Add another '04 to the list
Just did my '04 at 181k miles. Had the waterfall sound from the firewall for tens of thousands of miles-- with no other symptoms I thought that was normal and people saying it's a symptom of HG failure was BS-- but they were right.
Under normal conditions, no overheating, barely any coolant loss, ran like a top, BUT upon towing a small camper at highway speeds, after about 10 miles, the temperature shot up. Carefully limped without overheating to somewhere I could work on it, and it appeared the thermostat decided to go. New t-stat. Coolant flush to ensure no clogs, new Toyota coolant, bled air, etc. Few weeks later, another towing test, less than 3 miles at highway speeds, lower radiator hose blows off radiator and there goes all the coolant.
Diagnosis by me: exhaust in the cooling system, under high revs/engine stress it was over-pressurizing the cooling system. Probably caused the t-stat to fail, and the lower hose to pop off. The last time I towed was a year ago and back then had a leaky radiator- I assume any exhaust pressure in the system was able to leak out that way. With new radiator, cap, and t-stat the pressure had nowhere to vent. Topped off with coolant, it started to blow out the overflow tank cap consistently. Finally diagnosed noticeable gas/exhaust smell when the rad cap was opened cold, and always low on coolant (in radiator, not overflow tank).
About $500 in parts from Toyota, 6 full days in the garage, plenty of foul language, cuts and bruises, and it's running great. Took a whole day to get the exhausts off the heads, which had to come out of the car with the heads.... If you're in the rust belt and never touched the exhaust headers before, do yourself a favor and buy new nuts and studs.. and a stud extractor tool. Had to special order these from the dealer who needed the VIN because there were THREE different part numbers for the studs for 2004 (wtf?).
I should add that I got the overhaul gasket kit and a new water pump. They don't tell you but the water pump and the overhaul kit come with a water pump gasket, so don't order an extra like I did. Could have done the valve seals, but didn't. Replaced every seal and gasket I could identify with stuff from the overhaul kit during reassembly. Also cleaned the crap out of the heads, the intake manifold, the throttle body, and discovered the #6 injector screen was mostly clogged. I think I went through 4 cans of carb cleaner, a can of sensitive electronic cleaner, and a can of brakleen.
Big thanks to a fellow forum member [MENTION=112658]nossliw[/MENTION] for this post on his process:
http://www.toyota-4runner.org/4th-gen-t4rs/174451-new-member-need-help-p304-misfire.html#post1806657. I had the FSM but wasn't completely convinced to tackle it myself until finding that thread.
Both head gaskets had obvious leaks. One between cylinders 5 and 6 and the coolant passage, and at least one cylinder and the coolant passage on the other side. No leaks around the oil passages. With the new gaskets in, no more waterfall sound and the coolant has stayed up to the top of the radiator after one air bleed/top off. Been about 500 miles, towed the camper and really thrashed the engine; no problems.
My first car was a 1988 BMW 535 which I sold at 250k miles... with no major engine work. A Toyota truck shouldn't blow its head gaskets when it's so young-- or EVER.