What happens when you don't replace radiator cap?
 

Was looking at my radiator cap the other day and I saw a warning to replace cap when replacing coolant. What happens if an old cap isn't replaced? Does the radiator explode? Over heating? or just another way to extort money from us more often? Just wondering why the warning on the cap.

It just wont hold system pressure and the coolant will just go into the overflow tank because the spring is too weak to keep the seal from sealing. If the vehicle is older say 1990 you might wanna get a new one. But you shouldnt be too concerned about it.

For the sake of disscusion lets say you buy a 39 year old car that was sitting for 4 years give or take. The cap seems to be a few years old and has that little red lift tab to relieve pressure. You replace the water pump and most hoses and leave the thermostat out for fear it would get clogged with years of gunk. If the cap is bad pressure won't build and the water will leak out. Now lets say this car doesn't have the overflow bottle anymore. If the system leaks water because of a bad cap could enough water be lost to cause a low coolant over heat? Just wondering cause my car over heats and I haven't had time to trouble shoot all problems yet radiator gets rodded this weekend I hope and a new cap will be put on. I also replaced the water pump and bought a new thermostat to put in but left it out when I saw the previous onwer have left it out also. I just wonder if the bad cap may have started a chain of events that led to the overheat.

Lets say if you have a 15 PSI cap and the spring is getting weak. It lowers the boiling point of the coolant cause coolant under pressure has a higher boiling point. the higher the pressure of the cap the higher the boiling point.

Not haveing the thermostat is more than likey the issue of overheating. Without it, there is very lilttle restiction, and the water doesnt flow slowly enough through the water jackets to absorb heat. Even race cars run restrictor washers in place of the thermostat, just to slow the water down.

Yes.. an engine with no thermostat will over heat.

If you have a bad cap... one of three things will happen.

1> Coolant will leak out due to a bad cap seal

2> The cooling system will not properly pressurize due to a worn relief spring in the cap... resulting in lower boiling temperature... causing an overheat

3> The cooling system will OVER pressurize, blowing out hoses and other cooling components (rare, but i have seen it happen)

I put an engine in a caravan once due to it being severely overheating and seizing. Got it all done.. ran it... all was good.. put the cap on.. went to wash my hands (left it running).. came back.. sucker was overheating.

The upper hose had collapsed and was restricting coolant flow because the cap would not allow any pressure buildup.

Which means they spent thosands on a new engine.. because of a bad $8 cap.

Radiator caps are cheap.. when in doubt put a new one on.

I second that. My sister had a car that completely ruined the engine when the rad cap went bad. Kept blowing the radiator hose off. After a few shade tree fixes it ended up over heating to the point of blowing the head gaskets and the car never ran again. Not to scare anyone here just saying it how it happened. For what it's worth the car had the 3.1 GM motor.