I saved a 94 GS300 w/ 192K on it from the scrap yard cause it had good tires, fresh battery and a new radiator. Bubbles and steams within a couple minutes of a cold startup, will overheat when pushed with a/c on. I replaced:
H2O pump cover & timing belt
T-stat, but am running the original gutted, and rad cap.
Cap, rotor, plugs, and wires that were all original.
Combustion gas check in radiator was negative, did it 3X and verified with tail pipe, no color change.
Cooling system pressure check lost 8 psi in 30 min, same as my 4runner thats good. I pumped it up to 10 psi and ran it, no pressure increase.
I have flushed the cooling system multiple times, ran it w/ vinegar for 2 days, flushed out rust. Pretty confident I have burped it.
How do I verify this is or isnt a headgasket, which i have already ordered making this a $700 300, but dont want to lean over this car for 10-15 hrs if I dont have to.?
Yes I forgot to do a compression test when I did the plugs, (vodka). Thanks to all, and the help this forum has already given me!

 

Compression test is the easiest way to tell really. Do this first
Look at your oil if its milky then you probably need a gasket
Bubbling coolant
Smoking exhuast
poor idle
These engines are bulletproof when maintained. Does it drive at all? Is your fan blowing the radiator?

 

Car runs and drives great, idles smooth, good steady power, no smoke on startup.
I would think pressurizing the cooling system would push it into a cylinder and I would see some white smoke, no luck.
Drained the oil, (previous owner told me she had just changed the oil w 9 qts, i fixed that) no evidence of water or on the bottom of the fill cap.
Poured a couple oz's of gear oil in the fan clutch, it spins for sure now, may have lost a couple hp's off the starting line!
The other 2 electric fans cycle.
If the HG leak is small into the cooling jackets, how much of a psi difference am I looking for? Thanks.

 

Update, compression test revealed #'s 1,2,3 @ 130. 4,5,6, @ 150, dry. WD40 bumped it up 20 psi, but the tell-tale was the WD mist spraying out of the #1,2, or 3 cylinders that didnt have the tester screwed in it. Pulled the head, this is not fun for a guy with a bad back, but is doeable for your average shade tree, its not back together yet either. Head gasket was intact but obvious blow over between cylinders, half the cooling jacket ports on the front 3 cylinders of the HG were plugged with what looks like used to be a thermostat o-ring, too thick to be gasket material, probably why they had to replace the radiator. Shop only took .004" off the head.