Radiator cap hissing - how to test if it's actually holding pressure?
After yesterday's drive and being around some other peoples' cars for a change, it occurred to me that Jane's radiator cap is unnaturally... hissy.
Whenever the engine has been run for enough to build up a bit of heat (not even to temperature, just to something above "stone cold"), you can hear the radiator cap hissing after you turn the car off. It also sounds a little bubbly almost, like it is digesting I guess.
No coolant coming out and there's no evidence of exhaust gasses in the coolant (in the form of visible bubbles in the radiator), so I don't think it's an issue with pressurizing from a leaky head gasket or anything. Coolant levels aren't dropping at all, and yesterday I put 450 miles on the car at pretty rigorous speeds so if it was going to lose any coolant I would have noticed by now.
I've always been able to pull the cap off pretty reasonably without any kind of drama when the engine is hot - that is, I can let a little bit of pressure bleed off for about 10 seconds and then remove the cap without it trying to go flying into outer space. This has led me to wonder if the radiator cap is maybe not holding pressure the way that it should. Not sure if it would be an issue with the cap going bad, or if maybe the Northern radiator wants a certain type of cap - when I first put it in a couple years ago, it came with a cap but I just stuffed my old one on because I liked the way it looked better. Never bothered to see if the seal was placed the same or anything.
Car doesn't overheat, but does tend to fluctuate in temps with ambient temp / speed more than she used to before installing this radiator. Yesterday it was 80*F out and on the highway she was running at 185*F with a 192*F thermostat in (below operating temp). At idle, I was looking at 195*F (normal temp). In past warmer days in traffic, she has gotten up to 210-215*F pretty fast but does not continue getting hotter after reaching whatever equilibrium she seems to prefer. Last year I did have to deal with one "major issue" day when we sat at a near-standstill in 110*F ambient temps in the sun, surrounded by semi trucks, for 3 hours, and then the motor seemed to want to hang out at 230-240*F. Again, same thing - didn't continue getting hotter and hotter, just seemed to want to get to that temperature and sit there. I didn't let it do that long term (ran the heater and ran RPMs up to keep it to 220*F), but when I did let it go that's where it sat.
So here's the question: Does that sound like a radiator cap issue, and how can I test the cap? What should I be looking for in a new cap? I don't know why it's never occurred to me that this is strange before, but... well, it's Jane, and Jane does weird things and that does not even rank on the list of top "weird things Jane does".
1966 Modified Fastback - Driven semi-daily!
and rebuilt even better
289 v8, 4-speed, 3.25 9" rear, goodies and stuff.
See my travel blog here for my adventures: http://mapandamustang.blogspot.com/
Over 50,000 miles of North American roads driven on road trips since 2014. More always in the works - stay tuned!
When I show up at a car show and my car is covered in mud, it's probably because I drove 2000 miles to get there