rad cap overflow at high 4,000 rpm on hwy

[Doog 427]

Hello Hope all are well

Having trouble with rad cap overflow at high 4,000 rpm or higher revs on highway even for short spurt, changed stock cap from 12 lb to 18 lb now have 25 lb, still so much water pressure cap releases and rad fluid blows out drain hose near bottom of car but makes a mess. Ideas?

Thank you
Steve

[deathsled]

No expert here but something sounds plugged in the engine where the coolant flows. Water jackets? Bad water pump?

[shelbydoug]

Pressure test the cap and the radiator.

It sounds like you may have a bad seal between the cap and the radiator neck.

[6s1802]

Mine was blowing water and making bubbles over pressurizing the system. Pulled the heads and had them checked for cracks. Turned out the water pump was bad and was cavitating causing the pressure. put on a O' Riley's cheap ass water pump amd the problem was solved.

[DGSOH]

Don't overlook the simple stuff - make sure the radiator isn't overfilled. I used to fill mine to just below the bottom of the neck and it would invariably push coolant out during/after a run. Then I read somewhere there should be just enough coolant to cover the core and no more. So now I fill and maintain to 3/4"+/- above the core and it never overheats, never overflows... problem solved.

[sg66]

Classic symptoms of a blown head gasket. Pull the radiator cap, start the car and let it idle. Once warmed up and the coolant is flowing, if you see bubbles coming out, it's a high probability that you have a blown head gasket or although less likely, a cracked head or block. If you do see bubbles at idle, I would do a compression check on each cylinder to see if one is obviously out of range of the others and go from there.

[98SVT - was 06GT]

Classic symptoms of a blown head gasket. Pull the radiator cap, start the car and let it idle. Once warmed up and the coolant is flowing, if you see bubbles coming out, it's a high probability that you have a blown head gasket or although less likely, a cracked head or block. If you do see bubbles at idle, I would do a compression check on each cylinder to see if one is obviously out of range of the others and go from there.

Just watching for bubbles at idle doesn't always work. There is a kit you can buy that will check it quick. It goes on in place of the cap and any exhaust gasses changes the color of the liquid. You can get the kit many places or stop by your local radiator shop.
https://www.harborfreight.com/combustion-leak-detector-64814.html?cid=paid_google|||64814&utm_source=google&utm_medium=cpc&utm_campaign=&utm_content=&gclid=CjwKCAjwtdeFBhBAEiwAKOIy5_aoeBdqnuiJKgWZ5qcpDnKR1MpPNlUlpL0M-_M26nQz4rB4aqVEthoC_lAQAvD_BwE

[Doog 427]

Ok thank you many replies let me check into this appreciate it

[Doog 427]

Hi Everyone, appreciate feedback on rad cap overflow

Motor and water pump new did test for bubbles and head gasket anyway, was OK, I took the easiest route first, lowered the fluid level more than usual, to barely above fins, also put a small overflow container on end hose off rad cap if it pukes would catch it and then maybe suck fluid back in when cool, switched higher pressure rad cap, added water wetter and so far seems OK, car runs cool

Thank you
Steve