Usually indicates a problem in the emission control system - have vehicle scanned to determine the problem
No there is not a water jacket in an intake manifold they are around the cylinder walls in the engine block.
The water temperature sending unit is on the front of the engine. It is next to the top water hose inlet connection.
An engine scanner will give you exact water temperature and gauging by it you can tell.
The water pump is run off the timing belt. did you have the belt installed by a Toyota certified technician? if not i would definitely check the water pump seals.
have no idea
It should last at least 100,000 miles or more but there is no guarantee.
A 1999 Toyota Tacoma typically requires about 2.5 to 3.0 gallons of coolant for a complete system fill, depending on the engine size and whether the system is completely drained. It's essential to use the appropriate type of coolant, usually a 50/50 mix of water and antifreeze. Always consult the owner's manual for specific recommendations and guidelines for your vehicle.
I used to do alot of wheeling and any part axels, engine, tranny that was submerged in water the fluids must be changed.
On a 1998 Toyota Tacoma with a 2.7-liter engine, the thermostat is located at the engine's front, near the water pump. To access it, you'll need to remove the upper radiator hose, which connects to the thermostat housing. Once the housing is unbolted, you can replace the thermostat. Make sure to drain some coolant before starting the replacement to avoid spills.
It's best not to wash an engine unless you have to. If you have to it's best to use plastic bags to cover electrical components such as the alternator and try not to spray high pressure water into electrical components eg: intake, fuse boxes etc..
This engine has a timing belt. Needs to be changed at 90,000 miles. All seals and water pump should be replaced at the same time.
Try replacing the thermostat.