Anomalous cooling

[driveslikejehu]

I have an STI engine with stock swirl pot and Wayne hack for engine manifold to pot hose; an overflow tank next to the pot.
Normally it all works well around town on hot days. Overflow might fill 1/4 and return to system when cool. Fans come on about 205deg.

At the track, after 2-3 sessions, it fills the overflow tank and starts leaking out. I usually smell it, look down and see a big temp spike, also note higher oil temp. Look a bit later, all looks normal. After the session, the overflow tank is full.

Not sure what’s happening. I replaced the swirl pot cap in case it is weak, no change. That amount of overflow can’t be just thermal expansion of the coolant. Is a localized flash to steam somewhere causing the cap to release? Is the cap releasing, allow brief low pressure and a steam burp?

I feel like it is well filled with no air pocket cuz the manifold hose and filling slowly with a bucket deal. Stock system calls for a 16psi cap on the swirl pot and 20psi on the radiator. Its tough to believe the pump or thermostat are an issue.

I could try the 20psi cap on the swirl pot? For the moment, I’m relying on the dash temp gage. A water temp gage is in the works. I’m wondering if the ECU drives the gage in non-linear manner. I know my BMW’s were notorious for holding the gage at Normal within a fairly wide range and jumping up to the red when really hot.

I’m planning to add an oil cooler just to lower the heat load on the system.

Any thoughts relative to tracked cars would be appreciated.
202961

[blomb11]

I have not gotten to the track yet so take my thoughts with a grain of salt. However, I know some Subaru track people have issues with the different rad caps and I switched mine out after this Flatirons Tuning video. I now have a high pressure cap on the front radiator and the OEM cap on the swirl pot. They even interview Sgt Gator in it and get his take. Also from my research into these forums for the track users it seems like an oil and trans cooler are a must on these cars due to the heat load/soaking that builds up with the engine in the back. I think it is paramount to try to get as much air to move through the engine bay and exhaust it out the back to try and keep temps down. I am working on this now as I add coolers and work the aero pieces.

https://www.youtube.com/watch?v=HzjBJz1Zdmw

[driveslikejehu]

Thanks for that, the video is helpful. I knew the filler tank cap was 2-way, but didn’t realize the radiator cap is not. Makes sense to run lower pressure, as stock, on the filler tank and higher, 1-way on the radiator.

What I’m seeing is weird; it doesn’t seem to be “overheating.” The indicated temp will spike then immediately drop back. It seems transient, not like the whole engine is at high temp. Related to the cap opening probably, lower pressure would lower temp, unless there is a steam flash.

Maybe just a Subaru thing. But anyway, more cooling can’t hurt.

[mikeb75]

Any pattern to when the heat spike is encountered? High load, after, random? Frequently, high temps are a latent air bubble in the cooling system, but if you've seen this a few times one would expect the air had worked its way up to the upper reservoir & into the overflow.

You have the rad caps installed correctly; the high pressure cap stays on the radiator, OEM style radiators have no pressure bleed into an overflow. The lower pressure cap is on the upper reservoir; high pressure there will escape/vent to the overflow tank/siphon back.

How did you handle the T connection off the crossover pipe, the heater core loop and the accessory heating of the throttle body and the Oil "cooler" at the OEM oil filter location?

[driveslikejehu]

Everything on the engine is stock; oil/water cooler, throttle body. The heater pipes are connected with a Cyl 4 mod. Not sure which crossover pipe you're referring to.

It happens when fully warmed up, 2nd and later sessions, which are 20mins. Lots of WOT, full boost; a couple of part throttle longish, double apex corners and 2 good straights. Fully wring out 4th and 5th; just hit 6th on both straights. I didn't comprehend if it was a regular place during the lap.
203002

[Sgt.Gator]

Have you done a CO2 sniff test? If you are lifting the heads just enough to allow combustion gases into the water jacket intermittently you will have those symptoms.
We run one after every track day, it will warn you that things are going wrong.

https://www.amazon.com/UView-560000-Combustion-Leak-Tester/dp/B000NPDL76

How we burp the 818R:

203421

203422

[driveslikejehu]

So after a year, still not resolved but seems to be a head gasket leak allowing pressure into the cooling, not overheating.
I have new rad caps and a larger recovery tank. Around town recovery tank fills some; which completely returns on cooldown. At the track, on a 65deg day and nominal temps on the gages, recovery tanks fills more (~30oz) and doesn’t all return on cooldown. Also getting a few ounces in the rad overflow, which shouldn’t be getting any.

A bit of research shows this is a “Sube thing” that others have seen as Gator noted. My old school experience says a bad head gasket would cause oil and coolant mixing, but Sube folks have seen the pressure issue without mixing.

I like the idea of the CO2 test...
Planning a leak down test but not expecting much as the problem only manifests at the track, with sustained boost.

218644

[driveslikejehu]

I was wondering what this winter's project would be, now I know; head gaskets
Thanks to Gator and the Sube community I did the combustion leak test and yeah, looks like it. Not the pure yellow as some see, but I'm convinced given the small sample I had.
219958
It makes sense as I never had "cooling" problems, only pumping out coolant. Apparently plenty of WRX/STi folks have seen this on the flat fours. Looking at the coolant and oil passages, head to block, it fits.

[blomb11]

Hopefully that is the smoking gun so you can be ready for spring!