Why does my raditor resovoir tank drain, yet the raditor is full

i keep topping up my reserve tank, yet m y raditor in my 99 camry stays full

You probably have a small leak, in a pressurized system with a non pressurized coolant bottle such as your Camry has, when the engine heats up pressure builds in the cooling system and as the radiator cap maintains a certain pressure a certain amount of coolant is expelled to the overflow tank. If there is a leak in your cooling system when the engine cools off it will draw more antifreeze from the tank than it earlier expelled resulting in coolant loss in the tank but it radiator will still be full due to the fact the loss was made up from the tank.

Reservoir, overflow, expansion. Some tanks have 1 line in, they allow fluid in/out based on coolant temp. Other cars have tanks with in and outlet lines and they act as settling tanks and are a means to de-aireate the coolant. All tanks have a fill line. If yours is low, than you have a coolant leak

Stop topping off the Res tank and see what happens…Sooner or later the issue will present itself. You either have a leak in the Res tank…which should be rather obvious or easy to ascertain and identify. Or your engine is burning coolant…or the rad is leaking… SO THREE possible things could be happening here…OOPS…Make that 4… The Head Gasket could be also leaking into the Oil… I covered the burning portion of Head Gasket failure…I always overlook till last …Coolant in the Oil…

Like I said…STOP filling the Reservoir and give it a couple days of driving… See if your radiator is now low. SOMETHING is happening here. The coolant does NOT evaporate essentially…So see what happens in a few days of driving… Your problem is within the three possibilities I’ve listed above.


Check your oil right away. If it looks like a chocolate milkshake instead of normal oil, stop driving and check back here with us.

Yes…What the Mighty Lion said… I SECOND this suggestion and with very good reason.


First question, define top off. If you are filling the overflow tank all the way to the top, then that is the problem. There is a max fill line on the overflow tank and that is the highest that you should fill too. If you go over this line, then when the system warms up, the coolant will expand and push some out the drain. When it cools back down, you will see that the level has dropped.

If you are not overfilling the reservoir, then the overflow tank has a leak. I was first going to suspect that the hose between the reservoir tank and the radiator might be the source of the leak, but if the radiator is staying full, that is probably not the case. It would be the tank itself.

It could also be an internal leak in the engine such as a blown head gasket. Do check the condition of your oil, but even if it is good, that does not rule out a bad head gasket, it just means that the head gasket is not leaking near an oil passage. The coolant would be drawn into the combustion chamber during the high vacuum of the intake stroke and exit as steam.

I would start by putting a paper towel under the coolant reservoir and checking it everyday for a couple of days to see if it has any signs of coolant on it. This is the best way I know to find very small leaks other than using a special UV dye. Cheaper too.

Try using paper towels under other areas of potential leaks, especially around the heater hose just under the distributor. The distributors have a history of developing an oil leak around the shaft housing that leaks onto the heater hose underneath. The oil causes the heater hose to swell and soften, developing a small leak at the clamp.

If you find this, the rubber O ring for the distributor is about a $3 item and only takes a few minutes to replace. The distributor itself is not affected and does not have to be replaced as some mechanic will tell you. You may be stuck with a minimum labor charge of one hour to change the o ring. The o ring itself can be bought at the dealer. Autoparts stores don’t usually carry these.

If you can’t find a leak, then remove the spark plugs and see if any of them are unusually clean. If there is a coolant leak into the combustion chamber, it will steam clean the spark plug tips.

As your coolant heats up, it expands. When the pressure exceeds 16psi, it gets pushed into the reservoir, pushing any excess out the reservoir’s overflow tube. When the fluid then cools and contracts again, it draws fluid from the reservoir to compensate, leaving an air space in the reservoir. The level in the reservoir will continually go up and down as the engine heats up and cools, keeping the engine full. Any extra fluid used to “top off” the reservoir when the engine is cool will be pushed out when it again heats up.

If the coolant in the reservoir goes below the lower “cold” line molded into the bottle, you have a problem. The problem could be as simple as a bad radiator cap, but it will need looking into.

If the coolant in the radiator isn’t “clean & green”, you have a problem.

If the radiator (NOT the reservoir) doesn’t stay full but drops its level of fluid, you have a problem.

Does your car exhibit any of these symptoms? If so, describe it to us and we’ll walk you through a troubleshooting process.

If you do not have any of these or other symptoms and are simply wondering why the reservoir won’t stay topped-off, your explanation is in the first paragraph above.

4 posts were merged into an existing topic: Overflow bottle