home in the evening or don't drive it.

 

I should also have pointed out that the coolant loss will allow air in the system, you may hear bubbles rushing through the heater core under the dash. Topping off may not purge this air, if there is enough of it your temp sensor will enjoy a steam bath, which is actually cooler than the pressurized hot coolant, and temps can look normal, but low coolant actually make engine a lot hotter. Coolant conducts heat away a lot better than plain old steam. Point - you know you are having a coolant loss, don't trust the gauge if it says everything is "normal".