Coolant boiling out the rad cap!
 

Hey gurus,

Having a serious problem with my 05 GT Shinka.

Yesterday I was driving along normally and my coolant light flashed on for a second. Shortly after I could smell burning and when I pulled up to the light, saw smoke coming out from under my hood. I pulled over and popped the hood and a crap ton of coolant had boiled out the rad cap! It was splashed all over and slowing started running down the street in a long line of liquid bad news... This was the 2nd time this had happened (although much worse than the 1st time) and I had already replaced the rad cap suspecting it was the culprit.

I looked everywhere and couldn't spot a leak. It really looked as if it all came out the cap. I figured I might have replaced the cap with the wrong model (didn't get it from Mazda) so I had a friend deliver me some coolant and we topped it up. I idled the car for 5 min or so and the temps climbed to normal but not beyond. Coolant light remained off. Nothing seemed to be leaking. So I decided to limp home. Just as I was pulling onto my street the coolant light flashed briefly again and when I pulled into my driveway there was more smoke. Popped the hood and I had another boil over (though less this time).

Any idea what could cause it to boil out the cap like that? At 1st I thought I must have bought the wrong replacement cap but I checked with a buddy who also has an RX8 and the caps are rated to the same pressure (13lbs). The model I bought was listed in a database for 05 RX8s at my local auto parts store.

I'm not the most mechanically minded but I have a friend who is. We might take a crack at it today so I was hoping you guys would have some ideas. Otherwise it's off to the mechanic. :(

Some more background. Here's why I replaced the rad cap in the 1st place...

Happened once a month ago. I did a high rev pass and saw the coolant light fire up for about a second and smelled burning. I was slightly overdue for an oil change and hadn't checked the levels in awhile so I figured coolant may have been low. Temps were fine and it didn't happen again.

The next day the light came on and off a few times but temps were Ok. I went in for an oil change and the guy noticed burned coolant smell and dried coolant residue on the engine cover. The rad reservoir cap seemed to have had a blow out when I did that high rev pass. He topped up the coolant and said I should look into it. My buddy had a look and we couldn't see a problem but the old cap did look like it was wearing out so I replaced it. Seemed to be problem solved until yesterday!

Any advice greatly appreciated!

I'd definitely get it looked at as you really don't want to be overheating this car.
could be anything from a bad radiator, to thermostat not functioning properly to a bad water pump.

Are you topping off the coolant in the reservoir?
If so, what do you use to gauge that the reservoir is topped off?

I'd recommend having your cooling system pressure tested to start. That should tell you if it's the cap, a cracked or loose hose, or a bad gasket/seal somewhere. If it passes that, look at your water pump and thermostat.

If there is no leak in the system, but it's boiling, then most likely your reservoir is going out, lost pressure.
The lid of the reservoir is known to fail, same thing happened to mine.
Plastic + heat + time = fail.

Just replace the reservoir and you should be fine, cost around 90.
You can get one without the sensor, and just reused your current sensor.

just my .02