Hello!

If your tahoe is losing coolant, obviously it IS leaking somewhere, or it would remain full. It sounds as if the repair shop you took it to just did not do a very thorough job of finding the reason for the coolant loss.

Coolant can leak in 2 directions: either externally (to the outside of the engine) or internally (into the engine oil or into a cylinder). The process used to diagnose this type of leakage typically begins with a cooling system pressure test. In this test a tool similiar to a bicycle air pump with a pressure gauge attached is connected to the coolant fill on the vehicle; air is then pumped in to pressurize the system up to it's rated operating pressure and it is allowed to sit for several minutes to monitor pressure drop. If pressure is lost, there is obviously a leak somewhere. At this point the technician will usually raise the vehicle on a hoist and inspect the underside of the vehicle to see if there is any sign of liquid coolant on the external surfaces of the engine while it is under pressure. If the coolant leakage is very slow and hard to locate, the next step is toperform a UV dye test to locate smaller leak points. In this test, a small amount of dye that flouresces under UV light is added to the coolant and the vehicle is returned to you to drive for several days or untill the level again drops. At this point the vehicle is brought back to the shop and inspected with a high intensity UV light, to cause any accumulated dye around a leak point to glow.

If the pressure loss appears to be INSIDE the engine, then things get more complicated. If it is suspected that coolant may be entering a cylinder past a faulty head gasket or a crack in a casting and being burned, then the technician can perform a cylinder leak test. In this test, the spark plugs are removed and a special tool is used to introduce a measured amount of compressed air into each cylinder in turn while the valves are closed. The technician then watches the coolant fill point to look for any air bubbles that would indicate air was seeping past a head gasket into the cooling system. If any bubbles are seen, then the engine needs to be partly disassembled to positively identify and repair the cause of the leakage.

If any coolant appears to be entering the oil pan, this would be a sign of an internally leaking head gasket or intake manifold gasket (very common on GM engines). This would be identified by visually checking the engine oil and any visible internal surfaces of the engine (such as the valve covers) for any milky or white residue that would be the result ofcoolant mixing with the oil, or watching for any traces of steam exiting the oil fill hole when the engine is started cold and allowed to run and warm up with the oil cap removed. The presence of steam or vapor exiting the engine as it warms up would indicate that some coolant had accumulated inside the engine overnight and was now evaporating as the engine heats up. This condition also requires partial engine disassembly to confirm the diagnosis, and must be corrected to prevent severe internal engine damage from occurring due to oil dilution.

I would suggest taking the vehicle to a different repair shop, or the GM dealer service department. Ask the shop how they will go about diagnosiing the problem for you; they should mention the steps above or some combination of sim,iliar testing to help them scientifically and methodically test for and eliminate potential causes for the disappearing coolant.

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