what 'psi' cap for metal macsradiator surge tank?

Hi all;

For the macsradiator surge/pressure tank/bottle, what kind of cap should one use?



I called them and they said use a regular 13psi radiator cap. I have seen threads talking about 6psi cap for closed system and 16psi cap for open system.

In using the metal surge tank, I assume one is still keeping the "closed system" configuration, so shouldn't the cap be 6 psi?

TIA .. Maz

anyone?

I'm going out on a limb here. I say use the 16 psi cap. The "closed" cooling system (all modern engines) uses pressure to reduce boiling, but obviously, there's a limit of how high you can go before something in the system blows apart. 6 psi sounds like what we used on our '40 Ford's true "open system" (coolant just bypassed the cap and drizzled to the ground). Just observe that the XJ "surge" bottle is really an expansion bottle. It is not filled to the top, but has air in it to allow the coolant to expand when it gets hot. The "modern" system has a two-way radiator cap that allows expanding coolant to "leak" into a coolant resivoir, then to be sucked back into the engine when it cools.

Maz said:

Hi all;

For the macsradiator surge/pressure tank/bottle, what kind of cap should one use?

I called them and they said use a regular 13psi radiator cap. I have seen threads talking about 6psi cap for closed system and 16psi cap for open system.

In using the metal surge tank, I assume one is still keeping the "closed system" configuration, so shouldn't the cap be 6 psi?

TIA .. Maz

Click to expand...

6 PSI sounds way too low, where did you 6 psi? I have never seen one less than 13 psi on an open system. Have you asked a Chrysler shop what the OEM cap is rated for? Where you even find a cap for less than 13 PSI????

perhaps the 6 you saw was typo?

Ecomike said:

6 PSI sounds way too low, where did you 6 psi? I have never seen one less than 13 psi on an open system. Have you asked a Chrysler shop what the OEM cap is rated for? Where you even find a cap for less than 13 PSI????

perhaps the 6 you saw was typo?

Click to expand...

I have an '89 with the closed cooling system, and in searching the Forums, I came across threads that talked about the cap on the "surge" tank being rated at 6 psi. It could've been typo, I don't know. That's why ask.

Can anyone confirm please?

Replacing the plastic bottle with the metal bottle does not change the cooling configuration, so if the cap on the plastic bottle (surge tank) is indeed 6 psi, then I would thing I need to find a "regular" radiator cap rated at 6 psi.

I am trying to verify and decide what kind of cap to get for that metal surge tank.

Thanks for the replies.

Maz said:

I have an '89 with the closed cooling system, and in searching the Forums, I came across threads that talked about the cap on the "surge" tank being rated at 6 psi. It could've been typo, I don't know. That's why ask.

Can anyone confirm please?

Replacing the plastic bottle with the metal bottle does not change the cooling configuration, so if the cap on the plastic bottle (surge tank) is indeed 6 psi, then I would thing I need to find a "regular" radiator cap rated at 6 psi.

I am trying to verify and decide what kind of cap to get for that metal surge tank.



Thanks for the replies.

Click to expand...

I understand the logic, but going to a metal bottle with a standard radiator cap may force, or allow you to use a 13 lb cap now that the plastic bottle is gone. If they used a 6 lb cap on the plastic bottle it would have been because the bottle could not handle higher pressure, but I would think these engines would over heat with only 6 lbs caps, as the pressure raises the boiling point and these engines run hot and use 195 F thermostats as oem standards.

Have you even found a 6 lb cap to fit the metal bottle?

I see no reason not to go to at least 13 lb on metal bottle.

I just checked the 1984-2000 model Haynes manual and it says the Coolant caps for in line 4.0s are all 16 to 18 lbs!!!!!! And the V-6 and four bangers are 12 to 15 lb caps.

I can't see any reason not to use a 16 lb cap now that the plastic bottle is gone, and haynes indicates it was at least 16 lbs with the plastic bottle.

I am pretty sure you will boil over with only 6 lbs.

Cherokee's with the open system used a 16 psi cap from the factory. I believe the pressure range of the XJ open cooling system was 14-18 PSI.

Now, I swear I heard using a 6 psi cap with the closed cooling system was infact normal.....Not sure, I don't own one with a closed system.

I do know though, that the local parts stores have a 13psi cap listed for the XJ which is wrong. Coolant boiled over twice or three times for me without reason until I realized 13psi was the wrong pressure rating! (for the open system, at least.)

the closed system is a 6 pound system.

now, that said, I don't think that the radiator is the limiting factor in the system.
It was the plastic bottle that would have popped with more pressure.
What flavors of caps do they make for those?
I'd go up a little, maybe 9 pounds?

From what it looks like, the metal containers use a regular-styled metal radiator cap....

I believe the plastic bottles used a plastic threaded cap?

The pressure cap on my closed system says 1.2 bar or about 17,6 PSI. I can't recall an automotive system with less than around 12 PSI.
My table says a system with 6 PSI and a 50% coolant mix will boil at around 235 F, 12 PSI around 250 F. At 16 PSI a 50% coolant mix, will boil at around 260F. The temperature gage, isn't the hottest spot in the system, it's likely near the cylinder sleeves someplace or maybe the exhaust ports.
Ar around 30 PSI many radiators will expand to the point they spring leaks.
The main difference between the closed system (which is a misnomer, the cap actually vents) and the open system is the closed system vents air and the open system vents coolant and the recovers it during cool down.
My cooling system specification table says (88) 4.0 L thermostat temp. 192-198 and Rad. cap ? PSI 12-15 PSI.

my vote- 16.

I really don't see how they can keep the closed system sealed and closed at only 6 lbs while operating at 210 F or worse 230 F under heavy A/C, towing and 100 F plus ambient tempurature loads, without it boiling over. The rubber hoses handle 16 to 18 lbs, so I do not see why that extra thick plastic bottle can not handle 16 lbs.

The metal bottles I have seen pictures of look like they use standard radiator caps. I have never seen a standard radiator cap with a rating less than 13 (or maybe 11 ) lbs. The plastic bottle cap is threaded. The metal one is not.

87 manche, do you have reference source for that 6 lbs or have you tested one (new threaded plastic bottle cap) with a pressure gauge?

I checked mine and it does not list a pressure on the cap. If I had to guess at the operating pressure of my plastic bottle closed renix (87) I would say it is running at at least 13 to 15 lbs, and I use a 165 F thermostat. Mine has yet to get over 200 F in the summer with the A/C on since I replaced everthing. I have an after market 3 row metal radiator. I would say it is geting near 16 lbs internal pressure at its peak of 200 F, roughly.

I am just not buying that 6 lbs (yet) value yet.

Also, 1 Bar is 14.7 lbs so tha math on your 1.2 bars being 17.6 PSI is correct. Based on that, and the fact that you have eliminated the weekest part, the plastic bottle, I would go for broke and use an 18 lb cap like the Haynes book says. I just might save from a boil over someday.

The system will hold the pressure fine. Some people convert from the closed to an open system just by splicing a T connection into the upper radiator hose......and using a normal radiator cap with a higher pressure rating. No problems.

I wouldn't use an 18 PSI cap though.

Yes, it may save you from a severe boil over someday.....but you shoulnd't let the temp get that high in the first place where an 18psi cap would make that much difference over a 16psi cap.

The system will hold the pressure fine. Some people convert from the closed to an open system just by splicing a T connection into the upper radiator hose......and using a normal radiator cap with a higher pressure rating. No problems.

I wouldn't use an 18 PSI cap though.

Yes, it may save you from a severe boil over someday.....but you shoulnd't let the temp get that high in the first place where an 18psi cap would make that much difference over a 16psi cap.

Use a 16-psi cap. Personally if you are on the original radiator, it might be more cost effective to switch to the open setup by buying a 1992 radiator for $130 shipped from radiator barn instead of paying $75 for an aluminum tank. The original radiator was only good for about 150k miles.

The 1988 Jeep FSM says 12-15 psi, but the replacement cap I got for the plastic football from the dealer had 6 psi stamped in it. That bottle popped within a year on the first really cold morning of the winter (-25*F). The second replacement made it less than a year, after which I converted to the open setup and it's been great ever since.

For straight water, 6-psi is a boiling boil of 230*F which is marginal especially if you have any trapped air. 16-psi gives you gives you 260*F.

I have trouble believing that a 13-psi cap versus a 16-psi cap would cause you to boil over since a 13-psi should still give you a boiling point of 251*F. Maybe it was a defective cap?

I don't know where the source of 6 psi is from. I was trying to verify what I had read and make sure I can use a regular radiator cap with that bottle / tank before ordering.

I dug up my '89 FSM and in there it said:



So I guess I'll get the metal bottle/tank with a 16 Psi rad cap. Is there any recommended rad cap brand?

BTW, thanks for all the replies.

I don't know where the source of 6 psi is from. I was trying to verify what I had read and make sure I can use a regular radiator cap with that bottle / tank before ordering.

I dug up my '89 FSM and in there it said:



So I guess I'll get the metal bottle/tank with a 16 Psi rad cap. Is there any recommended rad cap brand?

BTW, thanks for all the replies.

Sorry about the double post

Stant seems to be pretty good, or a dealership cap.

Actually I've never had any problem with the AutoZone CTS caps either, minus the fact they were 13psi which was wrong.

If I was a gambling man, I would bet that 6 lb label on the cap was a factory printing (casting) error, just missing the 1 in front of the 6. As someone said below, the temerature sensor and the thermostat do not see the hottest spot in the cooling system, they see more of an average engine block coolant temperature, so one can probably count on the gauge temperature being 10 to 20 degrees less than the hottest spot in the coolant side of the block. So at 230 F (which is what I hear many reporting their gauges to read under heavy loads) it is conceivable they are closer 250 F at the hottest liquid coolant / metal engine surface interface point which is where you want to avoid gas forming (boiling).

I have a bottle of antifreeze next to me that shows a 50/50 mix (not just water) having a boiling point of 265 F with a 15 lb cap!

lawson is right below, (his numbers surprized me so I had to check them) Pure water does boil at 230 F at 6 lbs of gauge pressure. I was also surprised to see that does boil at 250 F at 15 lbs, so antifreeze only adds 15 F degrees of differential to the boiling point at a 50/50 % mix, while reducing the thermal transfer efficiency since antifreeze has a lower heat capacity and lower heat transfer coeficient than water. Hmm.

Of course many people use antifreeze to avoid freeze damaged plastic bottles:bawl:.

If the 6 lb cap on the closed system was ever used (and maybe it was) and was not just a typo error, perhaps the open systems which vent liquid back and forth need a higher pressure cap than the closed systems which only vent gas (when working properly), and thus can operate at a lower system pressure (?). Also the older jeeps (closed systems) were not the newer HO's and I think they ran a little cooler than the newer ones (?).

Here is link to a nice site with a complete table of water boiling points versus absolute pressures. Remember to add 14.7 psi for atmospheric pressure first to get gauge pressures. The absolute pressures are what is listed on the tables.

http://www.engineeringtoolbox.com/boiling-point-water-d_926.html

Also, unless I am mistaken, the lower atmospheric pressure at high altitudes like Denver makes higher pressure caps necessary there as it is the absolute pressure (cap pressure plus atmospheric pressure) that actually determines the boiling point. The pressure in Denver is only 12.2 psi (14.7 - 12.2 = 2.5 Psi drop). Mexico city is 11.1 psi (3.6 psi less than sea level). I sure as hell would not run a 6 lb cap in Denver or Mexico city!


http://www.kinequip.com/pressure_terminology.asp

I tried to find a 50/50 antifreeze mix pressure (cap) versus boiling point table, but could not find one.

If there ever was a 6 lb cap for the plastic bottles, perhaps 5-90 can find it listed in one of his older FSMs????? 5-90, are you there????

I just checked my 1987-88 FSM and it lists 12 to 15 lbs for the pressure cap.