bad radiator cap? or worse?
 

bumped an older thread, but i think it was too much to read through, and i'm not getting any responses.. SO:

today was the first really hot day in Chi, over 90 degrees. was driving pretty hard all day, in stop and go traffic on the way home. when i got home, i smelled coolant, so i popped the hood and coolant was bubbling out from under the edges of the cap on the overflow tank, NOT via the overflow hose.

i let it cool down, topped it off, ran it awhile w/o the cap in case there was any air, though the reservoir was not completely depleted, so i don't think that was needed. kept an eye out for any bubbles forming up in there, didn't see anything.

jumped on the highway and ran the hell outta her for a good 15 minutes :naughty:

kept my eye on the temp gauge the whole time, didn't budge from the middle. ran the heat, and it was quite hot, so i think circulation is good. popped the hood when i got back and everything looked good, but when i put my ear close to the overflow cap, i could hear a slight fizzing sound like it was slightly bubbling.

so... is this just a case of a bad radiator cap or does this sound normal? i've never had a radiator cap that wasn't actually on the radiator itself, so this strikes me as kind of strange. i've only had the car since the winter time, so i am guessing it is possible i was slightly overfilled and it just bubbled off, today being the first hot day.

thanks guys

Coolant should never come out around the edge of the cap.

Check that the little overflow hose is clear and not melted shut.

There is a rubber ring that seals the 'hole' and a second ring around the top of the cap that seals the cap to the bottle. If this second gasket is missing or badly damaged, it's time for a new cap......

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thanks for the reply. i did inspect the cap and the gaskets look fine on both parts. but i think i will replace it anyway since it shouldn't be leaking in any event. hope this fixes it...

anyone have the part number? all i see in the diagram is 15-205, but that's just the last part of it, and is the same trailing digits for multiple radiator caps for other mazdas as well.

tried to get a generic one from the parts store, but it didn't fit as snugly and i don't trust it

well, replaced the cap and thought that was the end of it, but here i am again on a 90 degree day with the same issue, only seemingly worse. coolant is pouring out of the overflow hose, bubbling out from the edges of the cap, and i can see it boiling over in the overflow tank.

was not overfilled, just right between the high and low mark. i am not overheating.

growing concerned that this may be a compression issue...

thoughts?

wow, no one has any idea at all..? :grouphug:

What is the ratio of your mix? 50/50?

Stuck thermo cap, water pump going bad.

get down on your knees and check the coolers. make sure they are not blocked.

Jack up the car look at radiator. make sure it is not blocked.

When was the last time you flushed?

Check to see if your radiator fans are coming on normally...if they don't...it will overheat at low seeds...and cool off again when the car is going faster

Look for rubber grommets in the way of the fans...and burnt out motors......

The overflow often soaks the connectors with hot coolant, and the connectors corrode and fail :)

Also, look at the lip of the tank. I know I had mine crack, and then finally fall off. Had to replace the tank. If it's starting to crack, that might cause the coolant to come out as well. If it is cracked, get it replaced ASAP...heat+seals=dead motor. That's how my RX7 died on me.

i doubt it's a stuck thermo, since this is only an issue on really hot days, and i have never overheated. i will have to look at the radiator and coolers.

never been flushed so far as i know and not sure about the mix since it is as it came when i bought it 6 mo ago. maybe this is the first thing to try..

fans are working fine, they run sometimes when the car is shutoff, which i thought was really weird the first time. overflow hose is actually routed into the bottom of the intake tray since i have an MS CAI

tank looks fine...

An aftermarket guage or an OBD2 display for ECT is always a good thing. The guage is a dummy guage and doesnt move until over 230 deg, which is a little late.

Dont think you have a compression issue, atleast not from the symptoms discribed.

If the coolant pressure bottle cap is holding pressure (new it should) then it should not boil until and unless you are overheating. However, when ran hard, after you shut the car off, because the coolant is no longer circulating, it can boil a little.

You said it was not overfilled, however if you fill the bottle above the cold line even a little, the coolant will expand and overfill on hot days or when ran hard. So when cold make sure its only at the cold level and then next time pop the hood and check it before shutting the car off. You might also want to let it cool down a little (low RPM driving) before shutting it down.

holy @#$! i dunno why this never occurred to me, but I do have my carPC setup with OBDII reader, not hooked up right now. i was just thinking about how i could figure out what my actual water temps are. i will definitely get that hooked back up again.

i'm not sure if i fill this thing up right now. if i'm not mistaken, there is no hot and cold mark like a traditional overflow and instead there's a low mark and a high mark, and it should be filled to the high mark when the engine is cold, yes?

regardless, if it were overfilled, it should overflow until the proper level is reached and no more than that, correct? after it overflowed and cooled down, there was barely anything left in the overflow, way below the low mark.

Your right about the markings, Low and Full. If you overfill it will overflow. But if you run it hard on a hot day, then shut it off before letting it cool down, its not uncommon to have it overflow a little.

well just a little update. as we've been in the middle of a heatwave, it's been pretty good to put the rx through the paces. i hooked up my odbii reader to the carPC, so i've been able to directly monitor water temps at all times, which has been awesome :rock:

it takes a while to get up there, but my car seems to settle right around 210. at times i've seen it climb as high as 219 briefly, but always moves back down to 210ish. haven't really tested how it does just at idle only, this is simply the average through city driving. i think this is ok..?

whenever i've been overflowing, which has been only occasional, i've just been replacing with water instead of 50/50, which i think has been helping. yesterday i overflowed only very slightly, so maybe my mix was too coolant heavy. also getting time for a coolant flush i suppose.