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'79 GMC Sierra Grande 350 losing coolant?

coolant level is dropping very slowly maybe 1"-2"/day-truck is parked outside temps are around 30-50 degrees–temp gauge on dash moves up to first mark but never gets up to normal. my mechanic said the gauge/coolant level are related.Does this sound like theres a slow leak somewhere? Should I get the radiator pressure tested or just keep dumping in the coolant? thanks!

Where are you measuring the 1" - 2"? In the radiator or the overflow bottle?

If the engine is not warming up you need a new thermostat, but this is a separate issue from the coolant loss.

If the coolant is leaking externally you should be able to see puddles under the truck or some evidence of a leak somewhere under the hood, unless the leak is in the heater core. Can you smell anti-freeze inside the truck?

If there is no external leak you have to consider the possibility of leaking head gaskets. Hope and pray that the problem is an external leak.

You need to get the coolant system pressure test done first, not just the radiator. This should help locate the leak. If you don’t see it under the truck, chances are it may be internal. Coolant circulates throughout the engine block, cylinder heads, and intake manifold. Gaskets are suppose to keep the coolant from leaking into the cylinders, oil pan, and air intake ports, and out of the engine. But, gaskets go bad. So do hoses. There are a bunch of hoses that route the coolant to different parts of the engine, to the radiator, coolant overflow bottle, and the heater core. All of these need to be checked, and it is best done with the system under pressure when the leak is not evident. A system pressure tester can do this safely with the coolant at a safe temperature.

There is a leak, and it needs to be located. Just dumping in coolant is only a band-aid, and soon, the small leak can blow out, and leave you stranded somewhere. Most likely late at night, or during rush hour in traffic, or on the highway 10 miles from the nearest exit.

The temp gauge situation is probably due to a faulty thermostat that is stuck open. This is cheap and easy to fix. Replace both the radiator cap and thermostat once the coolant leak is found and fixed. Couple this with a good flush with clean water, and the system should be good for the next 5 years.