No-pressure radiator cap trick

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An old mechanic taught me this one, which is useful when you have a small coolant leak and you're trying to limp home, or to a place for repair.

Radiator caps have a spring loaded seal that keeps pressure in the cooling system. If you remove that seal from an old radiator cap, you have a cap that will keep the coolant in, but will not retain any pressure. A small leak leaks much more slowly when there is no pressure in the cooling system.

The seal is not meant to be removable. A little mild violence with something like vice grips will usually allow you to remove the pressure seal while leaving the rest of the cap intact. I'll take pictures tomorrow, when it's light.

I thought my cooling system was finally intact, so I took it on a test drive. I was wrong. The lower radiator hose leaked at the radiator. I pulled into a bookstore parking lot, leaving a trail of coolant behind me. (Yes, we still have bookstores in New England.) I tightened the hose clamp, and it broke. I've mentioned elsewhere that I use torque wrenches a lot because I break things... Fortunately, I had a spare clamp with me.


I pulled the front battery to gain better access. Palpating the hose, I noticed a dent in the rim of the round fitting on the radiator that fits into the radiator hose. I had placed the clamp over that dent, resulting in an incomplete seal. No amount of tightening would have fixed that. I put the new clamp much closer to the radiator, away from the dent. I used 1/4" sockets in hopes of not breaking anything. I used my no-pressure radiator cap, as extra insurance.

I keep an old rifle-shooting mat in the truck, as something to lie on when I have to go under the truck. It's somewhat padded. I folded it in half and laid it across the radiator and brush guard, so I could lie full length along the front edge of the engine compartment, and use two hands to install my hose clamp.

I got home with no evidence of leaking. When I topped off the coolant, it took about a gallon, so I lost quite a bit.

Tomorrow, I go buy more spare hose clamps. I had only one. If I get to the store without leaking, I'll put on the regular radiator cap and see what happens when the system builds up pressure.

This is the second time I've used my no-pressure radiator cap to limp home. I thought others might find this notion useful. The cooling system is designed to work at pressure, but if you have a leak, it's better to have no pressure rather than losing your coolant.

I imagine this trick applies to the bigger trucks too, but I have no experience with them.

 

[Keith_J]

We did that trick during the cross country (TX-WI) recovery of my M1031. Used a half gallon of water every 400 miles. Dang plastic radiators..

 

[frank8003]

This old time used car trick also works in reverse. I got a nice old Cadillac, still real nice at 180,000.
so I am doing the PM's and put on a new 12lb pressure cap. That pressurized the system so it blew the whole side of the radiator out onto the freeway. Did I tell you the one about in Miami where they charge the AC with propane and sell it quick as "cold air" here in the summer?

 

[tommys2patrick]

sometimes on older vehicles, the bayonet style gas caps are interchangeable with the radiator caps on a temporary basis. not really pressure rated but depending on the situation your in it might be good to know. modern vehicles not so much.

 

[CUCVLOVER]

Good trick.
I have just loosened up the cap for the same effect.

 

[98G]

I limped a 5ton with a cracked radiator 1400 miles by loosening the cap. Leaked about 1/2 gallon of water every 200 miles.

 

[MarcusOReallyus]

Good trick.
I have just loosened up the cap for the same effect.

Yep. No need for vice grips. You won't lose much this way.

 

[rustystud]

That why radiators have that second lip on the neck of the radiator. As you loosen up the cap you hit a spot where you must push-down to bypass it. If you just loosen up to this spot you will have no pressure. All radiators have this.

 

[M543A2]

I am definitely not a fan of pressure caps on older vehicles including my M series trucks. Most of them have tall and/or wide radiator tanks, lots of surface area there to move in and out as pressure rises and falls. This action weakens and fails the tank metal and the solder joints, causing leaks. Also if the little fins between the tubes are deteriorated the tubes then are not properly supported and can fatigue and leak if there is pressure in the system. Most later model radiators have smaller end tanks to better handle the pressure. I do not run pressure on any of my older vehicles or farm equipment. I keep the cooling system in good order, radiator cleaned out of debris, and have never had an overheating problem. I do not run pressure on my 99 Durango pulling a camper and car hauler trailer. The only time I had an overheating problem was when the fan clutch failed. All the pressure does is allow the system to reach higher temperatures without boiling the coolant. If the system is right on vehicles that do not need the higher engine temperatures mandated to meet emissions they should never reach those high temperatures anyway, running around 180 to 190 degrees. I really do not like one of our local radiator shops because they say they will pressure test the old radiator for leaks. Every time you can bet they will find one, suspect due to their over pressuring it and creating their own profit margin. So, therefore, I do all of my own radiator repair on the older ones.

 

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Good trick.
I have just loosened up the cap for the same effect.

Yep. No need for vice grips. You won't lose much this way.

That why radiators have that second lip on the neck of the radiator. As you loosen up the cap you hit a spot where you must push-down to bypass it. If you just loosen up to this spot you will have no pressure. All radiators have this.

Last night I had a chance to test this. On my CUCV, there is a big difference between loosening a radiator cap and using a no-pressure cap where the internal ring has been removed. With the no-pressure cap, you tighten the cap fully, so no fluid leaks at the cap. Any pressure just pushes coolant out into the overflow tank. With a partially tightened cap, lots of coolant will leak (gush) out from under the cap.

In the pictures below, you can see the two concentric rubber seals in the intact cap on the left. The inner seal seals pressure into the radiator. The outer seal seals the cap to the radiator. To make a no-pressure cap, you want to remove the inner seal, and fully tighten the cap so the outer seal keeps the coolant in the radiator and the overflow tank. The cap on the right has been mangled to do that.

Before I fixed it, my leak was a small drip in the lower radiator hose. As an experiment, I tried the loose cap approach, and I lost far more coolant from under the loose cap than I did from my hose leak. With the no-pressure cap on the right, I lost no coolant at all from under the cap.

So for me, the no-pressure cap leaks no coolant, while a loose stock cap leaks a lot of coolant. I suspect that's why the old mechanic made me the no-pressure cap while I was limping around with a slow hose leak.

 

[rustystud]

Try it with a new cap jpg.

 

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New cap installed. I didn't realize how dried out and hard the old cap's seals were until I compared it to a new one.

 

[rustystud]

New cap installed. I didn't realize how dried out and hard the old cap's seals were until I compared it to a new one.
View attachment 591825

According to the manufactures you should replace a radiator cap every two years. At the transit dept. we tested them every 6 months.

 

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According to the manufactures you should replace a radiator cap every two years. At the transit dept. we tested them every 6 months.

I have never replaced a radiator cap before. And I'm old! Thanks for the lesson!