My radiator fluid level is always low. I can't seem to find a leak anywhere and have no indications of running hot or heater core problems. What would cause it to be burning through fluid. Should I fill it to the top with antifreeze each time it is low or just add water? Thanks.

 

The rad should be full at all times (cold) ensure the rad cap 13 lb is good and there is no big indentation on the sealing ring. The coolant reservior should be clean, 3/4 full and the hose to it should be secure at the rad and allow free flowing in both directions.

Any fluid added should be a 50/50 mix of antifreeze and distilled water, SCA's to be added when required by test strip indication.

Leaks can be in several places.... heater core....inside pax floor or externally at the heater hose connections. Rad cap, water pump weep hole, any one of several frost plugs in hidden areas. Also check the block heater area....back of the heads on both sides.

Please put some truck info in a signature......

 

It could also be going thru the cyl liner and being burned off when firing.

 

Don't forget the oil cooler.

 

A rather small leak can cause this.
What happens when the engine warms up, the fluid expands and move to the reservoir.
When the engine cools, vaccum sucks the coolant back in the engine from the reservoir.

But a tiny leak lets air in instead of sucking the coolant back in.

A couple heat cool cycles, low coolant in the radiator.

Look at the back of the intake around the glow plugs and on the exhaust manifold 2 minutes after you shut the engine down.
That is when the cooling system pressure will be greatest.