Radiator cap pressure rating

[NewDog]

Tracking down a coolant leak, which I think is my rad cap. It's not leaking enough to even drip onto the ground, but just enough to leave small amounts of coolant on the battery tray and frame rail. Hoses in the area seem all good and clamps have all been tightened up.

As info, I am running a 302 with c4 auto, with a WH Staggered 4 Core rad (connected to coolant overflow tank) and separate trans cooler. Truck is a 71, street cruiser, lives a pretty pampered life now.

The current rad cap looks to me like it has a bit of a nick in the rubber (see photo), so am replacing it. It shows an 18 lbs rating.

From what I can find, most rad caps for the EB's seem to be rated at 13 lbs. Not sure why mine is 18 lbs.

Is there any reason why one would want to run a higher pressure rating than 13 lbs?

Thanks,
David

 

[73azbronco]

Because it leaks? Might have a mating surface issue on radiator

 

[langester]

I have always run 13 lb caps. Not sure why you would want any higher, I am no expert either. The seal on yours looks shot, if it was mine I would get a new 13 lb cap and see what happens. As was said double check the mating surface on the radiator for damage as well.
Good luck with it!

After a little searching, here is an interesting article. It explains some of the reasoning behind the different pressures.
Hope it helps!
https://www.allpar.com/fix/engines/cooling-caps.html

 

[B RON CO]

Hi, I have used the Stant Lever vent radiator caps forever. # 10329 fits the stock Ford radiators. Good luck

 

[Pa PITT]

I did not read much of that but i'll add my 2 bits to this.
Rad cap are about the cheapest thing out there . Just replace it.
& I'D Put a 13 on it.
... Now water boils at 212. You basically don't want it to boil in the motor or radiator.
SO For each pound of pressure cap you add 1 1/2 degree to your 212 boiling point .
SO a 10 pound cap adds 15 lbs. So 212 becomes 227*.
& A 13# BECOMES 4 1/2 MORE OR 227+4.5 MORE SO 231.5*
... & I SUPPOSE YOU COULD TAKE A FLAT FACED FILE & draw it across the rad neck top.
I'd think that'd flatten the surface & show you what you got.
GOOD LUCK.

 

[NewDog]

Thanks all for the feedback. The mating surface on the rad looks totally fine. I bought a new 13 lb rad cap, will try it out and report back.
Thanks again.
David

 

[DirtDonk]

Tracking down a coolant leak, which I think is my rad cap.

These can leak at the hose fitting under the cap too. Not as likely on a newer radiator as an old one, but it's still possible so keep an eye on that.

As info, I am running a 302 with c4 auto, with a WH Staggered 4 Core rad (connected to coolant overflow tank) and separate trans cooler. Truck is a 71, street cruiser, lives a pretty pampered life now.

A '71 would not have had an overflow/recovery tank originally, so make sure any cap you get new is compatible with a recovery system. If they still sell caps that are not, the one normally listed for a '71 might not have that capability.

From what I can find, most rad caps for the EB's seem to be rated at 13 lbs. Not sure why mine is 18 lbs.
Is there any reason why one would want to run a higher pressure rating than 13 lbs?

Yep, no A/C and no really heavy duty cooling, other than what was listed on the window sticker. Which most of them said they had!
And less likelihood of a leak in the under-engineered system that would come back to haunt them.

Someone either just got the cap that was available, or wanted more pressure to raise the boiling point. Maybe they were even having a problem, or thought they were having a problem with boiling over.
Along those lines, what size is your recovery tank by the way? If it's too small it will puke fluid all the time and might lead one to believe the engine is running hotter than it is.

That was an interesting article langester. Thanks!
I have even had several older cars with heavy duty cooling and A/C and I'm pretty sure even some of those ran just a 13lb cap. I can't remember the last time I saw an old car with a 16-18 lb cap. Just newer cars.
But it makes sense. More pressure, better cooling from fewer flow issues deep inside the jacket, higher boiling point for when it does heat up. Makes total sense.

Paul

 

[NewDog]

Thanks Paul. Always appreciate your insights.

Along those lines, what size is your recovery tank by the way? If it's too small it will puke fluid all the time and might lead one to believe the engine is running hotter than it is.

It looks to me like the standard size recovery tank, whatever that is. Haven't had any issues with it overflowing. Will keep an eye on it though.

 

[NewDog]

These can leak at the hose fitting under the cap too. Not as likely on a newer radiator as an old one, but it's still possible so keep an eye on that.

A '71 would not have had an overflow/recovery tank originally, so make sure any cap you get new is compatible with a recovery system. If they still sell caps that are not, the one normally listed for a '71 might not have that capability.

I did make sure the replacement cap was compatible with a recovery system.

But to your first point, now I wonder if there is too much bend or kink in the hose, where it exits out of the rad? I'm thinking maybe that bend impedes the flow of coolant out of the rad to the overflow tank, to the point where coolant is forced out of the cap instead. What do you think?

 

[NewDog]

Hi, am happy to report that a new rad cap (13 lb) solved the problem.

Took it out for an hour or so drive, temps stayed where they always do (190-210), and no annoying coolant leakage anymore.

Love it when the fix turns out to be simple (and cheap)! No cans of worms got opened this time...

Thanks all for your help.

David