what to look for after driving low on coolant
 

After toyota changed the valve springs in my 2012 GT86 it got a massive coolant leak. I didn't notice until i checked after it struggled to start all of the sudden. So i have probably driven quite a few miles while low on coolant, but i haven't noticed rising temps or any warning lights. It drains the tank from max to min in around 70km, and i have probably driven at least twice that between the spring change and when i refilled.

After i refilled it starts perfectly fine, and i can't tell that there is something wrong while driving. It's going back to toyota today to sort out the coolant issue. Is there any specific test they should do to check if overheating has caused any kind of long-term damage to the engine?

I don't think you'll have any problem if the coolant is at min mark even if it's slightly below that, this is less than 300-400ml coolant!

It probably burped. You have topped it up, just keep a eye on it.

Warning: someone might say to sell it and buy a Civic before it explodes, this explosion may not in fact happen.

I think i just explained a bit poorly. When i first checked the coolant, it was completely dry. Not a drop of coolant in sight. I refilled to max, and after 70km it was almost empty again.

So if i drove it 200km after it first started leaking, it was probably empty the last 100km. I drive mostly short trips, that's probably why i haven't noticed overheating. It just scares me that it was difficult to start, but that happened only once.

Yeah nah, that could be real bad, I would contact dealer and tell them, if it keeps losing coolant, get it to the dealer/mechanic.

The dealer has actually been really helpful so far. They took the car in today.
I'm just afraid that they will fix the leak, but leave some kind of damage that will reveal itself later on. Like prematurely worn piston rings and what not.

With no warning lights the coolant temp has probably been fine, they hold over 7 litres, missing a bit, whilst still not real good, shouldn't cause any major issues.

Was the temperature gauge reading anything at all? If so, you are probably fine.

The reservoir (expansion tank) would only empty when you turn off the car as coolant gets sucked back into the system, so filling it isn't a good indicator that your coolant is at capacity. The amount that would great sucked back in depends on the expansion of the coolant, which depends on how hot it gets (as well as how much air is in there acting like a spring). Guessing the rate of leakage from the expansion tank doesn't really work.

As long as i compare reservoir levels in the same conditions each time, shouldn't it be a decent indicator? The levels rise and fall with temps, but at the same temp it should stay at the same level. In this case i filled it to max when cold, and checked again when cold.

The gauge has been reading normally.

If your leaking coolant, you should be able to tell pretty quick by a big puddle under the car. Could also put cardboard under the car to help locate where its dripping from easier. If its dripping from the engine your probably screwed, but chances are if you do have a leak it is at one of the hose locations, and a clamp was put on wrong or not tightened enough, if so easy fix.

The other scenario the dealer could have flushed all the coolant to do the valve springs, refilled it but forgot to bleed it. In which case as you drive you would slowly bleed it and emptying your reservoir in the process. If not bled, you could probably fill reservoir to max and fully drain it multiple times before all air is out and its fully bled. In which case you don't have a leak just needed bleeding.

Mine wasn't fully bled after the valve spring recall. I had to top off the reservoir twice. Try topping off the radiator first, with the bleed screw near the back of the engine.

This. I also, after the second time they replaced my engine, immediately had to go back and I asked them to give me a free jug of coolant so I could keep topping it off. Took a couple cycles to get it consistently between min/max

The expansion tank is connected to the rest of the system by a valve. When the coolant in the system contracts (cools down) it reduces pressure and sucks coolant from the bottom up the expansion tank up (yes, literally up, follow the hose) through the valve into the sealed system. If there is air in the sealed system, that is allowed to expand when the coolant's pressure is reduced through cooling (gas is compressible), which reduces the pressure difference available to pull fluid in the expansion tank through the valve. There is still some pressure though, so eventually the expansion tank will empty, but it will take more time than it normally would.

I have refilled a full litre. Could it have been that much air in the system?