Radiator Caps - they're all not the same ya know?

Cobra20646 · #1 (**permalink**) 09-30-2001, 05:24 PM

I guess when you buy any previously owned auto, you'll find just about anything that has been done to it, has been either done right or very wrong. Such is the case with my car.
During the past years restoration of my car I've had a few run ins with the cooling system. I tracked down a leak here and there and thought I had the thing all tightened up. But when checking the coolent level in the radiator, I noticed the level was always low. It was always visable in the upper tank, but never full to the brim. I'd top it off, run the car, and after a day or two, I'd notice the water temp sitting a wee bit higher than normal. I'd then let it cool off and check the level and it would be low again.
It wasn't until this weekend that I noticed that the level of the coolent in the external reservoir (pressurized coolant recovery system) was increasing!
These systems will push coolent out of the radiator and into the reservoir when the coolent heats up and expands. When the radiator cools down, it is susposed to "suck" the coolant back into the radiator from the reservoir, keeping the thing full.
Well, seems my radiator cap was one way only! I compared it to one off my regular street ride and saw immediately there are two valves that must be on the cap to pass coolant both ways.... the wrong cap was put on the Cobra.

Now to replace it with the correct one........... and here's where I'll need some help.
These things come in various pressure settings. The one on the car now is 13 pounds. I know the pressure effects the boiling point of the coolant, but what other criteria does one use to select the proper one?
I went to my local auto parts store today and the counter guy couldn't offer any advise......... seems his computerized parts reference guide didn't list a 82 Unique Cobra with 302 cu in engine without A/C..

If it ain't in the computer......... well you know!
I noticed two caps that he had hanging on the wall that were rated at 16 pounds, that looked identical to one another, but had different part numbers. What kind of slight differences are we talking about here?
So, all you coolent guru's, what's my next step? Or do I just pick one and try it on for size?

- Jim -

poorboy · #2 (**permalink**) 09-30-2001, 05:43 PM

Jim;
What type of cap is on the coolent recovery tank? It should not have a gasket on the smaller of the 2 sealing surfaces, also the vent has to be open to the air. A number of people put a good sealing cap on the recovery tank, and plug the vent. This stops the coolent from being able to return the radiator. I would use a 15# cap. The hose from the radiator to the tank needs to go in the bottom of the tank.
Poorboy

Cobra20646 · #3 (**permalink**) 09-30-2001, 06:11 PM

Poorboy,
The radiator cap off the cobra only has ONE valve. This is the one controlled by the large spring and takes some force to press back. This valve will allow the pressure built up in the system to open it up and move coolent into the reservoir via a small hose. This cap is obviously not the correct one for a recovery type system. The cap off my regular ride has TWO valves. The aforementioned one, plus a smaller one that open if it sees a vacuum in the system, and allows coolent to be pulled back into the system.
The reservoir tank has a vented cap, and the connecting hose is positioned at the bottom of the tank so it will not suck air ( unless the tank is empty

)
I'll try the 15 pound cap like you suggested. It's gotta be better than whats there now

Thanks
- Jim -

Bob Putnam · #4 (**permalink**) 10-01-2001, 04:29 AM

Jim,

There will only be a few degrees difference in boiling temperature between 13 and 16 psi. As long as both 16 psi caps you've got are the reverse-flow type they should work if they seal on both the inner and outer rims of the tank. There are a couple of different heights from inner seal to outer, so you'll have to look carefully for the match. The cap must have sealing gaskets for both the upper and lower surface.

Steve Siedlecki · #5 (**permalink**) 10-01-2001, 04:03 PM

Boy, what a simple thing that I almost overlooked?!?!?!!? I am now in the process of hooking all this stuff on my cobra. I'm running a griffin radiator in front of a 351 windsor.
I was told to put a 20lb cap on the radiator and
use a 20 lb cap on the remote fill. I too am running a goza recirculating catch can. I have to make some kind of plug for the radiator overflow.
Am I going in the right direction???? What about this reverse flow cap??? Do I go to the auto parts store and tell them that?? Do we have a pt # maybe??? Thanks....
Steve

Bob In Ct · #6 (**permalink**) 10-01-2001, 06:36 PM

Hey Jim:
I was having a similar problem in my SPF 351W. Everytime I checked the fluid it would be down the same amount. After adding about a gallon of coolant I replaced the cap. Put on the identical cap. Ever since then the coolant level is always right to the top. I guess I had a bad cap.

Bob In Ct

Bob Putnam · #7 (**permalink**) 10-02-2001, 05:06 AM

You can tell the reverse-flow design by the center of the bottom gasket. Some may have a rivet holding the gasket. If you can pull the brass piece up from the gasket surface, its a reverse flow - or place your mouth over the bottom gasket and suck. You should get some airflow.

Cobra20646 · #8 (**permalink**) 10-02-2001, 05:32 AM

Steve,
You made the light go on!

I completely overlooked the cap on the radiator. I have a remote fill that is only about 6 inchs away from the radiator. The top of the radiator is hidden by the bodywork. Last night I pulled the radiator, (only way to get to the top) found that cap and it is a reverse flow type rated at 16 pounds!
Appears that the lower pressure cap on the remote fill was letting the coolant excape into the reservoir, but the other cap on the radiator was just letting in air. Boy do I feel dumb for not thinking of a second cap

Like you said, I'm going to plug the radiator overflow tube so this won't happen again.

Bob,
When I first looked at the 13 pound cap, and compaired it to the one off my regular ride (Pontiac) they looked very different, and the reverse flow valve was very prominent. On closer examination, thanks for your description, I found that this cap was indeed a reverse flow. I guess my whole problem was the cap on the radiator..... its reverse flow valve was opening instead of the other one. Really feeling dumb now!!

Thanks to everyone for helping me get straight on this one.

- Jim -

Steve Siedlecki · #9 (**permalink**) 10-02-2001, 08:53 AM

Jim, glad to make the light go on.......
What if we use a non reverse flow on the hidden cap. Would we still have to plug the opening???
Steve

Cobra20646 · #10 (**permalink**) 10-02-2001, 10:59 AM

Steve,
My only thoughts on this would be to maybe have different pressure rated caps. If they were both the same, if your luck is anything like mine

, the wrong one would open and shoot juice all over the place

So far with mine, the 16 pounder on the radiator and the 13 pounder on the filler have not had that problem (knock wood) as far as I can tell anyway.
I'm sealing the tube to prevent sucking air back into the thing. I just made up a 2" piece of hose and filled it with RTV. I'll hose clamp this to the tube and this should take care of the problem. If the things builds up pressure and tries to vent it through this orifice, I'll have the only Cobra that shoots rubber bullets!!

- Jim -