Hey Ken==The radiator cap isn't sealing. Replace it. GoodluckJoe
If replacing the radiator cap is not enough then look at your radiator as the problem. Your radiator when the car is running should be the same temperature top to bottom. Run your hand down the radiator (careful you can burn yourself) and see if it is cool toward the bottom. If it is cool at the bottom but hot at the top your radiator is clogged. Flush does not work so save your money. I would start looking online for a new radiator their cheap enough. Good luck.
If it still vents with a new cap then the excessive pressure could be due to a blown head gasket pushing combustion pressure into the coolant system, if the vehicle has been overheated recently it is a good possibility
I had the same problem. Check your upper and lower radiator hoses. You might also need to replace your thermostat. The thermostat is a spring seal for coolant. These get stuck open and closed often for civics in the 1990's. I had to replace mine 2 times within a month. Part of it was my fault, because i drove it like i stole it. When your engine gets hot, the thermostat opens up and lets coolant in. When it is too cold, it closes and doesn't let coolant into your engine so it will warm up. Your thermostat probably got stuck closed. Also, check the little hose running from the reservoir tank to the radiator, it might be clogged.
Why does the radiator fill up the reservoir
A cracked radiator would leak the fluid out, leaving no coolant for your engine which would cause your engine to overheat and be destroyed pretty quickly. So the answer is NO.
It sounds like you have a blown head gasket. Combustion gases can enter the cooling system through either a blown head gasket or cracked cylinder head. The gases (under pressure) force coolant out of the reservoir. This also causes a void in the upper part of the cooling system , leaving no coolant at the thermostat and the coolant temperature sending unit. The result is a erroneous temperature Reading and will even prevent the thermostat from opening.
Allow the car to cool. Remove radiator cap. Add water or coolant to fill radiator. Start engine leaving cap off. check to see if coolant is circulating in radiator. Locate upper rad hose, and squeeze to move air bubbles out, or to ecourage circulation. If large air bubble works its way out, add coolant to fill. Once coolant starts to expand and spill out, replace cap. Watch temperature gauge to esure the problem has been rectified. You may have to repeat this procedure.
It could be a damaged or loose radiator hose, a damaged radiator, a faulty radiator cap, blown head gasket. Have a look around all the hoses and see where the water is coming from and then use common sense.
Lower radiator hose.
lower radiator hose
TO MAKE A LONG STORY SHORT I HAD THE SAME PROBLEM. I FOUND THAT THE COOLANT OUTLET CAP WAS NOT PROPERLY SEALING DUE TO CORROSION ON THE SEAT LIP OF THE OUTLET. THIS WAS CAUSING A WEEPING OF COOLANT TO THE RESERVOIR AND A LOSS OF PRESSURE REQUIRED TO PREVENT BOIL OVER. MY CHEAP SOLUTION WAS TO CAP THE COOLANT LINE TO THE RESERVOIR. PROBLEM SOLVED. If you replaced two caps then I doubt those caps are the problem...there is something else causing the pressure to be too great and thus flow out. You could have a bad head gasket, which allows the pressure from the combustion chamber to flow into the cooling circuit creating to great of pressure for the cap, and thus, flows out. Just capping that line cripples the functionality of your cooling system, it is designed to make use of the overflow tank, you shouldn't just disable it like that. IF your head gasket IS going out, the blocking the overflow tank will cause EXTREME pressure to build up, now your going to blow the radiator and your engine. Cheaper to fix the head gasket and keep the old radiator then replace a radiator and a head gasket. On a cold start, stand behind your car, someone starts the car, smell for coolant. When the car gets cold, the higher pressure in the cooling circuit could cause coolant to flow in the engine, so on a cold start, you may smell it. After it starts though, the coolant may not flow into the cylinders. Get a coolant tester kit to check for exhaust gases in your coolant, you can find them at your automotive stores. You may not see white smoke coming from the tail pipe after the car has warmed up because the coolant may not flowing into the engine combustion chamber while it is running...but you might see white smoke. I'm diagnosing my fiance's car right now with this same situation...new water pump, new radiator cap, etc etc...head gasket is the next possibility.
The expansion tank retains coolant in a reservoir that's been forced out of the car's radiatior under pressure. As you run your car, the temprerature of the coolant circulating through the water jackets naturally increases causing increasing pressure in the cooling system. When the pressure builds to a high enough point, the radiatior cap allows that pressure to escape into the coolant reservoir through a small black rubber tube, called an overflow tube connected by the neck of the radiator to the expansion tank. Most of the coolant that is forced into the expansion tank returns to the radiator naturally after a few hours or when the engine is cold. If you had no expansion tank on your car, then the coolant that was released under pressure would spill out onto the road. It also helps by dissipating air bubles that have accumulated in the system. If you keep having to fill the expansion tank with large amounts of coolant every day, more than a cupfull, then there is likely a small leak somewhere else in the system. That could also be due to a faulty radiator cap (which you should always replace first before worrying youself stupid), a faulty radiator (blockage), water pump or hoses or in the worst case scenario-a blown head gasket or cracked block. If in doubt, have the radiator pressure tested at 15psi to determine where the leak is coming from, also have the radiator coolant tested with a special plastic tube contaning blue liquid that sticks in the radiator neck to measure whether exhaust gases are mixing with the coolant. It takes seconds to do and turns yellow if the result is positive leaving you with a very expensive repair for a new head gasket. If not, thank your lucky stars.
check to make sure there is coolant in the radiator. if not, fill it, and bleed the air from the system by leaving the radiator cap off until it has warmed until normal operating temperature. Keep topping off the radiator, then cap when it stays full.
Most of the time the radiator fluid fails to complete its circuit and is probably clogged leaving it to overheat -------------- Bad thermostat Bad head gasket Cracked head Low coolant level Debris in the radiator fins Cooling system may need flushing
The thermostat is key to maintaining relatively constant temperatures. It is basically a temp. sensitive spring operating a plug. It can be open or closed or partially open, depending on the engine temperature. When the car is cold, the thermostat almost completely prevents coolant from entering or leaving the engine. As the engine warms up to operating temperature and higher, the thermostat opens and allows coolant to circulate from the engine to the radiator. If the engine becomes too cool, the thermostat will close again. Coolant circulation is aided by the waterpump. When the engine's heat contained in the coolant reaches the radiator, it passes through a maze of finned tubes which allow the heat to transfer to the air flowing through (sometimes aided by a fan) the radiator. Now the cooled coolant circulates back into the engine to absorb more heat and repeat the cycle.