[Blue Bowtie]

Let's do the math...

You have a '99 Taco (that's seven years old). It has under 100,000 miles.

You've owned it for two years (since it was five years old).

PLUS - The required change interval for DexCool is 5 years/100,000 miles.

Unless there is proof that the previous owner had the system flushed and refilled just before selling/trading it (highly doubtful) you should have flushed the system two years ago.

PLUS - It has been operated at a low level (almost a gallon) for some undetermined amount of time. Operation with any air in the system accelerates the formation of solids, especially true in OA coolant systems (DexCool).

EQUALS - If you haven't drained, flushed, and changed the coolant by now, it's well beyond the due date.

The Wedge-Block Man is all over it. This is an excellent time to pressure test the system, since the old coolant needs to be drained anyway. Any repairs determined necessary as a result of the tests are an opportunity, not a problem. The Vortec V-8s don't seem to suffer head gasket problems like some of the 60º V-6s, and don't have as many intake leaks. It's possible that leakage could be anywhere from a water pump seal to a heater core.

Does the system create and hold pressure once the engine has been fully warmed? If not, there is likely a problem somewhere.

Does the engine oil have a milky appearance or higher than normal level? Is there excessive moisture (steam) in the exhaust stream even after the vehicle has been warmed and driven? Those are also indications of internal coolant leakage.

That pressure test can help reveal if and where that leakage may be occurring.