Packard Forum Index » V-8 (1955-56)

noticed water overflowing from overflow. First thought thermostat was sticking. Took out thermostat and ran w/o. Started car ran with cap off at first and saw a good stream of coolant going to radiator. Put cap on and still saw coolant leaking from overflow. What should I do next?

Could be you have the wrong cap. If it doesn't seat at the base of the neck, you will get water going out the overflow.It happened to my friend with his 54 Cavalier and that was the problem.He obtained the proper cap and the problem was solved.

GTO has probably nailed the problem, a radiator cap with the wrong depth from cap to seat - a very commonly encountered problem.

What is proper radiator cap pressure for my 55?

12 psi.

For the possible incorrect cap length others have mentioned, I wonder. 54 and earlier needed a long reach cap and that is a frequent cause of the overflow if someone has installed a modern readily available standard cap. Perhaps others can elaborate more on the cap. I thought they changed to a standard length neck in 55 and if the radiator is stock, the need for the extra reach cap was not a issue with V8 cars. Am I in error?

Is your present cap in good condition and are you filling to approx 1" below the cap sealing surface. Leaking cap and overfilling are also causes of water overflow.

Newer cap from auto part store (supposedly correct they looked up in their parts book). My fault for not taking old cap in to match.

Thank you for info.

Thank you for info.

I had the incorrect cap on my 56 when I first bought it, kept blowing coolant from the overflow. If appearance matters you can buy a 13 psi AC-licensed and exact replica of the original factory cap ($33.50 at last look). I believe you want the RC-6.

http://www.oldcarstuff.com/radcap.html