When was the core last cleaned?
The reservoir wouldn't bubble - the release cooled the fluid.
Check the fan is working, adequate coolant, pump working and thermostat working - if you are not comfortable with that it may be a garage job to find the cause.
The radiator could of been a bit low. When the engine is cold, fill both the radiator & the expansion tank to the top. Drive a day or two, then re-fill just the tank - DO NOT even open the radiator. Re-fill just the expansion tank every couple of days until it stats full.
Since you do not mention overtemp on your coolant gauge when you noticed the steam can we assume the gauge read normal??
Has you said it could be a weak spring in the radiator cap ,check to see if the spring has a broken coil in the radiator cap .
Yesterday on my way home at a stop I noticed a bit of steam coming out from under my hood. Not pouring out, but enough to notice. I stopped to check under the hood. There was spots of dried coolant on the engine near the cap, some coolant in the crevices on top of the radiator, and some coolant in various places. The reservoir was low, so I added more coolant.
I never saw specifically where the coolant was coming from, but the spots near the cap as well as the top of the radiator point to that.
All hoses seemed fine, none were collapsed. I got plenty of heat from the vent with no smell. Car drove fine, no issues. Coolant looks fine, nice clear green. Temperature gauge did not go high.
This morning I doublechecked to make sure I had coolant and drove into work. Everything seem fine. Good heat from the vent, car not getting hot, no loss of coolant, no wisps of steam.
Everything seems to point to a bad radiator cap. But I'm wondering if there's something else I'm missing? I never saw coolant coming from under the cap, but it was a couple minutes between noticing it before I had a chance to check under the hood. Could it just be a release of pressure under the faulty cap was able to seal again?
