Radiator coolant bottle

1990’s Toyota Corolla. I was driving in San Francisco last month when I noticed a lot of fog. Well, being SF, I wasn’t surprised, so I continued along my merry way. The fog was so thick I had to turn the windshield wipers on! Then I noticed the fog was only in front of the car, not in the rear view mirror, or out the side windows! Back and side I could see miles. Front? 10 feet! Directional fog? Yikes!

Not being the sharpest tack in the drawer, it took me awhile, but eventually I figured out the fog was coming from the car, not the rarified SF atmosphere. The radiator had sprung a leak. Upon inspection, the top seam, where the plastic part connects with the metal, had come apart and leaking steam. Voila, instant fog.

I replaced the radiator and everything’s seems to be working fine. But I wonder why the radiator leaked in the first place? From the top of the radiator comes a plastic tube which goes to a plastic overflow bottle. There’s a T connection there on top of the bottle, so apparently if the overflow bottle becomes full, it won’t burst the pipes and radiator, instead the excess coolant will leak out onto the ground. It’s not supposed to ever beomce fully plugged in other words.

So I took off this tube from the top of the radiator, and I took the top off the coolant bottle, and blew into the tube, plugging the tube that otherwise extends into the bottle. Hard to explain. 3 connections to a T, right? Input, Overflow, and the one that goes into the bottle. I blew in the input side, plugged the one that goes into the bottle. I expected it would blow straight through to the overflow.

But no. It acted like it was plugged. I couldn’t blow hard enough to get any air to pass through to the overflow.

Is this normal, or could it be the reason the radiator blew out in the first place?

The radiator failed because it’s made out of plastic and failure is part of the design…With the overflow tank, the three ports are not connected together…One, the tank overflow, is isolated, connected to the tank only…See the little hole?

First, let me start with a warning. Antifreeze is extremely toxic. One ounce can kill you. It destroys your kidneys. Be careful what you are putting in your mouth.

You are not clear as to what is plugged and where you are blowing. If you are blowing into the radiator and the radiator cap is on, you can’t blow thru. If the radiator cap is off, you should be able to blow thru. I got a new radiator once where the fitting for that small hose was plugged up. If this is the case, the fitting must be drilled out.

The hose from the radiator goes to the port on the overflow bottle that has a hose on the inside that goes to the bottom of the bottle. The other port is an air vent so that the bottle does not draw a vacuum or become pressurized.

When the coolant in the engine gets hot, it expands and pushes any excess coolant into the bottle. When the engine cools down and the coolant contracts, it draws coolant back into the radiator. This keeps air out of the radiator. If air gets into the radiator, it gets mixed in the coolant because of the turbulence created by the water pump. That air would reduce the efficiency of the cooling system.

The plastic and aluminum radiators are more efficient than the old copper and brass radiators, especially on an aluminum engine. The rubber seal between the tank and the core will deteriorate over time and eventually leak. About 12-14 years is the lifespan for this gasket.

If anything is ever responsible for excess pressure in your cooling system it will be the radiator cap which is supposed to open up & allow coolant out to the overflow somewhere in the neighborhood of about 15-16lbs of pressure. (It probably gives a psi on the cap).

But its likely that this radiator was just old.

I’d also check/replace your thermostat. A thermostat that is stuck partially closed will cause overheating and a buildup of pressure in the cooling system.

Weeeell There are a couple of reasons this may have happened… In addition to what the other guys have mentioned.

One…the rad just failed, they do this, not surprising. Two…a faulty cap keeping in too much pressure, not venting into the overflow…which is rather rare, but certainly can happen.

The last one is a failed head gasket…over pressurizing the cooling system…it would then rely on the cap to vent this over pressure…and if it didn’t…Boom…it will blow the rad up like a balloon…and bust the tank seal on your rad, like you saw.

Hopefully the rad just failed… If it didn’t…then you have some work to do to determine if the rad cap is the culprit (any garage can test it for you) or the dreaded head gasket. Have you ever overheated this vehicle prior to this issue?


Thanks everyone. I think I understand. The output port of the T which vents to the air only goes into the tank, and not connected with the input or bottle port. So I shouldn’t be able to blow from the input to the output. I’ll check that out to make sure it remains open.

No, car has never overheated, and not overheating now.

The reason it leaked is that the radiator fan connection had come loose I think. When I replaced the radiator I noticed it wasn’t a good connection. Loose. I fixed that when I replace the radiator.

Thanks for good advice & info