Where should the coolant level be after the car has cooled down?
 

I'm trying to rule out a (bad) O-Ring problem. After the car is completely cold, where should the coolant level be in both the filler neck and my aluminum AST? It was reasonably cold overnight (about 55), and when I check the levels in the morning. The level in the filler neck was about 2 and 1/2 inches from the top (the coolant level sensor was still submerged), and the AST was about half full. I topped the system off through the filler neck until the AST was full. Is this normal, or do I have a bad O-Ring problem?

Thanks!

Check your exhaust when you first start the car in the morning. If your o-ring(s) is on the way out, you "should" smell coolant burning when you first start the car. Plus, you'd probably be losing a good deal of water, and it doesn't sound like you are. :)

Really the best way to diagnose an o ring problem is to remove the radiator caps while the car is cold, start it, and watch the coolant through them until the thermostat opens, if you get "champagne bubbles" when it opens then you have a problem.

The coolant should be full in the filler neck and will probably have some air in the AST. You probably have a small leak somewhere else

When I start the car in the morning, there's a little smoke for the first couple of minutes. The exhaust doesn't smell like coolant, it smells very rich (like gasoline).

BTW, does anybody know what pressure cap I should be using on the AST? I've got an OEM 16psi on the filler neck and a generic 13psi (with pressure release valve) on the AST right now.

Thanks!

.9 bar (13 psi) is the current (post Mazda recall) AST cap...

A little white smoke is normal when starting, especially if its cold.

From this and the rest of the advice others have given, it doesn't sound like an o-ring seal, good news!

The Filler should have a solid cap (no spring) and the AST should have a 13<>14psi 2 way (vent / return) cap. the Filler neck should ALWAYS be FULL to the top the AST should be full to the same level as the top of the filled neck (near to the top the AST should be the HIGHEST point of the system). the Expansion tank should have 1-2" in it when COLD and 3/4 full when HOT.
White STEAM w/ no odor is normal when starting, Sweet Smelling SMOKE that lingers is BAD. a Slight acrid GAS smell is OK also, but indicates a need for new Plugs &/or Plug wires. Gray Lingering smoke indicates worn OIL control rings, &/or Turbo Oil seals

That 16psi cap on the filler neck is probably your problem. I noticed they dont seal very well there, they seem to work for a month or so then start to leak. You will have to spend about $20 for the correct cap at the dealer but I would bet this solves your problem.

I saw bubbles in the filler neck when I started the car, so I figured that my RX has an O-Ring problem.

I decided to try to make it through the summer without a rebuild, so over this weekend completely flushed/degreased the cooling system and put 2 cans of K&W Permanent Metallic Block Seal into the car (and drained it after driving it around for 30 minutes or so). I just checked the coolant level after my morning commute and I've got about a 2" air gap in the filler neck, but the AST is full. I also put a 13 lb Stant pressure release cap on the AST. I'm guessing that the block seal lessened the amount of exhaust gases getting blown into the cooling system, but didn't seal it completely.

Can anyone without an O-Ring problem check how much gap they have in their filler neck?

Mine is always completely full in the filler neck. I check it at least once a week lately.

Weird if I open up both caps and try to fill the filler neck all the way up to the top, coolant will overflow the AST and equalize when the coolant is about 1 to 2 inches form the top of the filler neck... The only way I can fill it to the top is to put the cap on the AST does yours work this way?

Your AST is too low I think.

I had bubbles in my coolant from a turbo coolant line. I had a slow leak there and it was probably 2 weeks before I saw smoke from under the hood.

Mine does the same thing because the AST is lower than the filler neck. You need to put the cap on the AST before topping off the filler neck.

i was chasing a similar problem and it turned out to be a pinched overflow tank hose. the car would get hot and there would be positive pressure in the system, forcing coolant into the overflow tank. then, when the car would get warm, the coolant system would start to creat a vacuum. normally, this would result in the coolant in the overflow tank being sucked back into the system, but since there was a pinch in the hose, the vacuum would completely collapse the overflow hose and prevent any coolant from going back into the system. thus, every other day i would have a coolant warning alarm going off in the morning. after a tip from another forum memeber, who's name i don't remember (thank you), i figured out my problem. and fyi, i thought i had champagne bubbles and everything else that indicates a bad coolant seal, but it turned out to be the hose. i think my pettit intake had something to do with it as the metal heat sheild requires re-routing of the overflow hose, which i initially did half-assed. so if you have that intake, it is even more likely to be your problem. in any event, i have driven about 3000 miles after finding the pinch and the coolant level has remained constant. hopefully, this is your problem.