Toyota Repair: 92 4-runner overheating, radiator shop, temp guage

Question
Dear Ted,
I have a 92 5-speed 4wd 3.0 6cyl 4-runner 230K miles. I am having a problem trying to figure out why it is over heating especially when the A/C is turned on.I have replaced the thermostat(twice)and flushed the radiator. It doesn't start to overheat until I've driven about 20 miles or so. Normal errands around town doesn't seem to affect the temp. guage, but anytime I drive for more than thirty minutes it starts to act up. I don't have a leak and when it does overheat I hear a "perculating" noise comming from the antifreeze reservoir. Your site has helped me with  many problems but this one I'm stumped on(my mechanic couldn't find anything wrong despite several tests and failing to get it to overheat). Thank you very much--Jim

Answer
There are two possible causes for this, it's either a clogged up radiator or a leaking headgasket both of which are not uncommon, flushing the radiator doesn't clean out the core if it is clogged badly with rust, the percolating noise is caused by the coolant rising to a temperature that will open the radiator cap and let coolant into the reservoir because the coolant has reached the boiling point, there are two things you can do at this time, take out the radiator and have a radiator shop disassemble it and clean the core or take it to a shop to have it checked for a blown headgasket, this can be done by doing a block test. Unless there is a lot of steam coming out of the tailpipe I would say it's probably OK but the only way to be sure is to do a block test, your mechanic should now how to do this.