The past few months I've noticed that there is occasional wetness around the base of the overflow bottles cap. Never enough to pool on top of the bottle and spill down the sides, but it's confusing the hell out of me.

Before anyone starts running around waving their hands in the air yelling 'bad coolant seals!', I'll say that there is zero evidence of bad seals and the engine was rebuilt a handful of years ago.

No champagne bubbles, no need to add coolant now and again, it doesn't push coolant into the overflow bottle abnormally, doesn't overheat, and it holds pressure when tested with a cooling system pressure tester just fine.

The flat cap is new. The pressure cap is of unknown age and is located on the radiator where the overflow nipple resides (as the two need to 'follow' eachother, location wise).

I'm just a little miffed. I feel like maybe the seal for the overflow bottle cap could be the issue here, but it is intact save for deterioration that has certainly happened over the last 30 years. I'm tempted to grab another overflow bottle cap from the salvage yard just to see if that stops it.

Anyone else have a better idea or explanation ?

I'll probably sift through the parts fiche and call my mazda dealer tomorrow, but I'll bet the part is NLA.

 

The recovery tank (or "overflow bottle" if you must) is not pressurized, so the seal is mostly irrelevant to system operation.
The level should never rise high enough to overflow the stock relief port and come out the cap.
Have you verified the interior hose is good, the intake port is clear and the hose not cracked (remember, the hose between the rad and the recovery tank sees no action at all till the rad cap pressurizes beyond 13psi)?

 

Hoses are all intact.

The overflow bottle doesnt fill up like you suggested.

Maybe its a compounding problem? Pressure cap that is letting a bit through at times at a much lower pressure and the seal on the overflow bottle cap not doing so hot.

 

Perhaps I'm being dense but if the tank isn't filling up how can it leak from the cap seal?

 

You're not being dense, I'm not sure myself. The level doesn't change. Obviously it'll fluctuate a hair under normal operating conditions, but the level always returns to the same starting point, and theres always coolant to the top of the filler neck.

 

Just to follow up...

I purchased both a new OEM overflow bottle cap (comes with dip tube, discharge tube, cap w/seal) and a radiator cap. The weird leakage has stopped. Not sure which one was the problem for sure, but I reckon the overflow cap was. The new cap snaps on with some force and is an extremely tight fit whereas the old one snapped on with very little force and you could spin it around a bit.