radiator cap recommendation

[roadking]

im still a little confused to what the proper radiator cap to use, i have stock crate 350 non-a/c and no recovery tank with a new 4 core modine raditor . im currently useing a 13 psi cap, under (hot condition) and i get just a few drips of anti freeze out the overfill hose, ive also ran a 16psi under (hot condition)i have no anti freeze leaks but i feel i may be over pressurizing my system? sometime in the near future i will be adding aftermarket a/c. what is the correct cap recommendation for non a/c and with a/c systems? thanks


note: (hot condition) 95 degrees plus humidity

[Tim240Z]

OEM is 16lbs I believe. That is what I am running, although I am using a 99 L31 engine.

[StingRay]

I have a very good friend with a rad shop. He puts nothing more than a 7 on any rad he sends out. If you look at the boiling point of a water glycol mix it is higher at 7 psi than the overheating point of the engine. At 7 psi there is alot less stress on cooling system components. If it worries you at 7 run an 11. I just looked at mine and it's a 7 from my good friend. I have no problems with it even in 100 degree temps.

[cdowns]

i'm one who believes in the 7pound cap

[fastwillie 696969]

15 lb cap is used with no a/c and just recovery bottle with a 180 thermostat

[fastwillie 696969]

get a eletric fan put on makes a big difference i seen my drop down as much as 30 deg.

[tomatocity]

I am not at elevation but what about the members that live at 6200 ft, is there a difference? Will a 7 psi cap handle slow stop and go traffic on very hot urban streets. This might sound like an extreme but both Tim240Z and I drive our trucks in these conditions and I am sure many of the other members are also in the same situation. I would like to run a lesser cap but I want to do the home work so I don't have to do a roadside cool down on a 105 day. Hot August Nights (5500 ft) is in a couple of weeks and that will be a great testing ground with support. Thanks.

Originally Posted by StingRay

I have a very good friend with a rad shop. He puts nothing more than a 7 on any rad he sends out. If you look at the boiling point of a water glycol mix it is higher at 7 psi than the overheating point of the engine. At 7 psi there is alot less stress on cooling system components. If it worries you at 7 run an 11. I just looked at mine and it's a 7 from my good friend. I have no problems with it even in 100 degree temps.

[StingRay]

Pressure is pressure. When you cap the system you ristrict the parameters to that of the caps pressure. The boiling point is what it is at that pressure regardless of altitude. If that boiling point is actually hotter than the heat damage point for the engine then you have enough cap. Ask a few independant rad shop guys that are a bit older what they would run and I'm guessing you'll get he same answer as you will around here. If you are having any stop and go heat issues and the ambient temperature (outside) is quite hot then the chances are that you need to consider a new 180 degree thermostat ( I run a 195 most of the time with no problems but when I head for Southern BC i put in a 180), proper fan shrounding ( a topic all on it's own), a big enough fan ( best is a large clutch fan) and a large enough rad that is in good working order.

[tomatocity]

That is what I wanted to hear. Tell me if I understand you correctly. "By changing the psi rating of the radiator cap you are effectively changing the boiling point of the cooling system."
I am in the process of putting my cooling system together and I will try a 7 psi cap and carry 16 psi as a backup. I also plan on using a coolant recovery system. Thanks.

[StingRay]

Yes that is why water will boil at lower temperature when you are at a higher altitude. The ambient air pressure is lower. Also there are different caps for the recovery and non recovery types of systems.

[roadking]

im just trying to make my mind understand this better, a 7 psi cap would be the best recommendation because it would relieve the cooling system at a lower boil over point if you were to have over heating issues?

[Fred T]

Adding to the boiling point query, when the pressure increases the boiling point goes up. Higher altitudes lower the air pressure, but the difference will only be about 3 psi at 5,500 ft, 14.7 psi is standard at sea level. You cap increases the atmospheric pressure by it's rating. Now that you're completely confused....

What your see as leakage is actually due to expansion/contraction of the coolant, engine and radiator. It is completely normal, and once the coolant level corrects itself with the overflow, no more coolant is lost under normal conditions. The overflow tank and proper cap allow coolant to be sucked in and pushed out of the system as the temperature changes. It does keep the antifreeze out of the environment.

[Fred T]

Originally Posted by roadking

im just trying to make my mind understand this better, a 7 psi cap would be the best recommendation because it would relieve the cooling system at a lower boil over point if you were to have over heating issues?

The reason they are recommending the lower pressure cap is to reduce the pressure in the radiator and heater core. In theory, this will extend the life of those parts. However, they usually fail due to corrosion caused by poor maintenance before they fail under fatigue from pressure. Best to keep the a/f at a 50% mixture and flush the system every two years, in my opinion.

[rage'nrat638]

fyi the lowest pressure rad. cap summit sells is a 7 psi cap.....


go ask the nerest parts guy....after he can,t find it in the book......and his thumb is still up his a$$........

call summit.......
i agree with fred t.......

[Radman]

I am in the Rad business and i recommend the 13 lb cap. You can find it anywhere. You will not be able to find a non recovery cap any more.(obsolete) Your rad is more than adiquate for your situation. I would highly reccomend a overflow bottle. This then makes your system a closed loop. as your cooling system is now when your truck gets warm it will puke a little a/f on the ground. when you shut down the engine the coolant will cool and contract. When you check your rad in the morning you will see it is down about 2-3inches. You now have air introduced into your system. This is the #1 cause of corrosion starting. In a closed loop system your cooling system stays topped up and makes it more efficent.

Antifreeze is crap nowadays!!! you must flush your system every 2 yrs and i personally use distilled water in every flush i do for my customers.

Water is the best coolant. If i lived in the southern states i would run distilled water with a additive like water wetter to stop any corrosion and dump it every yr.
But since i live in the great white north i mix 50/50.
For every pound of cap pressure, it will raise your boiling point 2 degrees. So a 13 lb cap will raise your boiling point to 238 degrees f. I personaly would be pulling over and letting it cool at 210.
Fred t is right corrosin is the #1 cause of cooling system failure. I personally test all rads and heater cores to 35lbs pressure before leaving my shop.

Some rad shops tell their customers to run 7 lb caps simply to end warranty problems!!!!! Why else would they do it? No automobile manufacture has had a 7 lb cap since the fifties!!!! Since the closed loop system was adopted by them the lowest has been 13 lbs!!!

Hope this clears up some Questions
ttyl
Rick

[tomatocity]

I am not an expert by any means but I do live in a very hot valley and have had overheating problems with every recent vehicle except my 4.3L TBI S10. I could start that up on a hot day, turn on the AC, lock the doors, and walk away. eventually it would run out of gas but it would not overheat. Someone loves me

I know there has been some very valid posting of using a lower 7 psi radiator cap to keep from over pressurizing your cooling system. In you case you gave yourself the answer. When you use a 13 psi cap it releases pressure which means your motor was running too hot. When you use a 16 psi it does not overheat and you do not release coolant. Changing from a 13 psi to a 16 psi raises your boiling point but it does increase the cooling system pressure. Have you checked how good your coolant is? Have you flushed or cleaned your cooling system to remove deposits that can create heat? Have you checked your timing?
I am currently cleaning my cooling system and got some uglies. Tomorrow I will back flush the system to get all of whatever is in the cooling system out. Then I will replace both radiator hoses and heater hose, then add a 50% mixture of coolant and water. A new thermostat will be installed though I am not sure if I am going to run a 160* for the summer and a 180*-185* for the winter. I am going to try and run a lower psi radiator cap though I will carry spares.
Ps - try adding a recovery tank. One of the members is offering them if you can find the cap.

[StingRay]

I think some of the rad shops say use a 7 because at 7 psig the boiling point of a 55% glycol water mix is 119.5 degrees C or about 250 degrees F. Much hotter in your own opinion than an engine ought to be. If it was only pure water starting at 212 then it would be a different story. As I have understood it the only reason the vehicle manufacturers use the higher pressure caps is to keep customers from complaining about the sound of coolant passing from the rad to the overflow and vice versa when they don't understand what is really happening. The higher pressure caps tend to delay this till the owner is not around to hear it. Check for yourselves the boiling point info is available on the net Link to charts
If you only really need 7 psi then why fatigue your system pumping it up higher than it needs to go. The less pressure the less likely it will find a weak point.