will coolant evaporate from a closed system?
 

Just curious if coolant can evaporate from a closed system. My non MB coolant is about an inch below the minimum mark and it has no leak that I can detect and UOA shows no coolant in the oil. I dont know if I have a problem or not and the mazda forum peeps only seem to care about what color lights to put under their cars.

nope.
if level is down, you either have a leak,or a weak radiator cap. (which would be a leak)

Hmmm. Thats what I thought. I guess its time for a pressure test. I screwed the cap off and it did release pressure but who knows. Thanks

... was the car cold when you removed the cap? pressure release when cold is a HUGE telltale sign of a bad head gasket...

No it was hot. I know it isnt a blown head gasket. The car only has 54k on it and the oil analysis showed no signs of coolant in the oil. I really havent looked too much into it. I dont know if it has been getting hotter than normal because there is no temp gauge. I cant tell what it runs normally or now, but there are dummy lights and they have not lit up. The level is only about an inch low but I know last time I looked (8k ago) it was full. The only reason I noticed it was because I was checking the oil as I recently did an oil change and I wanted to make sure everything was fine after a week.

Answer
 

NO.

There must be a leak.

http://www.peachparts.com/shopforum/general-information/299739-cooling-system-pressure-tester-kit.html




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Have the rad cap pressure checked. If really substandard and your engine was pretty hot sometime in the last eight K it might have lost a little coolant.
you may have a Car that is designed to operate at higher coolant temperatures.

Or as already mentioned a very small leak perhaps. Your electric fan is functional?

Wouldn't a bad radiator cap just blow excess coolant into the overflow tank and not allow a leak?

There is no cap on the radiator, its on the overflow tank just like the MB.The only difference is that on the MB there is a line that vents to the engine bay for overflow and the mazda there is not one.

pressure or vacuum?
 

I just did my water pump, thermostat and short bypass hose.
Been worried about the head gasket because it was driven for a while
with a leaking water pump and lost a lot of coolant.

Been checking the level and topping off as needed.
Checked last night and noticed the level was down an inch.
When I removed the rad cap, there was a whooshing sound and
the level jumped back up to normal.

I am taking that as a sign that the system is tight and I do not
have a blown head gasket.It's getting colder here at night and I topped
off in the day when the coolant was still a bit warm; all that cooling would
create a bit of vacuum.

Check the level before and after you remove the cap.
Whooshing sound can be pressure or vacuum.


Jeff

whoshing sound and the level jumps UP?

um, that means your system was under pressure, and you took the cap off, and the hoses shrank back down to normal size...
it's a pretty good sign of a bad head gasket... sorry.

hmmmm. think about it. how could vacuum cause the level to drop?
under vacuum, the hoses would be collapsed and it would reduce the amount of space for the liquid, raising the level, not lowering it...

OH NO!!
See what I know. What should I do to confirm bad head gasket?
There's no smoke that I see.

Jeff

yes, bad... if the level jumped up WHEN THE ENGINE WAS COLD, it's a sign of combustion gasses in the system remaining pressurized after the heat expansion has equalized...

My Thanksgiving weekend just took a turn for the worse:(

Jeff

best way to confirm is with a combustion gas analyzer. some auto parts stores sell them, maybe napa, it's little more than a tube that has a reactive paper in it, that you hook to the coolant area when the car is running.