I am getting ready to send in my first sample at 164,000 on a 1997. I am not currently having issues, aside from a slightly rough idle, or miss. but the dreaded head gasket is always on my mind. When you guys say "Watch that overflow tank, when I noticed that mine was slowly dropping I knew I had a problem." What are you watching? The overflow tank is supposed to fluctuate a little. How much is to be concerned? I have done the combustion leak detector test with negative results.
What I do is when I open my hood and check my oil (at least weekly because mine is a DD) and I look at the level in the overflow tank. I keep it right at the "full" level. If the tank lowers over time, that means I have a leak because I'm checking it when cold, first thing in the morning.
If the level continues to go down over time it could be:
Head gasket
O-Rings leaking on the bypass pipe
Perforated hose
PHH failure
Heater core failing
Oil cooler leaking
Other tell-tale signs will show based on which other issues you see.
Milky oil means it's either HG or oil cooler.
Oil in the overflow tank means it's either HG or oil cooler
Strawberry milkshake in the overflow or radiator is the transmission oil cooler in the bottom of the radiator.
Strawberry milkshake in your transmission oil is the transmission oil cooler in the bottom of the radiator.
If the level never changes, even when hot, you need to check the level in the radiator by removing the radiator cap. Because, if you have a leak in a hose, you may not be able to have enough vacuum in the cooling system to draw the coolant back in from the tank because a perforated hose is allowing air to seep back in very slowly.
Also, if your overflow tank continues to fill up, it could be the same issue. It pushes fluid out, but can't suck it back in.
So, I check my oil and coolant overflow tank level at least once per week and my radiator cap monthly.
Check you transmission oil monthly or if you start feeling weird shifting patterns. Always check after having driven a minimum of 15 miles with a warm engine running, on level ground, in park.