hey all, as the title states im in a 98 limited, 320k miles (original motor), and think im coming up on a blown head gasket/cracked head.

the truck has never overheated under my ownership, but has been steadily drinking coolant. i recently did a 600 mile round trip and before i left, i filled the coolant up to the full line, and when i came back, it was halfway between full and empty.

i performed a UV test but didnt see leaks externally (hoses/clamps/etc). i also rented a radiator pressure testing kit from oreilys and pumped it up to 13 psi, it very slowly lost some coolant, again no signs of external leaks. oil cap doesnt look too milky, and i didnt see bubbling when looking into the radiator on a cold start. i have also had a pretty rough idle on cold start a couple times, with the truck feeling as though its bucking for a minute or two before settling down.

i've found a shop near me that quoted me 4k for a jdm engine + installation, with new seals, water pump, timing belt, spark plugs, thermostat, oil filter, rear main seal, and front crank seal (all oem parts). if something is wrong with the motor, they have a 3 month unlimited mile warranty and will cover swapping the bad motor with a good one.

with no external leaks, and the pressure in the radiator dropping slowly, is it safe to say that its something with the head/gasket? im prepared to drop the money to replace it, but just wondering if you all think its worth getting a compression/leak down test before committing.

Peform a leak down test. That is by far the most reliable way to know if it's a bad head gasket or not. I had the same thing going on with my 97, once I pressurized cylinder #3, I had air bubbles coming out of the radiator.

On face value, it does sound like you have a cracked head and/or bad HG.

Good luck