Dan and I have also been discussing this offline. I'm not really sure what to think here, except that I have 100% negative experience using the dangle-valve caps on any Mopars without recovery tanks in Austin Texas. I'll look at the P/N on that old rad cap off my Satellite tonight. I *know* its the original because I kept it just because it was one of the few original equipment replaceable parts that were still with the car when I took it over in 1980.

Here are my experiences, and these are all in hot climates:

1949 Plymouth- runs fine without any radiator cap, and the recommended cap is only 4 psi, so its obviously in a different league. Its also got a 160-degree thermostat and 7:1 compression.

1966 Polara- originally a 383, now a 440 Magnum clone, non coolant-recovery. Always had a springy cap, but the OEM cap was long gone before I ever got the car. It will generally be OK with a partial-pressure (dangly) cap, but will sometimes spit coolant after shut-down.

1969 Coronet R/T (440 Magnum, non-recovery)- Almost non-driveable with a dangly cap. Will puke coolant after shutdown even in the dead of winter, and in summer will often begin to boil over madly before the valve slams shut and pressure builds.

73 Satellite 318, non recovery- mostly works with either type cap, but usually spits coolant after shutdown in the summer with a dangly cap

93 Vision TSi not really a "recovery" system, its a remote pressurized reservoir system- won't even idle at full operating temperature in the driveway without boiling coolant out unless you use a 14 psi springy cap. Ditto for my folks' modern Mopars with 195-degree thermostats.

All of these cars run perfectly happily with full pressure caps, none overheat at all.

As for stressing the radiator with full pressure all the time- I'd rather do that instead of risk cavitation damage due to microboiling at hot spots inside the engine all the time. Regardless of what Chrysler originally did, ALL my cars have been running full pressure for at least the past 26 years, and I've only had to repair one upper tank separation (yes, it was a round-top radiator on the Polara). And my wife's "one-strike" plastic radiator is fine at 240,000 miles.... maybe I'm just lucky (or maybe itll blow a tank tomorrow). I don't like plastic radiators, but I gotta admit this one has lasted about 4 times longer than I expected it to.

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I'm not makin' a holy war out of this (look where that's getting 'em in the Middle East!), just enjoying a spirited discussion. Fact is, I've run "springy" (full-pressure) caps on old Mopars, too; they've cooled just fine and otherwise behaved normally, haven't puked coolant, and I haven't kept careful enough track of radiator repairs to know if the incidence of rad failure increased significantly when I was running full-pressure caps. It's probable that the boldface warnings never to use a cap with a spring-loaded vent is maybe overstating the case. It's not like the car blows up or stops running or anything.

That said, air flowing past the free-hanging vent valve will close it very easily and securely. I've got a Chrysler 3781 830 cap in front of me; that P/N was introduced in 1974 and used through about 1995 (on the K-based cars; the LH cars, as you note, got a different system with full-pressure cap). I find it very easy to make its free-hanging valve slam shut securely just by "kissing" the cap and exhaling, with the cap held upright. That suggests to me that just the steam coming off a well-short-of-boiling coolant load will securely close the partial-pressure cap's vent valve—no raging boil required—and that matches what I've seen over the years.

I do feel more comfortable following Chrysler's recommendations on the subject, especially since even the aftermarket (e.g. Stant) specifically lists partial-pressure ("vented") radiator caps for the old Mopars...and you know how the aftermarket loves to "consolidate" replacement parts to ridiculous degrees, since most of the MBAs running the aftermarket companies consider it beneath them to actually know anything about cars and parts. To them it's all just "product". And yet, for slow-moving 40-year-old applications, they still list and manufacture the partial-pressure caps...h'mm.

One thing I can say for sure: Never use DexCool or any other OAT coolant! It rapidly and aggressively eats solder joints and attacks many gasket/seal materials. I wasted a lot of money learning that lesson in '60s and '90s Mopars.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

OAT coolant???

What's OAT stand for?

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

OAT = Organic Acid Technology. It was GM's dumb idea in the 1990s, to replace silicate corrosion inhibitors. Even in GM's specially-made cooling systems, this stuff is widely blamed for many, many expensive failures (head gaskets, intake gaskets, cooling systems, etc. And that's in systems specifically designed for it. There are consumer actions and class-action lawsuits and everything. Using it in systems not designed for it = disaster, just ask my wallet

See Here and Here for an in-depth discussion of coolants for older cars. Do a Google search with keywords Dex-cool damage and see what you find.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

just the steam coming off a well-short-of-boiling coolant load will securely close the partial-pressure cap's vent

I don't know how effective that is, because the destructive knocking in a borderline system comes from local boiling in the water jacket at the exhaust valve seat or spark plug boss. Steam already contains too much heat and doesn't readily absorb any more (Raising water at 211 to 212: .556 calories per gram. Converting water at 212 to steam at 212: 300 calories per gram, 540 times as much), and its density is far below water, so it not only does nothing for cooling the spot it covers but prevents water from contacting it.
The water leaving that site is 212 but liquid, and won't close the vent.
The only practical method of keeping the coolant liquid at the hot spot is more pressure.

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Nor am I. YOU know be better than that, but I thought I'd make it official-in-writing for everyone else
I cannot see ANY benefit from running partial pressure caps, other than a (marginally at best) reduced risk of radiator failure. But since the system HAS to pressurize to prevent microboiling, you only "benefit" by avoiding pressurization a small fraction of the time you're driving the car. Assuming the partial pressure cap works as advertized, the only time you won't reach full pressure is in very optimum low-load, low ambient temperature conditions. Of course I've said that my experience is that it doesn't work as advertised- it fails to pressurize properly much of the time, resulting in nagging coolant loss at best, coolant puking and boilover at worst.

Nor can I see ANY detriment to running a full pressure cap, and I see lots of benefits. Better control of microboiling even under non-demanding cooling conditions is one example. Plus I have repeatedly correlated partial-pressure caps with "mystery" coolant loss and with coolant puking, that much I can say with absolute certainty.

It costs them more to put the spring in....


Amen, preach on brother!

I can also report that my experiment with G-05 (a HOAT coolant recommended for Fords and modern Mopars) in my rebuilt 440 is looking really, really good so far. It seems to lay down a thin blackish-grey coating on all the internal parts (particularly obvious on the thermostat I changed out a couple of months into using the coolant), but the iron and brass parts looked very nice. There's not much aluminum in that engine at all (just the water pump iteslf), but then again the big problems with OAT are the coppers (brass alloys) and solders, not the iron or aluminum. The radiator looks great inside, no scary fluff forming at the tube junctions like people report with OAT in copper-containing radiators. One aggravation with G-06 is that if you spill a drop on the top of the radiator, when it dries it leaves a really obvious white spot- sorta like ultra-hard water would. Not a sticky spot like silicate coolants do.

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Couple of follow-ups (can we beat a topic to death, or what? )

First, I checked my '66, '69, and 72 manuals (I don't have a 73 manual to match my 73 car, but for some reason I have a 72 manual... don't ask, I don't know where it came from. Anyone want to trade?) and they indeed say EXACTLY what Dan quoted. Not like I doubted him, but in ~30 years, I've never bothered to read the FSM entry on radiator caps.

Second, I found the cap that I've always thought was the original to my 73 Satellite, but now I'm not so sure its an OEM because I can't find a P/N similar to the ones Dan mentioned. Where would the P/N be stamped? I may have to do some careful bead blasting to see anything on it at all. From the top, it looks exactly like this:

http://tinyurl.com/eephf

Except I don't see S M C O for sure- maybe a "C O" but the rest surface is in bad shape. Furthermore, I buy Dan's explanation of 16lb = HD cooling or AC, 14 lb= std cooling and no AC more than Year One's "There's no agreement on which is more correct" weasel statement.

From the bottom- well, its so deteriorated that its hard to say for sure if its a spring pulling the valve up, or just the deteriorated rubber. The valve hardly moves at all anymore. I'll have to concede that Dan may well be right and my memory wrong- the OEM cap could have been partial pressure. Given Dan's track record, he almost certainly IS right.

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i run a 195* thermostat and have never had over heating problems. i pulled the mech. fan off ond put an electric fan on. sometimes i wonder why i put the electric one in. i hardly ever use it, even here in san diego. only when i hit traffic doing less than 20 mph do i need the fan. the reason i put the 195* thermosat in was to keep the coolant in the radiator as long as possible to help it cool. and keep in mind that 95% of all cars made today use a 195* thermostat. thats for emissions, though.

as for your cooling problem, make sure airflow is good and like was said, the hoses aren't collapsing. if you like to run those speeds, i would definitely think about upgrading to a larger radiator. i have the 26" wide model in my dart that came out of a 74 dart,V8 with a/c. i had to drill one hole to make it fit. the lower hose outlet is 1 3/4" but there are hoses that will fit.

zedpapa

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1970 dodge dart w/225 /6 bored .040" over, holley 390cfm w/vac. sec., compcams 252s, clifford shorty headers w/2.5" exhaust w/flowmaster, f-body 11" front discs, aluminum A-833OD, 8 1/4 w/3.21 SG
soon to have 5 gears!!!

Not sure the P/N was stamped on the '73 caps. Together with a Pentastar, it was stamped on the inner "ring" element of the top surface of the cap (the next-smaller circle to the one where "Keep Tight" and "Open Slowly" or "Do Not Open Hot" are stamped).
Yeah, "no agreement" my butt. It's right there in the parts and service manuals, year for year and model for model. Whaddya expect from Year One, who go around claiming the 198 is an LG engine, the A-body started in 1964, and other imaginative things. They also go around charging $30 for a radiator cap!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I don't follow this line of reasoning. Please explain?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I can't really think of one, either, but then...why such adamance from Chrysler on the subject, and why the continued aftermarket support?
Dunno...the partial-pressure caps have a weight atop the vent valve stem that the full-pressure caps don't have. Probably just about a wash.
That is great to know, especially since old-fashioned green stuff (silicate coolant) is getting hard to find.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Huh? Here in the midwest the green stuff is by far the most common stuff that you can find. I guess things are significantly different in Canada?

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Used to own:
'82 Dodge D150
Erson 270 Cam, O/S valves, mild port work, ~9.5:1 compression

Currently fighting with an '85 VW Cabriolet

My other passion

Are ya sure you're really seeing old-fashioned green stuff? Both up here and when I've looked in at parts stores on visits down South over the last few years, Prestone is selling pretty much only their "All vehicles/all years" formula, which might or might not be G-05. What brand of regular green stuff are you finding?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Well... when I worked at O'Reilly's in MO, we usually had at least 20 cases of the regular stuff in the house brand and at least another 10 in the house brand pre-mix. Now, if you wanted to go for a name brand, yes, you ended up "all makes, all models," but at least half of the stuff we had in stock was the house brand ethelyne glycol. I've found it to be similar at most of the other parts stores I've been to. I've never seen a convincing argument that "off brand" antifreeze is any less effective than name brand, so I just run the house brand in mine.

On a similar note, whenever anyone asked for Dexcool, I always reccomended the "all makes, all models" stuff because I'd heard of Dexcool's nasty reputation. Would you reccomend this substitution?

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Used to own:
'82 Dodge D150
Erson 270 Cam, O/S valves, mild port work, ~9.5:1 compression

Currently fighting with an '85 VW Cabriolet

My other passion

If your "180 thermo" is faulty and not close when it should, you will get a constant waterflow in the engine and raising temperature.

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Lasse

Signet -66 225
Holley 600 Offy-5270 Dutra Dual,regrind. cam 272 0,420 110, head shaved 0.100 slight.ported
MSD6A
A904
7 1/4 3.23