Toyota Repair: Overheating, toyota 4runner, temperature gauge

Question
 Hi, I have a '92 Toyota 4Runner, 3.0 V6 engine, 186k miles. It was running fine but recently the weather got cold and I realized that the heater wasn't working and my temperature gauge was staying on cold. While replacing the thermostat, I discovered that the one in it was missing the whole inner piece and all that was there was the outer ring. I can't find the rest of it so that has me wondering if the previous owner cut it out to cover up this problem.
 Now that it has the new thermostat, it will overheat shortly after I start it. I thought it may be faulty but I tested it and that isn't the case. It has a new water pump and radiator, and it doesn't leak any antifreeze. I'm running out of options. Any ideas? Thank You

Answer
The engine should be tested for a leaking headgasket, most repair shops or the toyota dealer can do this for a diagnostic fee, leaking headgaskets were very common a few years ago on this engine and toyota had issued a special service campaign which may still be in effect for your truck if it's never been done before, contact the toyota dealer and give them the VIN# and ask them to check if
SSC VO-6 applies,if so you get the headgaskets replaced at no charge, there is a small chance that it does but in case it doesn't  make an appointment to have the headgaskets tested.