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Cooling systems: the surprisingly wide range of radiator caps

by Daniel Stern (part 2 of a series)

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The radiator cap seals the coolant off from the outside air, and allows a certain amount of pressure to build in the cooling system to raise the boiling point of the coolant in the system; the greater the pressure a liquid is under, the hotter it must be before it boils. Water expands and creates steam as it is heated; this is what causes the pressure in the cooling system-the pressure is not generated by the water pump! The cap has a spring-loaded disc valve that seals against the filler neck of the radiator. When the rated pressure is exceeded, the coolant will force the disc valve open against its spring and escape from the system (in modern cars, including most of those from the 1970s, into a collection jug rather than onto the street).

After the engine is shut down, the system cools off, the coolant contracts, and system pressure goes down. If the system was filled when cold, then some coolant will have escaped once hot due to expansion. When the system cools down, it will no longer be full, and if there weren't a way-a countervalve or vent valve that allows flow past the cap into the radiator-for system vacuum to bleed off, it could grow strong enough to spoil gasket seals, collapse hoses, and endanger thin brass radiator tanks.

Starting in the late 1950s, cars without air conditioning or heavy-duty cooling got a 13- or 14-pound cap as original equipment. Cars with air conditioning and/or heavy-duty cooling got a 16-pound cap. If you are using a water-based coolant, then you should stick to these ratings.

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You can use a 16-pound cap on a standard-duty cooling system if you want, and caps are available with pressure ratings of 17, 18, and even 20 pounds. These should not be used. It may be tempting to try to "upgrade" a cooling system's capacity with a higher-pressure cap, but the higher the system pressure the (much) greater is the stress on the whole system, particularly gaskets, seals, junctions, and seams. If your cooling system is insufficient, it should be properly repaired and upgraded as necessary; increasing the pressure beyond a couple of pounds above original spec is just not wise.

But neither should you take that idea too far and use a 7-pound or a 4-pound cap because you want to minimize stress on the system. This is another myth that just won't die. You may not detect any problem (the radiator doesn't boil over…), but there's a lot going on in the cooling system that we can't see because it's not happening up near the top/front of the system. Local temperature inside the engine, right at the metal, can and will be much higher than the boiling point of unpressurized water or any water-mix coolant. Local boiling can and will occur, even if we can't see it at the radiator. This can cause ping and other problems.

A cooling system in proper repair will not suffer under the pressure load it is designed for; go ahead and use the 13 to 16 pound cap the system is designed to use. If the cooling system is so weak that it will leak if the proper cap is used, then using a low-pressure cap is a stopgap measure until you properly repair the cooling system as soon as you can.

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The pressure rating is the parameter most of us are familiar with, but there are other important choices to make when selecting a radiator cap. One of them has to do with how this pressure/vacuum equalization is approached. In a "full pressure" cap, the vent valve is spring-loaded into the closed position. It only opens when a vacuum sufficient to overcome the vent valve spring develops and pulls the valve open.

A full-pressure system begins building pressure as soon as the coolant begins to heat up, and retains it long after coolant outflow would cease to push closed a free-hanging centre valve. This protects against localized boiling in the system, but at the cost of increased physical stress on the entire cooling system-hose junctions, seals, solder joints in the radiator and heater core, etc-because they're under pressure more of the time, including during system temperature changes.

In a "partial pressure" cap, the vent valve is free-hanging and weighted into the open position; it closes only when system pressure rises high enough to cause volume that can overcome the weight and push the valve closed.

Until the 1990s, Chrysler-built vehicles usually came with partial-pressure radiator caps with the free-hanging vent valve. In a partial-pressure system, the cooling system does not become pressurized until the system is hot enough that the coolant and/or steam wants to rush out of the filler neck. At that point, the coolant trying to flow out of the radiator pushes the vent valve closed, and system pressure builds. This means the system is not under pressure unless and until pressure is necessary to elevate the boiling point. This is easier on gasket junctions, seals, solder joints, hose connections, etc. It's little details like this, all over the car, that contributed to Chrysler's reputation for sound and dependable engineering.

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So, which kind is better? Usually it's best to use whichever kind your system is supposed to have. It won't make the difference between the system working or not, and it won't cause or cure overheating or other cooling problems. The car won't explode in a shower of sparks. But generally things work best when you use the correct kind of cap. For most Mopars before the LH cars (1993), Chrysler's recommendation of a partial-pressure cap is quite firm. Here's what the 1976 FSM (Factory Service Manual) has to say on page 7-8:

The vent valve at the bottom of the cap should hang freely. If the rubber gasket has swollen and prevents the valve from hanging loosely, replace the cap. Hold cap in hand upside down. If any light can be seen between vent valve and rubber gasket replace cap. Do not use a repacement cap without this vent valve that hangs freely. (boldface is theirs).
And here's from the 1963 FSM on page 7-7:

The brass vent valve at the bottom of the cap should hang freely. If the rubber gasket has swollen and prevents the vent valve from hanging loosely, the cap should be replaced. (same text in '64, '65, '67, almost identical in '68 and '71).
And here's from the 1992 FSM on page 7-19:

Hold the cap in hand, right side up. The vent valve at the bottom of the cap should open. If the rubber gasket has swollen and prevents the valve from opening, replace the cap. Do not use a replacement cap that has a spring to hold the vent shut." (boldface is theirs)
Factory caps are of nice quality; other brands are all over the map; some good and some lousy. Shop thoughtfully.

There's also the question of or not the cap is configured for a coolant recovery system. In a non-recovery system, the other end of the overflow tube is above the ground, so whatever comes out the tube spills and is lost. When the system cools down and the coolant contracts and the cap's vent valve opens, air, not coolant, goes into the system. If you don't understand this and keep adding coolant in an effort to keep the radiator full up to the filler neck, you'll go through a lot of coolant (and spread a lot of toxic waste on the ground).

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Starting in the early 1970s, coolant recovery systems caught on, first on vehicles equipped with heavy-duty cooling systems, and then on all vehicles. The system routes the overflow tube to the bottom of a plastic tank, usually right next to the radiator. When the system cools, it draws coolant back into the radiator from the overflow tank. This way the radiator's always full and we're not pissing expensive, poisonous coolant all over the streets and roads and driveways and garages. Nice! But you have to have the right radiator cap or it won't work.

A cap for a recovery system has two rubber seals: on the face of the main valve, which seals against the lower inner diameter of the radiator neck (all caps have this seal), and a larger seal, just under the cap's underside, which seals against the outer upper diameter of the radiator neck. This forces overflow coolant through the overflow tube rather than gushing past the cap, and when the system cools down and coolant volume in the radiator decreases, the suction is channeled to the overflow tube, which instead of sucking air draws coolant from the reserve tank back into the radiator.

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There's no problem using a dual-seal (recovery type) cap without a recovery tank, but a recovery tank won't be of any use if your radiator cap hasn't got the upper ring in addition to the lower one. No upper seal, no coolant recovery.

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So, leaving aside the pressure rating differences, and the physical differences (shape, size, length of center stem, etc.) there are four different types of cap:

  • With lower seal only, with free-hanging vent valve (Partial-pressure, open system)
  • With lower and upper seals, with free-hanging vent valve (Partial-pressure, coolant recovery system)
  • With lower seal only, with spring-loaded vent valve (Full-pressure, open system)
  • With lower and upper seals, with spring-loaded vent valve (Full-pressure, coolant recovery system).

Thanks to Stant USA Corp. for use of their photography.

Part 1: thermostats and optimal heat ranges <> Part 3: Radiators

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Great article, and I've got a question regarding my particular modified application. I'm running a big block Mopar 440 in my 2002 Dodge Dakota, and I am trying to determine which cap is best suited for my particular application. I am using a new/replacement 2002 Dodge OE radiator and heater core, and a coolant recovery tank from a '98 Jeep Cherokee.

According to the parts store application guides, 2002 Dakotas list for a 20 psi full-pressure (FP) cap/system and recovery tank (spring-loaded recovery valve), and the 70's big block (and the 98 Jeep) list a 16 psi PP cap/system and recovery tank (weighted recovery valve). I am inclined to 'match the cap to the engine'. To further buddy the waters, I have the option of either a PP or FP cap in the 16 psi range.

The kicker is there are 16 psi PP and FP caps available for my radiator...but I am inclined to think I should match the cap to the engine, which means going with a partial-pressure system, but I can also use a full-pressure caps for the same application.

Seems the PP system is 'easier' on the cooling system in general, and I cannot find any solid reason to go with a FP system. "Boiling points" come to mind, but I don't profess to know everything involved with that process other I understand it...a PP system would take longer to build 'pressure' because it requires a certain amount of flow out the valve before pressure can begin to build, thus delaying the boiling point of the system. In my case I'm building a serious off-road vehicle that will be at the lower rpm's most of the time when driving a trail but will also be at sustained highway speeds for hours to/from the trails, so a proper cooling system is paramount, and mine has the OE 19" mechanical clutch fan and mated shroud.

So I guess my quandary boils down (pun intended) to whether to go with a PP system or a FP system - pros and cons, etc.

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