Coolant boiling and smoke coming out of engine bay

Question

I have a 1995 Toyota Camry SE that has 195,000+ miles that has started to develop some serious problems. Aside from what I believe to be an issue with the transmission, the car had been running fine.

On the way to the store this morning I had been stuck in traffic. While idling, I thought I heard a bubbling noise to my right side, and looked in that direction to see white smoke seeping out of the hood. I looked at the dash and observed that the temp was almost all the way to the H. After a few minutes, I took a turn onto open road and I noticed the temperature start to drop again. I got home, popped the hood in time to see the fluid in the coolant overflow boiling and white smoke (steam?) coming out from the cap thats on the hose running to the reservoir...

Googling scares me because all my searches have led to a gasket blown...any ideas?

EDIT - Thanks for all the informative answers in such a short time! It seems that the problem is a leak somewhere in the cooling system. I did as you all suggested (checked the electrical fans, coolant level, debris in radiator etc) and ended up seeing a small puddle of fluid underneath the car which wasn't there prior to starting it up...makes sense because the cooling system is pressurized? But what I don't understand is why the coolant reservoir is still full...

Answer 1

On Symptoms...

The boiling and smoke are a result of the high temperature due to a cooling system malfunction. The fact that your temperature dropped again is indicative that the system works sometimes. The question is, why didn't it work here?

Root Cause...

What you really want to understand is why the cooling system overheated in the first place.

• Was the system simply low on coolant?

• Is there a problem with the fans, and by moving, you were cooling things off by forcing air to move past the radiator?

• Is the water pump failing, and there's no coolant/not enough coolant flowing through the pipes?

As @JPhi1618 mentioned, you can visually watch to see if the fans come on, once the system gets up to temperature.

As far as the coolant level goes, inspect your reservoir. If low, you should fill it to full. That may not be the actual cause, since as you said, some has boiled off already.

With regard to the water pump, this is the most difficult to troubleshoot. It would probably present as a lack of heat in the cabin, or very low temperature heat. The coolant isn't flowing, after all, and that's what provides heat. Water pumps are prone to go out periodically, especially factory ones made of plastic: the fins physically dissolve. The other way that water pumps fail is in the seal; this manifests as a whole lot of coolant on the ground under the car.

But why does it sometimes work?

Maybe there's JUST ENOUGH coolant in there that the increased RPMs of moving cause JUST ENOUGH flow to cool things down.

Maybe the fins are very worn down, to the point of not being effective unless spinning at increased RPMs.

Why was it coming out of the thermostat?

Pressure has to release somewhere, and usually that's the path of least resistance. I've seen coolant vent at the reservoir before, but it looks like you probably have blown the gasket on the thermostat. That's cheap to replace... and usually pretty easy. A new gasket is usually less than US $10.00. Hopefully that's the limit of the damage, since the car was operating way above nominal temperatures.

EDIT 1

Per @Ben, "When Toyota water pumps go bad they make a lot of noise -> bearing failure. They very rarely have any other causes of failure."

Answer 2

Given that this happened while sitting in traffic and went away as soon as you got up to speed, I believe the issue is your radiator fan(s)

Since you boiled over, check your coolant when the engine has cooled down.

Start the car, let it sit and idle. Once the temp gauge gets past about 1/2, the fans should kick on. If it gets up to ~3/4 without the fans kicking on, there is a problem.

Possible problems:
Radiator Fan
Radiator Fan Relay
Coolant Temperature Sensor
Fuse
Wiring
ECU

Fuse is the easiest to check, so start there...

You can test the fan by unplugging it and connecting one side to ground, and the other side to the battery with a fusable link.

If that is good, I would look at the relay. Find the relay. It should have 4 connections - signal, ground, in (battery voltage) and out (to fan). Use a test light to verify the 'in' side has voltage, and the 'signal' side gets voltage when the engine gets hot. If the signal side has voltage, the relay should click when you connect it. It helps to have a schematic for this.

If the signal is not getting voltage, I would look at the CTS. You will need to measure the resistance across the pins when the engine is hot to verify it is reading correctly. If the ECU does not know the engine is hot, it doesn't know to turn on the fan.