Boiling coolant in a Miata (MSM)

It was 92 F driving home from work today, so I ran the A/C the entire 20 mile drive home. Traffic wasn't even bad for once, so I was mostly moving. When I shut off the car, I heard a gurgling sound coming from under the hood and knew immediately that the coolant was boiling. The temperature gauge read normal. The boiling was pushing coolant into the overflow tank, and it kept on for about 5 minutes after parking.

My first thought, maybe because it's the least expensive option, is a bad radiator cap. It's not original, but has about 50k miles on it. Once it cooled, I could hear air and/or coolant leaving and entering through the overflow while squeezing the upper hose. If the cap were sealing properly, I wouldn't expect coolant to reenter the radiator so easily. Removing the cap showed the coolant level to be normal, and no air could be burped out of the block. The overflow tank was at the low mark, so some coolant was pulled back into the system while cooling.

Other thoughts are a leaking original radiator with 110k miles on it. The tanks are just starting to turn yellow, but there are no obvious leaks yet. I was going to replace it with an aluminum radiator in the next few months anyway.

Of course, I could have a leaking head gasket or cracked head/block. There are no obvious puddles, but after owning a MINI I know that doesn't mean much. Small leaks tend to burn off. I also do not see any coolant trails or cooked-off puddles. Also, the coolant is clear with no traces of brown sludge.

Am I missing something to check?

The normal stuff.....

-Did you check for proper cooling fan operation? -Is space between and radiator clean and free of debris? -Foam gasketing directing air through coolers rather than around in place?

I'd replace that cap, sounds iffy...probably do the radiator too, since it was on the list anyway, it's old, and prone failure at some point if not now, besides, it's probably not working at its peak.

In reply to TGMF:

Both cooling fans on with the A/C going. There are no blockages unless you consider the rather large FM intercooler (at least compared to the tiny stock MSM one). I haven't added ducting, but also haven't had an issue in 98 F heat at an autocross with this setup.

Bad radiator cap. It's not holding pressure if it's boiling at normal temps and the leak is out the overflow, the cap's job is to control that leak so the system stays at a set pressure when it heats up.

I would also check the thermostat. Had that issue with my 99 Miata. It also would go into limp mode though, wouldn't go over 3k rpm. Gage always read in the normal range. Ran normal after cooling off. Pulled the thermostat and found it cocked inside. Allowed enough coolant through so the gage read normal but was actually overheating due to inadequate coolant flow. New thermostat fixed it and haven't had any issues since.

And the cap should ALWAYS allow coolant to easily get pulled back in.

The engine may also have just been running hot. There could have been boiling spots in the head that wouldn't affect the temp gauge because steam is a good insulator. Or the hot spots may have been on the cusp of boiling but water pump pressure kept it in check, until you shut the engine off, at which point it started boiling. (This was a normal cooling system characteristic with FD RX-7s, they'd boil and gurgle for a little while after you shut them off)

Of course, if you had a good radiator cap, that'd probably be enough to keep afterboil from happening.

Thanks guys. I'll pickup a new thermostat while at the dealer. The new radiator and hoses can hopefully wait until fall. I just don't have time to deal with it right now.

Knurled wrote: And the cap should ALWAYS allow coolant to easily get pulled back in.

I was talking about fingertip pressure forcing coolant in/out of the radiator cap. It's a 16 lb cap, so should hold some pressure.

Joe: Were the fans running when you shut the hot engine off? If not, there are three relays on the drivers side, near the diagnostic port, on a brass pedestal, that you should check. The relays are all identical, Imasen H270. If one fails, the fans will only run in certain modes, or perhaps not at all.

I know both fans would come on when the A/C compressor was turning (clutch engaged). It's possible that it wasn't coming on from just the temp sensor, but I can't say for certain. I'll check the relays.

Visual inspections first -- warm it up and validate that the cooling fan cycles and both fans are on with AC. Check the plugs for any that are really clean looking. Look for white residue on the plastic radiator end tanks...it could have a hairline crack. Check for oil / water on the dipstick, oil cap, overflow. You can dump the overflow into a drain pain to look for an oily sheen. Then you can pressure test the cooling system, and/or get a hydrocarbon test done on the coolant.

Otherwise I say fans, thermostat, rad cap, headgasket in that order.

Issue hopefully fixed. The fans checked out and still no traces of coolant. I installed a new radiator cap and no more squishy sounds when I squeeze the upper hose. Drove 6 miles to get gas with the A/C on and let it idle in the driveway without A/C for a few minutes upon return. At least now I have a new thermostat for when I replace the radiator.