Okay. I've done a bunch of searches and taken a lot of notes, but I can't find exactly what I'm looking for.

I have the weird "add coolant" light issue. The coolant level isn't low, so I figure there's a bit of air in the system, which I'll bleed out. What worries me, though, is the fact that the air might have come from a coolant seal beginning to fail.

The coolant also looks a bit dirty. Is that normal for old coolant? I'm going to flush the whole thing anyway... but what exactly does oil look like in coolant? is it always milky? I had a compression test done at Mazda, and it showed 8 - 8.5 across the board. Could there be a small leak? If there's such a small leak, would the coolant seal test I found in the FAQ show anything at all, or would only a huge seal failure make the coolant erupt?

 

Sometimes the leaks are so small that you practically have to tear down the engine to find them. Mazdatrix has a little writeup about such occurances.

For now, let's pretend you don't have an internal leak. Is your water pump seeping at all? Do you have to add measured amounts of water priodically? How is the overflow tank's level changing, if at all?

 

The coolant level sender wire is most likely broken... Wayne should know this..

 

that depends if the light is on constantly or only after the engine warms.

 

Nope, the water pump seems fine. The little plastic bit on to which the pressure cap [?(radiator cap that's not on the radiator)] screws has a bit of a leak, but it's very small. I do have to add a little, but I assumed it was because of that. Can a mechanic test my coolant for contamination?

 

So the cap neck leak could in theory account for all of your problems?

You can test your own coolant- you said you were going to change it anyway, just save as much as you can when it's drained, then look for any oil slicks on the top of the coolant, and any solid debris that may settle to the bottom...

 

It's not always on, and sometimes it lights when I've just started the engine. But usually if I move the car a bit or blip the throttle, it goes out.

 

Just in case Rat's right, visually follow the single wire from the sensor as far down towards the harness on the rail as you can. See any damage on the wire?

 

Not a bit. Though every time I open the bleeder screw, I let some air out. I hope to god it's just the cap neck. x.x I'm trying that next.