Coolant System Problem Options

[80gumdrops]

Howdy. Got a UK '96 ST202 3S-GE.

Firstly, when I start the engine I can hear air bubbles circulating around the cooling system (happens about 3 seconds from ignition).

I've checked that the coolant is fully topped up.

On two occasions I haven't heard any bubbles.. when that happens, no hot air comes in and the engine temp rises above normal. When this has happened I've felt the pipes and found that the top pipe and top of the radiator is hot and the bottom pipe and bottom of radiator is cold.

First time it happened I decided to get the thermostat replaced.

Second time, I added coolant and gave the pipes a squeeze, which must've dislodged some air, because it started working again.

Last night, I heard a few bubbles as usual and had heat as usual, but about 5 mins into the journey I gave it some welly up a hill, lost hot air and the engine started to overheat again.

After a 5 min rest, I started the engine and heard LOADS of air bubbles circulate.

Any ideas folks?

Cheers.

[80gumdrops]

When I fire up the engine I've noticed that it over-revs until I can hear the bubbles move through.

Could that be a pump issue?

Also I've noticed that if I drive with the heat all the way up, it will be cold for the first 4 mins of the journey and then suddenly (like in 3 seconds) start coming through hot.

[Galcobar]

When I fire up the engine I've noticed that it over-revs until I can hear the bubbles move through.

Could that be a pump issue?

Also I've noticed that if I drive with the heat all the way up, it will be cold for the first 4 mins of the journey and then suddenly (like in 3 seconds) start coming through hot.

That sudden switch-over isn't really a surprise if your thermostat is working well; the temperature of the water in the radiator has to hit a certain point, at which the wax in the thermostat transitions from liquid to solid and you get flow back into the engine -- and thus into the heater core. Any bubbles trapped in the heater core would prevent heat transfer from the engine to the core (and thus to the cabin air) until they were moved out by a strong enough flow of coolant.

The engine is designed to run at higher RPMs when cold.

Toyota designed the system to be self-bleeding, as long as you have a properly calibrated radiator cap and the appropriate amount of coolant in the reservoir bottle. If the overflow bottle's not properly filled the system will suck up air as it cools and the coolant condenses. What this means is that over time any bubbles should work themselves out; manual bleeding is to get the worst of the air out quickly so that you don't have to keep adding coolant to replace the volume of air removed.