Fans are not turning on due to air in the system, which is preventing the thermostat from opening.

 

I am assuming at this point you pressure tested the system? It sounds like its a really really small leak.

 

I took it to the dealership. They said they checked it out and couldn't figure out what was going on, but I needed a new radiator. Didn't instill me with confidence. They gave me a no and a yes on pressure testing the system, so I'm sure they didn't test the sytem. That will be my next stop, but I'm not losing coolant at all.

 

The rad cap an hold a certain pressure.
Go above it and it opens into the expansion tank.
In a sealed and completely filled system it will show a higher level in the expansion tank when hot.
Coolant expands.

How full does the expansion tank get?
All the way?
If so, that makes me think there is pressure entering the coolant system.
Not just the pressure you get from heating up & expanding but extra (higher) pressure.
The only source I can think of is combustion pressure = headgasket.

And how long does it take to fill the expansion tank?
How many heat cycles, how much time at operating temp?

If the rad cap would allow coolant out but not back in you should get a vacuum in the coolant system.
If it is sealed.

 

"If it is sealed." That last sentence is the key.

The head gasket is fine, no smoking and no coolant smell in the cabin. The problem is that the system is not sealed, that's what I've been saying all along. That is allowing a slow air leak in, which is causing the air bubbles plaguing my system, while allowing pressure to exit (which is preventing the expansion tank from draining back on its own).

The problem is that the system is completely sealed until I turn the heat on. When I turn the heat on, there is a leak somewhere. I'm just wondering where there could be a leak that would only be open when I turn on the heat.

 

Ok so I took it to a shop and here's the diagnosis:

They pressure tested the system and found two small leaks: one at the water pump and one at the thermostat housing. They replaced both (I had them put in a Mugen thermostat that I had purchased a month or two ago), and bled the air out with a vaccum thing designed for that. So far no issues, although the heat is not really that hot unless I get on it. I think it may be because the engine is running cooler and it's so cold outside?

They said the reason it was happening when I turned the heat on was that I kept bleeding out the system, but I didn't have the car high enough so there were still pockets of air collecting in the heater core. When I turned the heat on, that created the problems. I'm not sure if that makes sense or not, but I dropped $550 on a new water pump and thermostat + install, and now it appears to be ok. I opened the bleeder valve by the valve cover with the engine off, and coolant started shooting out (no air bubbles) after I was driving around with the heat on for 15-20 minutes. So it looks like problem is solved! Thanks for the help, everyone!

 

$550?? OUCH!! you could have done that yourself. It's not that hard especially on the s2000 since there is no timing belt.

 

Yeah I know that sucks. 2 hours labor on thermostat, 2 hours on water pump. I wasn't going to do the thermostat though, that was a monster of a job, it took the shop way more than 2 hours for it. Turns out someone had worked on the thermostat before (not me), and cross-threaded the bolts, so it was tricky on that part. Should've done the water pump myself though.

 

If the heater core is broken air bubbles would cause overheating all the times and on the passenger side mat should be wet. Check what ur coolant temp says. If its reading correct then ur cooling system needs to be checked out

 

They didn't say the heater core was broken, they just said that the air bubbles were collecting in there because the car wasn't jacked up high enough when I was bleeding the system.