Well to start off repair wise I'm going to replace the cap and hope it's the cause of the air. Then from there I'll order a set of gaskets and go at it I guess. Would a head gasket leak be intermittent though? I would think that once there's a way through it'll show at all times?

The very beginning can be , or at least seem to be intermittent. I'd keep wtchinh it closely for a bit, and see what happens.

Watching

Would a head gasket leak be intermittent though? I would think that once there's a way through it'll show at all times?

Yes, that can happen. They don't always show all the time. It's usually engine specific though and I've never seen those old-style headgaskets like the EJ22 has do it, but it does happen so I wouldn't be surprised for other engines to be able to have that failure mode, but it's very rare if it happens on EJ22's.

 

The EJ25D and EZ30 engines with failing headgaskets can have come-and-go symptoms, sometimes going months between symptomatic events. i've also seen them be directly related to ambient temperature and vehicle demands (idling, A/C on), etc.

the EJ25D's (1996-1999) would have randomly have overheating events - separated by months sometimes.

 

two H6's i've seen would only exhibit symptoms in the summer, and during high load times - hot, A/C on, mountain driving. which doesn't make sense in some ways - with internal combustion pressures and temps being so much higher than ambient, as well as a "controlled" engine temperature via the thermostat - it's hard to imagine ambient temperatures and loads affecting that, but i've seen it so it is what it is.

 

Keep in mind this is all just an answer to that one specific question you asked, I'm not suggesting this is the case for your exact issue.

Edited January 11, 20179 yr by grossgary

Well this only started happening once the temps got into the low 40's and it seems to go away when it warms up a bit outside but then comes back once the temps drop at night. I've never seen an issue like this in any engine but I'm used to Nissans and this is my first subaru. I plan on getting a baja after I sell this impreza, also something that's a manual.

So good news after a little bit of bad news! I fixed my issue! It took 2 replacement caps and some aggravation but so far it's holding pressure. The first cap had the same problem as the old cap. But the second cap works perfectly.

 

My issue wasn't the seals on the cap but it was the pressure spring in the cap itself. It would hold pressure until coolant reached full temp and then it would allow all of the excess pressure to vent into the resovoir until that overflowed which ultimately resulted in a low coolant level. Then once the engine cooled down over night it would suck the resovoir empty and since the resovoir overflowed it would then allow air into the system in place of the missing coolant.

 

So this sudden problem and huge headache was caused by a seemingly fine radiator cap with a faulty pressure spring.

Good job tracking it down, now you know!  and thanks for following up.  

Was it a Subaru or aftermarket radiator cap?
Was it the original radiator cap?

I have no idea if it was the original cap or not but it could have been, it has an orange sticker with Japanese writing on it.

 

The replacement was an aftermarket one, I can't remember the brand but its made in Japan so it's not a crappy cap. I originally bought it for my 87 pathfinder and these 2 vehicles happen to share the same replacement cap. It was like $6 I think.