I bought a pack of commodity 8mm plastic fasteners so I could replace old and brittle fasteners on my 2004 Mazda RX-8. I knew there were a lot of them holding the front bumper cover in place, fastened to wheel well liners and other adjacent pieces. And I knew I would have to deal with them because I noticed a puddle of coolant under the car when it is parked. There is a leak I have to track down!

The radiator is tilted forward roughly 45 degrees so the top edge is ahead of the bottom edge. Immediately above one side of the radiator is a coolant reservoir tank. From this pair, several hoses large and small lead backwards to the engine and cabin heater core. Given this knowledge, I was mystified by my observation coolant is dripping from somewhere in front of the radiator. I don’t understand how coolant got so far up front, but to get a closer look I need to remove the front bumper cover and several airflow management panels directing air through the radiator. About a dozen of those 8mm plastic fasteners later, I can get a clear look at the air conditioning condenser and the radiator immediately behind it.

Given the car’s age, I had expected to find coolant leaking from an old cracked radiator hose. It’s a common failure point and inexpensive to fix. But the forward drip location made me suspicious that might not be the case. Tracing dripping coolant back to the source, I found the leak and it was indeed not a failed hose.

The top of the radiator has cracked between two hose fittings, near the base of a hose fitting leading to the engine. The failure point is very inconveniently positioned for picture-taking. This is the best I can do, which required removing the battery and engine air filter box, and even then I know this picture lacks context.

Here’s an annotated version which might help. The camera is in the engine compartment looking roughly forward-right, but tilted at an angle. I have two arrows labeled UP and FRONT to designate those orientations. The crack is still very small and when the engine is cold, barely any coolant dribbles out flowing down the front of the radiator. However, when the engine is hot and cooling system is pressurized, a very thin stream of coolant shoots out of the crack towards the front. This stream will strike a piece of chassis metal ~10cm away before falling and that is why I saw coolant dripping in front of the radiator.

I’m glad I caught this problem before erosion expanded the crack big enough to drain all coolant and cause an engine overheat. I believe this is the original 20+ year old radiator. While the radiator core is made of aluminum cooling channels and fins, the top and bottom caps are made of plastic. Searching through RX-8 owner forums, old radiators cracking their brittle plastic is a known failure. Radiator replacement is the correct fix. Patching the crack will only be a short term fix because that old brittle plastic will fail somewhere else soon enough.