“Loud bubbling” after you turn off the car is the coolant being sucked back into the coolant system from the overflow bottle. Simple physics in play. Sounds odd but normal ime

 

I do not think that is normal unless the engine has been used hard in hot weather just before shutdown. I haven't heard that gurgling/bubbling since I changed to the Miata 180F (?) fan switch. As I said above it's coolant boiling on shutdown. When the engine is hot, the coolant continues to get hotter just after shutdown, especially at hot spots, as it absorbs heat from the engine metal. Coolant doesn't begin to be pulled back in until it's cooled enough that it's temperature/pressure reduces enough to cause a partial vacuum in the cooling system, which happens only after cooling for a significant period of time. And I've never heard that process make any noise.

Using a coolant mixture that contains lower than 50% antifreeze (lower boiling point) will make that boiling more likely.

 

delete - double post

 

I agree with everything you've said in this post but I feel like I need to point out that the cooling system will be pressurized to about the pressure rating of the cap which will raise boiling point, it's more likely if you hear gurgling that it is air pockets that couldn't have been bled before making their way out of the cooling system. Every year when I change my coolant I will hear gurgling for the following 4 or 5 drives despite using a lisle funnel, I can usually hear them frothing on the rear iron when the coolant makes that big U-turn from the bottom of the engine back to the top, and then again near the coolant filler neck.

On the original topic, coolant spilling out of the overflow can be a symptom of failed coolant seal but can just as much be a poor seal between the cap and the AST, or even a bad cap. I have also had problems using a combustion gas sniffer tester to test for coolant seal failure, though I'm wondering out loud if there would be better success using the sniffer tester at the opening of the coolant reservoir, specifically in this case where coolant is escaping out of it.

 

I agree with the above and should have mentioned pressure does increase boiling temperature.

 

You are right DaveW. I probably should have said "could be" instead of "is" as it would depend on what's happening with the specific engine and any issues it might have.

 

An interesting update, took time away to get a total hip replacement and just got back to replacing all the broken solenoids.

i removed the throttle body all the way and the lower coolant hose had a layer of sediment that plugged it completely at the rack. Was a relatively thin layer of sediment but was surprised to find.

Will be a bit until I get it back together but am curious to see if that’s what’s been causing my headache after all or not.

 

I wouldn't doubt it, but if you had some there, I would say there is a good chance of build up in other places as well.

 

New motor in. Old motor out. Take it apart. If its good inside - sell off the good parts.

 

no doubt, I’ve had everything apart but the engine itself now, and everything else does not looked bothered. Going to try and wrap up the vacuum stuff this weekend and see what happens. I have a good core ready to go if needed

 

I wonder what sort of high pressure coolant flush option there might be? My concern is the lower channels of your housings may be rotting away. You often see scrap housings damaged in such way.

 

Just posting to say I had the same AST cap sealing issues. Chased the issue for months when all it took was a little sandpaper on the AST sealing surface and never had the problem again.

 

Thinking I got lucky here. Had it running for 20 mins or so and the temp stayed nice and low, it was now blowing heat and the bubbling that was present before is now gone.

i’m going to flush one more time and then get her on the road for the real test.

I went ahead and replaced all the vacuum lines and solenoids, and put new plugs and wires and she is purring, still has a slightly high idle a tick above 1k but not overly concerned with that at the moment.

 

Feel the coolant hose to the top of the throttle body when its warmed up. If its not hot it has an air bubble and can cause that high idle.