Coolant overflow

benefield · Post by **benefield** » 23 Jun 2010, 17:10

Need some opinions please....within a minute or so of stopping in my driveway this afternoon, the coolant began belching up into the expansion tank. Within another couple of minutes it began spewing out from under the tank cap. While on the way home, the temp gage did not indicate overheating, but the low collant warning light did come on. I've been losing coolant for a couple of weeks, but have not noticed a leak until today. What would cause the coolant to run out from under the cap? Shouldn't any expansion of the coolant go back into the radiator?

Post by **vegasjetskier** » 23 Jun 2010, 19:27

You might have a bad cap. I'd try replacing that first. It controls the pressure in the system.

benefield · Post by **benefield** » 23 Jun 2010, 19:55

Thanks Vegas. I'll try that. There are some hairline cracks in the underside of the cap (black portion) - my mechanic said not to worry about them. How does the cap work? It looks like the white portion of the underside can be pushed up if the pressure gets too high. When that happens, where does the coolant go? Is it supposed to spew out from underneath the cap? If so, what would cause the high pressure?

benefield · Post by **benefield** » 09 Jul 2010, 19:59

Vegas has the mojo. Replaced cap and problem appears to be solved. Can't explain it, but I simply replaced to evap tank cap and coolant level remains steady. Go figure.

dtropnevad05 · Post by **dtropnevad05** » 17 Jul 2010, 21:39

The radiator cap has a spring preset to open at a set pressure like 14 PSI. As the car heats up the coolant expands and is pushed into the overflow reservoir. So if the black seal is damaged or the spring is weak to much coolant will go into the tank.

However if I were you I would have the thermostat replaced also, I don't think the cap is your only problem. If your car is more then 5 years old or has over 70K it's time to replace the stat.

Don

dwd · Post by **dwd** » 08 Aug 2010, 09:14

dtropnevad05 wrote:However if I were you I would have the thermostat replaced also, I don't think the cap is your only problem. If your car is more then 5 years old or has over 70K it's time to replace the stat.

Just to second this - it might be the thermostat, or it might be the temperature sensor (which is connected to where the thermostat sits) which has gone bad.

I was just this week having problems with a lot of leaking around my radiator expansion tank, apparently from the bottom where the larger hose connects into the bottom side of the tank. I assumed it was a leak in the tank or a crack in that hose. However, it turns out that the car was running too hot, causing WAY too much pressure in the expansion tank, which was forcing it to leak despite the fact that there are no cracks/holes. The culprit was the temperature sensor; despite the fact that my temperature gauge in the dash was reading normal (3 o'clock), it apparently wasn't sending the correct signals to the computer which decides when to turn on and off the fan - so the fan was never coming on, causing overheating on hot days with short (running errands) trips.

Had the sensor replaced (and had the thermostat replaced at the same time since it is cheap and in the same spot); now the fan comes on every now and then (as it should) and so far no more leaks from the expansion tank (though I'm still keeping an eye on it just in case there was damage done by the excessive pressure).

Just thought this info might help.
-d