I just flushed and replaced my radiator coolant, but the separate container on the other side of the engine that is labeled "engine coolant" is empty. What is the difference and what do I put in it (if anything)? I have a 1983 jeep Cherokee 4 door with a 6 cylinder in-line 4 liter engine.

 

It's an overflow bottle for the coolant. Fill it with the same green coolant to the level marked on the side of the bottle.

 

Thank you so much! Now I feel stupid because it's so obvious.

 

It happens. We've all been there at some point.

 

My reservoir builds pressure/vacuum. Is this a symptom of bad reservoir cap or something else?

 

What year? Older models used a closed system that built pressure in those bottles.

 

1989. So only open it when cold?

 

http://www.olypen.com/craigh/cooling.htm

You can also convert it to an open cooling system which some people do.

Edit: Whoops. I imagine yes don't open it hot.

 

Surely you mean a 1993 Cherokee? Not 1983? The XJ came out in 1984 and offered the 2.5L 4 cylinder and the 2.8L V6.

Fill the overflow bottle with 50/50 mix of coolant. There should be marked lines on the bottle.

 

Some aftermarket replacements do not have any fill level marks. In that case only fill about half way up.