Hi all. I just wanna make a post as a future reference for people that might have a similar problem.

I've had a massive coolant leak of then-unknown source that dripped down from the left bottom bolt of the plastic flexplate/TC cover for a while now. There would be some amount of coolant under the car right at that area after sitting overnight. The level in the radiator would also drop till it levels somewhere around the bottom of intake manifold. I was terrified it would turn out to be a freeze plug, for they're a real pain to replace.

Anyway, as I was replacing the lifters, I also replaced all the removed gaskets including the manifold to head ones and torqued down the manifold bolts to 30 ft-lb. No more leak, the coolant level hasn't dropped after purging all the air out. It turns out the rear driver side was leaking - not surprisingly because the bolts at that corner felt like hand-tightened by the previous person/mechanic who installed them...

 

An update for the issue:

A while ago, I had a leak from the heater core. Replaced it, then all was good for a while until the leak I mentioned in the previous post showed up. Replaced the manifold gaskets during lifter change, all seemed to be good for maybe a week. Then I start seeing coolant just under the middle of radiator after parked for a while, and also overflow tank bottoming out and air in the radiator after that after 3-4 long runs. Of course it starts to heat up a little more at this point, and the trans also heats up a lot more than normal in stop-go traffic.

Something must definitely be wrong... There is obviously too much pressure in the system and components keep failing at the weakest spots one after another. First I thought it could be the high flow water pump developing too much pressure at high revs but then I decided to try something else. I put the factory radiator cap back instead of the Mr. gasket one with the built in temp gauge. The leak stopped. Put the other cap, starts leaking again. Put back the factory cap, stops again. Btw, i run a 180F stat.

The mr.gasket unit is rated for 16 psi - the same as factory unit - but either it was faulty and building too much pressure or the spring in the factory cap became weaker after many years and it takes less pressure to let coolant into overflow reservoir. When I compare the two, the factory unit sits 2-3 mm lower and the spring is actually softer, and it is also easier than the other one to screw and unscrew.

In any case, I'll probably have to repair or replace the radiator eventually, because the crack is there now. But the problem is solved, seemingly.

P.S.: I did a mountain climb of ~2300 metres on a 50 km road in 2008 with 2 passengers and some luggage without any cooling issues. The weather was 40C at the bottom and around 15C at the topmost section of the pass. I had no modifications whatsoever to anything at the time, so I guess it's safe to say the cooling system is working as it should.

I'm sure somebody can make use of this info.

Can