Why Radiator cap almost popped out?

89 Civic LX automatic with 224466 miles. I had first overheating
problem after driving about 30 mins in freeway. The temp indicator
almost reached to red. I immediately stop the car and open the hood and
saw most of coolant was poured out. The radiator cap was half opened. I
simply don't get it how comes the cap is half opened where the
coolant got poured out. The hot coolant did not
flowed to coolant reservoir but it flowed out through half-open
radiator cap. I lost most coolant volumn. I waited 2 hr and bought new
coolant and filled up the radiator and able to drive back to home.
What caused the radiator cap half opened? Did my radiator cap was
"loose" before I
drove to freeway? is this normal thing?

 

Certainly not normal. Several possibilities:

Cap not replaced properly after servicing. Was the car serviced by
you or another recently?

Cap is damaged/worn out.

Radiator neck is damaged/worn out.

Either of the last too is rare (I don't think I've ever heard of this
type of problem), but possible. My suggestion would be to inspect the
cap and neck for any obvious problems. If none are found, replace the
coolant with Honda coolant (if necessary) and reinstall the cap.
Carry some water with you and monitor the situation carefully. If it
doesn't happen again, the cap must not have been on correctly.

 

Thank you for your input.
Nobody looked into my car except me. I was the last guy check the
cap.
I have had to occasionally check the coolant level since the
waterpump is little leaking...
I do make sure fill enough of coolant level...however I remember I
did not check the level three or four weeks which caused to lose
significant amount of coolant that must have caused the overheating.
I assume that I did not properly tightened the cap and drove the car
with loose radiator cap with not enough coolant. Big stupid
mistake....I checked the cap and radiator neck as you suggested and I
found nothing wrong with it. If the overheating
happen again, it must be faulty thermostate...

 

If the water pump is leaking it will soon blow all of its bearings
out. When that happens, it will seize up or allow the timing belt to
come off. When that happens the pistons will probably hit the valves
and the engine will be wrecked.

Assuming that you want to put the money into this car, you need to
replace the timing belt, water pump, idler pulley and refill the
system with Honda coolant. This will cost a few hundred dollars. If
the rest of the car is in good shape and you don't have any other
transportation options, you have to bite the bullet and do it.

 

=========================================

(assuming the 89 is similar to current Hondas) Whatever you end up
doing, start keeping the coolant RESERVOIR filled to the MAX mark, as
well as checking the rad. Honda designed the system to draw coolant out
of the reservoir to replace any that leaks elsewhere. Your owners'
manual would confirm.
'Curly'

 

No I wouldn't do it... It has been leaking more than a year. Leaking
is so small that
it will not hurt timing belt...I just have to watch the operation of
water pump by opening up
the the upper timing belt cover so I can see it.. so I just leave it
and I don't have to fix it right away...As long as radiator is full
then it's ok for 3 or 4 days....

 

The choice is obviously yours, but a leaky water pump is doomed to cease
eventually, and you won't get a warning from it (it is already giving you
the best warning it can).

When that bad boy ceases up, you could very well lose your entire engine.
Not a gamble I'd personally be willing to take...