BN4 Radiator Cap

[AUSMHLY]

Friend has a BN4 with a new Wizard brand aluminum radiator. It came with a 4lb radiator cap.
When showing up at my house I noticed a steam small white air bubbles coming out of the overflow tube. The bubbles disappeared showing the green antifreeze.
No oil in it.

Would a 4lb cap do that?. Shouldn't he be using a 7lb cap?

What's causing the air bubbles?
1. He didn't bleed the air out when filling?
2. Wrong pressure cap?

Thanks in advance.

 

[Bob_Spidell]

AFAIK, all 6-cyl cars should have a 7lb cap. 4-cyl cars have 4lb caps and maybe they're supplied out of caution since (I think) 4-cyl and 6-cyl radiators are the same (a 7lb cap in a 4-cyl car could have problems; I forget what, exactly). Unless your friend overfilled his radiator there should be nothing coming out of the overflow; stock radiators have a longer-than-typical 'neck; ' if the cap isn't correct you would likely get overflow. A little air in the system after a total refill isn't surprising, but should go away.

 

[AUSMHLY]

Thanks Bob. This radiator takes a short cap. Wondering why 7lbs vs 4lbs. Not enough pressure may create air bubbles?
I suggested he replace the cap with a 7lbs and see if that solves the issue.
It just got me wondering why our cars specify 7lbs.

 

[Bob_Spidell]

...
It just got me wondering why our cars specify 7lbs.

I think it's because the system as designed can't handle more; modern cars are usually 14lb give-or-take.

 

[John Turney]

The boiling point of water at 7 psi is 230 F.

When a 14 psi cap is put on a stock radiator, the head tank will bulge.

The radiator for the 4-cyl cars is different that the one for 6-cyl cars.