Radiator cap - should it be pressurized?

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Weak440

Weak440

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I am working on my '74 Duster. I am running a factory radiator and over flow/reservoir. It normally runs at 180 degrees... which is when the thermostat opens. The last two times that I have taken it out... it has climbed up to 195/200...

My thermostat opens at 180 like it is supposed to. I have not lost a head gasket, both the coolant and oil are clean and clear.

On my way home today it got up to 200 on the highway. I pulled off at a gas station and let it idle to cool off. What I noticed was that at 195 degrees my upper radiator hose was soft like there was no pressure in it. It eventually crept back down to 180 and I eased it back home.

I am using a 16lb Stant Lev-R-Vent radiator cap. When I got home It was back up to about 195. I tossed a rag over the radiator cap and gently pulled up on the lever only to find that there was NO PRESSURE behind the cap. I opened it up with it running varified that the coolant was flowing.

My question is, Shouldn't there be pressure behind the radiator cap in that situation?
 
Yes, it should have pressure. You can get a pressure tester on the tool loan program at most places to test both the cap and the system.
 
Sounds like the cap isnt right but i would think if it gets hot while driving something is clogged up, especially if it cools at idle.
 
Sounds like a lyin temp gauge to me.
How can it really get to 200 and not have any pressure, And not push coolant out?
 
This was about the time that I got home. It was running about 195... my transmission (727) was just under 150...

I didn't loose any coolant. Nothing boiled over, leaked out, or found its way in to the oil pan.

This is a newer electric Autometer gauge using a temp sending unit in the location of the factory original sending unit.
 

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does it have coolent in it after thatdrive?my dad said for me to always run a low pressure cuase of the steel plugs and parts the higher the pressure on older parts will cause breaks. i run a 7lbs
 
every 1lb of rad cap pressure you get a 2 degree higher boile point. So a 16lb cap gives you 32 degree higher boiling point so in turn your boiling point would be 244 degree under a 16lb cap
 
My thought is your radiator is not doing the job. Weather it is clogged or just not enough cooling capacity. Or your not getting enough flow for the heat to transfer through your radiator.
 
It should still be under pressure at 195 - 200 no matter what's causing the heating.
"No pressure in the radiator" just does not fit all the other statements.
 
I could be wrong but I don't think the caps with the levers are for systems with recovery bottles. Try using Chrysler p/n 5207 9880AA.
 
ESP47 said:
I could be wrong but I don't think the caps with the levers are for systems with recovery bottles. Try using Chrysler p/n 5207 9880AA.
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I use one, and it works fine with the recovery system.
As long as it has the little recovery valve built into it, it works just fine.
 
SGBARRACUDA said:
My thought is your radiator is not doing the job. Weather it is clogged or just not enough cooling capacity. Or your not getting enough flow for the heat to transfer through your radiator.
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I agree with SG here......sounds like the radiator isn't quite up to the task, although 195~200 is nothing!! It's not even boiling temp for plain water, much less coolant. The only thing that would bother me is if you really taxed that cooling system, like say on a hot day on the freeway with a carload of people going up a long grade. I think you'd be sweating bullets and staring at that temp gauge, which isn't too fun.

I've had cars that had zero pressure in the cooling system like that. I knew it wasn't right, but they were just putt-putt around town cruisers. It shouldn't affect the running temperature of the coolant in average day-to-day grocery getter vehicles.
 
I had the no pressure issue. I had a bad 16lb Stant Lev-R-Vent and was also loosing pressure at the top hose I'm guessing steam. Tighten hose clamp and replaced with new lever cap. Pressure is back.
 
Its not easy to tell by squeezing the upper hose. 18 psi aint much resistance.
I run the lever release cap with my recovery tank also. Works perfect.
New cap is what ? 8 bucks maybe ? A fan clutch or other would cost much more so lets hope its only a weak cap.
 
RedFish said:
Its not easy to tell by squeezing the upper hose. 18 psi aint much resistance.
I.
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Red, yur slippin here, LOL.

You should be able to easily tell by squeezing the hose.


Is it possible you got the wrong depth cap? There are different depth radiator necks for different applications, and of course the cap could just plain be defective.
 
those lever vent caps and most aftermarket style caps are junk and made for mostly GM; get an OER style cap and pressure test cooling system make sure it will hold
 
67Dart273 said:
Red, yur slippin here, LOL.

You should be able to easily tell by squeezing the hose.


Is it possible you got the wrong depth cap? There are different depth radiator necks for different applications, and of course the cap could just plain be defective.
Click to expand...

Maybe you have a calibrated grip. I didn't get one. :(
 
67Dart273 said:
Red, yur slippin here, LOL.

You should be able to easily tell by squeezing the hose.


Is it possible you got the wrong depth cap? There are different depth radiator necks for different applications, and of course the cap could just plain be defective.
Click to expand...

I don't know I have agree with him. I have no problem determining pressure on the top hose. And it has been over 5 decades since I had my grip calibrated.
 
believe me, you can fell pressure in a hose. I just had a "moment" with beater 2.0 (82 Maz B-2000 P/U) Im cruising up the 210 on my way to pick up some $20 leaf springs and the temperature gauge is never more than 1/2 between hot and cold, more like a tick above low range so Im good to go..la de da..oops, gotta pull over as my pedal linkage has slipped out of the carb (stupid design)..WTF is that steam coming from my hood? I get out and pop the hood and my DIY gatorade puke tank is full and the hose is flipping around like a fire hose shooting green radiator scum everywhere. DAng, I thought I was running fine? I limp it to a gas station and douse a beach towel with cold water and use it folded over about 5 times to uncap the radiator cap to its safety position, well it blows now big time but Im fine as I took precautions and it all just jets down onto the pavement. This thing was freaking HOT! Hmm, I start the motor (starts right up and idles fine) and start adding water slowly. It doesnt back up on my, just fills to the brim and I put the cap back on. Oh, and that upper hose was hot and rock hard BTW. So i drive another 4 miles to my buds house and check cap again, no leaks, no puke, seems ok. WE check the cap, clean both rubber gaskets with gojo and top off radiator, and he hooks me up with a real bottom feeder puke tank I strap to the side f the battery. I am looking at my sender and I notice the brass tit is wiggly, so much so that I pull it right out ! Looks like it broke or came unsoldered to the sender, thats why my gauge was only reading 1/2 as there was little or no continuity in the sender. Well, I drive 50 miles home in LA freeway traffic in 85 degree heat and it never even flinched. I think the cap was not on properly in the first place and the "no pressure" radiator promptly boiled over, loosing coolant and causing a chain reaction that left me high, hot and dry! Get a pressure rated cap, if your radiator starts springing leaks at 7-16 lbs, it was already shot and was about to go on you anyway. Boil over with a 16 lb cap and straight water is 244. boil over with 50/50 can be as high as 275!
 
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