Cooling System Pressure... 67 327/350 with AC

[Bill C.]

hey-

i just put a new 15# cap on my expansion tank. I was not getting any serious pressure in the system, so I figured
the cap was not good (being over 40 years old....).

ran the car real good with the AC running and still it will not build up pressure.

the car will run around 165 or so and then on the highway creep up to about 200-205
the temperature swings are very susceptible to the temperature of the roadway. I am figuring this to somewhat normal?????
the reason for a cap was on one of my trips, i stopped and killed the engine. it burped fluid out the hose. but there was no pressure at the time.

confused why no pressure build up, new cap and all?

any suggestions greatly appreciated.

Bill

[Larry M.]

Re: Cooling System Pressure... 67 327/350 with AC

There are three possibilities:

1. Cap is defective

2. Expansion tank cap sealing surface is corroded or bent or not flat

3. You have another system leak somewhere.

My 1967 327/350 HP factory AC car with a 180F thermostat will build a few psig pressure when running without the AC on, but will get closer to the 15 psig limit of the cap when AC is on and outside temps are high. This is mainly thru "feel" of the upper radiator hose. I have not attempted to install a pressure gage to get an exact number.

Check the cap with a Stant tester to see if it is good. Check the expansion tank filler neck flange surface closely for defects or corrosion. The Stant tester may also be used to check the cooling system holding pressure and to look for leaks.

Do you run a 50/50 or 60/40 mix of good coolant and distilled water in the system, and do you only fill the expansion tank up mid-way when cold?? Theses are also important.

Larry

[Bill C.]

Re: Cooling System Pressure... 67 327/350 with AC

larry -

the surface of the expansion tank neck looks very clean and not pitted etc....

i am taking it to the shop tomorrow - my friend will check the system to see if it is holding pressure.
fwiw .... there is no antifreeze drips showing up anywhere else on the car.

what I don't know is if the previous owner took the thermostat out.
I wonder if a system w/o a thermostat would not build up pressure ????

the antifreeze is a 50/50 mix, but not 100%. I can get him to test the fluid tomorrow also.

thanks for the suggestions..

Bill

[Joe L.]

Re: Cooling System Pressure... 67 327/350 with AC

larry -

the surface of the expansion tank neck looks very clean and not pitted etc....

i am taking it to the shop tomorrow - my friend will check the system to see if it is holding pressure.
fwiw .... there is no antifreeze drips showing up anywhere else on the car.

what I don't know is if the previous owner took the thermostat out.
I wonder if a system w/o a thermostat would not build up pressure ????

the antifreeze is a 50/50 mix, but not 100%. I can get him to test the fluid tomorrow also.

thanks for the suggestions..

Bill

Bill------


You can use a cooling system pressure tester to test the cooling system. Either it will hold pressure or it won't. If it won't, then you have to find the source of the leak. . If the system does hold pressure, then the cap must be bad and this can be tested, too, as described above. If you take the car to a shop, they should easily be able to pressure test the cooling system and cap.

You NEVER want to use 100% coolant. 70% is the maximum and I do not recommend above 60%.

[Edward J.]

Re: Cooling System Pressure... 67 327/350 with AC

Bill, the temp swing sound normal, if your car has a high gear ratio the engine starts working a little harder at hwy. speed. losing fluid out the over flow can sometimes just be a level in the tank that's to high, (common) I myself just run the level about 4 to 5 inch's off the bottom of the surge tank. I have tested some of the repro caps and they generally are about 15 to 15 1/2 lbs., always a good idea to test them so much stuff is made over seas and quality does suffer.

[Bill C.]

Re: Cooling System Pressure... 67 327/350 with AC

some info to report back ----

there were a couple clamps that leaked at 18# of pressure. tightened them up and car holds pressure.
the fluid - we are pretty sure is in the 50/50 - 60/40 blend range based on testing it.

I drove the car, with the new cap.
temps seemed a little lower, so thats good news.

but after I shut the car down (after good hard drive), it will not build up 15#s in the system. I can still compress the upper hose. it now has some pressure in it, but not anywhere near 15#.

I am going to take the upper hose off and check for a thermostat. it is the last thing I can think of.

thanks for all the great info....

Bill

[Larry M.]

Re: Cooling System Pressure... 67 327/350 with AC

some info to report back ----

there were a couple clamps that leaked at 18# of pressure. tightened them up and car holds pressure.
the fluid - we are pretty sure is in the 50/50 - 60/40 blend range based on testing it.

I drove the car, with the new cap.
temps seemed a little lower, so thats good news.

but after I shut the car down (after good hard drive), it will not build up 15#s in the system. I can still compress the upper hose. it now has some pressure in it, but not anywhere near 15#.

I am going to take the upper hose off and check for a thermostat. it is the last thing I can think of.

thanks for all the great info....

Bill

Checking for the thermostat is a good thing. You want a 180F thermostat installed in your car.

Not building pressure to 15# or the radiator cap set pressure is not a bad thing. In fact, it probably is good and shows me that your cooling system has good cooling capacity. As long as the system is tight and doesn't leak, the pressure will fluctuate to whatever is needed for your engine and ensure that the coolant doesn't boil internally. You just don't want to keep the cap set pressure from being exceeded, and generally want to also keep the engine temperature in the range of 180- 210 F when operating.

Larry

[Duke W.]

Re: Cooling System Pressure... 67 327/350 with AC

some info to report back ----


but after I shut the car down (after good hard drive), it will not build up 15#s in the system. I can still compress the upper hose. it now has some pressure in it, but not anywhere near 15#.

Bill

System pressure is a function of coolant temperature. A 50/50 glycol/water blend won't generate 15 psi in the system until the temperature rises to about 265F, at which point the coolant is boiling and the pressure valve opens and releases liquid coolant/vapor the overflow hose.

From what you've said, I don't see any problem, but I recommend you install a 180F thermostat.

Duke

[Bill M.]

Re: Cooling System Pressure... 67 327/350 with AC

My 67 AC 350 never goes above 180 I used to think there was something wrong now I see it is normal

[Leif A.]

Re: Cooling System Pressure... 67 327/350 with AC

My 67 AC 350 never goes above 180 I used to think there was something wrong now I see it is normal

Bill,

If you'd ever like to see your temp gauge go above 180 with the A/C on in the middle of summer, come on down to Texas where on a regular basis this time of year we suffer with ambient above 105 (feels like 110 with the humidity). If I can keep mine around or just below 210 I feel pretty darn good. Of course, 210 is no where near overheating...but they do run warmer down here

[Bill C.]

Re: Cooling System Pressure... 67 327/350 with AC

just checked - i do have a thermostat and it is 180.

so I guess all is good.... :-)))))))

i was using my 72 BB as a barometer for how a cooling system, would function.
That car always generated pressure and after 20 minutes of driving would get the upper hoser very pressurized.
never high enough to make it puke, but enough so taking the cap off hot would get ya in a big mess....

THANKS to everyone for all the great info....
I love this site!!!

Bill