I have a 1991 ranger 4x4 4L engine. I fill the radiator with engine cold and the reservoir up to the fill line. After driving the reservoir has filled way over the fill line. I drive everyday thinking the cooling system will now suck back the antifreeze back into the radiator. It does not. I will use a turkey baster and transfer the antifreeze back into the radiator on a cold engine and the whole cycle will repeat itself. The engine is running a little warmer than usual but not overheating. It's also not that warm here yet. I pressure tested the 13# radiator cap and it releases at 13#. I pressure tested the radiator and it held at 15# for 1 hour. I ran the engine with the cap of waiting to see if the thermostat opens and the water pump starts circulating but it was hard to tell if the water was circulating because the fill neck is on the side and at first the uncapped radiator would have surges of antifreeze coming over the top till eventually it calmed down ( I am thinking the thermostat opened?). I did see some slight residue of dried anti freeze in a place where the water pump meets the engine block at the gasket but nothing dripping even when holding pressure of 15#. I don't understand why the engine is not creating enough vacuum to pull the reservoir antifreeze back into the radiator. Could the water pump be failing? How would I test it? Any ideas would be very appreciated. I need to solve this before I can go on a road trip soon.

 

Might be a bad radiator cap, sealing too tight, not allowing the fluid back. The cap has to be designed for coolant recovery. If you open the cap after the engine cools does it have vacuum in the radiator?.

But the amount of overflow from the system to the reservoir kind of suggests a head gasket problem.

Here's some reading on caps.
https://www.tridon.com.au/products/T...overy%20system.

 

You might check the small hose that runs from the radiator fill neck to the overflow tank for a tight fit, if it's cracked or loose, it won't suck coolant back in.

 

it is tight and in good condition

 

radiator cap seems good. It has a pressure relief lever on it also.

 

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Update: I replaced the radiator cap and everything is working properly. I tested the old cap under pressure and it worked ok, releasing at 13 LBS. There is no way to check a cap for vacuum, or is there?