Radiator cap hissing - how to test if it's actually holding pressure? - Vintage Mustang Forums
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post #1 of 21 (permalink) Old 04-29-2018, 08:54 PM Thread Starter
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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".
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post #2 of 21 (permalink) Old 04-29-2018, 09:04 PM
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It's normal for an engine to build a little pressure after shutoff- Coolant no longer moves, but stays in hot spots. Naturally, the cap will release some of the pressure when this happens.

Now, during normal operation, when the engine is fully heated, the coolant should be stable, releasing little to no pressure.

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post #3 of 21 (permalink) Old 04-29-2018, 09:20 PM
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If no coolant is coming out even when the temps are up, then I would think it is definitely holding pressure. Caps are pretty cheap, so you could always try a different one to see if it hissed the same. I recently replaced my 20 yr. old 13 lb. cap with a brand new 14 lb. cap for $16 I believe.
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post #4 of 21 (permalink) Old 04-29-2018, 09:21 PM
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The same kit you can "rent" at the auto parts stores "loan-a-tool" program has a nifty adapter to test the pressure cap.

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post #5 of 21 (permalink) Old 04-29-2018, 09:24 PM
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I'd get a pressure tester loaner tool from O'reilly or AZ. I rented the one from O'reilly good quality unit only downfall is it needs to be returned in 48 hrs or they charge you for it. AZ covers more cars but the quality isn't as good but you can keep it for 3 months before they charge you for it. If you rent one make sure it's new if there's a chance you can't return it.

You can check both the cap & radiator with either one.

https://www.oreillyauto.com/detail/b...rer=true?pos=6

https://www.autozone.com/loan-a-tool...ure/449951_0_0

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post #6 of 21 (permalink) Old 04-29-2018, 09:28 PM
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Originally Posted by Bob & Sue View Post
I'd get a pressure tester loaner tool from O'reilly or AZ. I rented the one from O'reilly good quality unit only downfall is it needs to be returned in 48 hrs or they charge you for it. AZ covers more cars but the quality isn't as good but you can keep it for 3 months before they charge you for it. If you rent one make sure it's new if there's a chance you can't return it.

You can check both the cap & radiator with either one.

https://www.oreillyauto.com/detail/b...rer=true?pos=6

https://www.autozone.com/loan-a-tool...ure/449951_0_0

Bob
I rented the one from Or'Reilly last night. A real quality piece. Tested the system - It held 15 pounds for an hour. Cap released pressure at 16 pounds. All is good.

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post #7 of 21 (permalink) Old 04-29-2018, 10:08 PM
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If you go to O'Reilly and take the cap inside with you they won't even charge you the "deposit" to use the tester there in the store.
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post #8 of 21 (permalink) Old 04-29-2018, 10:17 PM
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If you go to O'Reilly and take the cap inside with you they won't even charge you the "deposit" to use the tester there in the store.
Will they let you check the radiator also without the rental charge?

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post #9 of 21 (permalink) Old 04-29-2018, 10:36 PM
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Will they let you check the radiator also without the rental charge?
I guess that depends upon the particular store. As long as the car was parked right in front of the store windows where I could see it I would let the owner use it without the deposit.
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post #10 of 21 (permalink) Old 04-29-2018, 10:37 PM Thread Starter
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Awesome! I will test it. I found another radiator cap laying around in my garage and that one was substantially more difficult to screw on. Think I'll give it a shot and just see if I still get the noise. If so, I won't worry about the cap and will resume worrying about the motor building more heat than it used to
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post #11 of 21 (permalink) Old 04-30-2018, 09:37 AM
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A radiator cap or radiator not holding pressure will cause it to run hotter, less pressure boiling point goes up. I would lookup your radiator to find the right cap for it.
You should be able to rule out the radiator you say it's not leaking do a pressure test on it to confirm maybe 15-18 lbs pressure for 10 mins. Then get the correct new cap.

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post #12 of 21 (permalink) Old 04-30-2018, 09:51 AM
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ugh 4 min edit rule

Might be as simple as a loose hose clamp I'd check all the clamps you can get to. Then get the correct new cap, test it also I've bought bad new caps before.

Bob

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post #13 of 21 (permalink) Old 04-30-2018, 07:48 PM
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A radiator cap or radiator not holding pressure will cause it to run hotter, less pressure boiling point goes up.
Close but backwards; boiling point increases with pressure. That's the whole reason automotive cooling systems are pressurized. The larger the temperature difference between the air and the coolant, the more effective the cooling system will be (up to a point). But once the coolant starts to boil it forms pockets of vapor, which don't transfer heat as well. This causes the engine to heat more, causing more boiling and more vapor. This is what happens when your radiator boils over; it's a vicious circle. To get the heat transfer advantages of a higher coolant temperature the boiling point of the coolant must be increased. This can be accomplished by using a different cooling fluid, but water-based coolant is much cheaper and easier to handle than the alternatives. Instead it's done by pressurizing the cooling system.

As to Kelly's question: if Jane's radiator cap wasn't holding enough pressure your radiator would be boiling over. It's normal for a radiator cap to make some noise; it's a two way valve that allows vapor and coolant out if the radiator pressure exceeds the cap rating, and allows air (or coolant if you have a recovery tank on your overflow hose) back in when the pressure drops below atmospheric (ie when the engine cools down after driving). Coolant and vapor flowing through those valves normally makes noise. Hissing is a bit odd; my Mustangs both burp and occasionally groan, but I suppose each car finds its own voice

The only things I've seen go wrong with radiator caps are degradation/hardening of the rubber seals, getting the valves all gunked up, and corrosion. Once in a while I've seen aftermarket radiators with poorly-made cap fittings that prevent good sealing. Sometimes the aluminum cap fittings get worn out too; the steel caps are much harder than the aluminum flange. Anyway, it basically costs you nothing to test the cap, and next to nothing to replace it, so might as well! Maybe then Jane will burp too.

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post #14 of 21 (permalink) Old 04-30-2018, 08:39 PM
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When you turn off engine, water pump does no spin. So heat is not removed from block with cooled rad coolant, possibly, making the coolant in block expand. This is why some cars overheat when turned off but usually when there are other cooling problems. If Janes block is clean (flushed) then I think that I would look for cap problem. Your cap/filler hole may be bad or misshapen allowing coolant out. It is normal for some "hissing/overflow" some time but should not be happening all the time IMO. When a car is running, colling system usually does not build up to max cap pressure, especially if cooling system is working well (unless you are running a car hard then it is possible). I hope I was not too confusing, easier to explain in person than write. Almost sorry I tired lol.

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post #15 of 21 (permalink) Old 04-30-2018, 08:48 PM
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I just looked at your operating temps and seems like they may be a little high but don't think so. I am almost thinking that you may need to flush or thermostat is getting flakey if the temp is fluxing so much compared to before. This would both lead to car heating up really quick. Rust in block would also make Jane Burp more often as block is not cooling as well.

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