What PSI radiator cap for a "68" - Pontiac GTO Forum
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post #1 of 8 (permalink) Old 09-03-2015, 11:13 AM Thread Starter
 
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What PSI radiator cap for a "68"

As the title says, what PSI cap should I use.

I have not been having any overheating issues, but the last couple of times I took the car out on 90+ days I noticed that after I shut it off I get fluid out of the overflow. About a cup or 2. I currently have a 7 PSI cap and was thinking of going with a 15 PSI. Will going to a higher PSI cap cause me overheating issues? Radiator was not overfilled. Planing on flushing the system. Thanks for the help.

One more thing, I currently do not have an over flow tank. Were they factory or added after. She is a driver so I don't care so much about originality.

John
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post #2 of 8 (permalink) Old 09-03-2015, 12:22 PM
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You should be running at least a 15psi cap. 16-18 is even better. 7psi is not correct for these cars.
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post #3 of 8 (permalink) Old 09-03-2015, 12:51 PM
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^^this^^
Also the '67 didn't come with a puke bottle, not sure on the '68 but if you overfill it then it will puke some out.
Not a big deal unless so much comes out that the coils on top inside the radiator are exposed then you have a thermostat or pump issue.
These cars came with a 190 degree thermostat, lots of opinions on what you should use but I stuck with the factory specs.
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post #4 of 8 (permalink) Old 09-03-2015, 06:14 PM Thread Starter
 
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Thanks for the info. Stopped by the local auto parts store and got a 16 psi. Not sure why the PO had a 7 PSI?
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post #5 of 8 (permalink) Old 09-03-2015, 06:14 PM
 
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Garage
Pontiac Service Manual for '68 says 15 lbs on the radiator cap, 190 degree T-stat, and the fill level should be 3" below filler neck.

The 15 lb cap raises the boiling point of the anti-freeze to 258 degrees before the pressure releases out the cap.

Personal opinion - I like a 160 degree T-stat for summer use. Not so good if you use your car in winter cold as it opens too soon and you won't build good heat for your heater/defroster. The 180 is my choice if I use a higher temp T-stat, but that is splitting hairs compared to a 190, just always found 180 a more common rating at the parts stores.
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post #6 of 8 (permalink) Old 09-04-2015, 09:57 AM
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Been running 160 'stats in all my GTO's out here in CA for almost 40 years and will continue to do so. If I lived in colder climes, I would maybe go 180. But never a 190. Cooler is better, and you have to remember: When the '68 Pontiac Service Manual was printed, the 100 octane fuel recommended for the same car was available, and 25 cents per gallon. Times change, and we must change to be sucessful. So, cooler is better.....
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post #7 of 8 (permalink) Old 09-04-2015, 11:16 AM
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Increasing the pressure also increases the boiling point of the AF/water.
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post #8 of 8 (permalink) Old 09-04-2015, 01:18 PM Thread Starter
 
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Not sure what T stat is in their now. On the list of things to investigate. Thanks for all the advice.

John
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO General Discussion

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