Runnin hot. no heat - 2000 Pontiac Sunfire

check the thermostat could be stuck ,
also check coolant level in the radiator (after engine has cooled down)
check for fan operation either fan clutch or electric cooling fan.

If by that you are trying to say that the alternator isn't charging the battery when the engine is hot then the alternator brushes are worn out. If you search in car shops you could find a replacement set of brushes, but installing them is tricky; just replace the entire alternator , it will be faster and cheaper in the end.

check water level.... change therastat.bleed system...check fan ......if still overheats hhave head block test done from a good garage (could be head gasket)

feel your radiator when the engine is hot, should be hot at the top and cooler as you go down with no cold spots, if you have a cold spot then it is clogged, and it will require cleaning or replacing. Check your thermostatic fan clutch when the engine is hot should be hard to turn, if it free wheels while engine temp is more then normal, it probably needs to be replaced. Hope this helps, let me know

Check the valve on the upper heater hose and make sure it moves,if it does then you need to have the heater core flushed.You can do it your self by removeing the heater core hoses from the heater core and runnin water through it with a garden hose but a rad shop can do a better job.If you choose to do it yourself pick up a bottle of rad cleaner first and put it in the rad a few days before you flush it.

please git that coolant system checked careful HOT it stays runnin because the block is so heated that u have preignition think british and secure yourself a service manual -cheers

You may want to try checking the fans. I have a 95 ss with the same problem. If you disconnect the MAF and then start the vehicle the electric fans will come on. Try driving it like that. That will atleast narrow down your search. If that makes it run cooler then its most likely a problem with the fans. Either a relay or fan switch. Blown head gaskets will usually result in the coolant and oil mixing. Check your fluids for signs of that. I have also read that the radiator cap not sealing properly can cause the coolant system to lose pressure. This reduces the systems ability to circulate the coolant causing it to run hotter. Thats an easy fix. I have not fixed mine yet. I just havent had time. That should be a good place to start. As far as getting a new engine or head gasket they are both really expensive fixes so try any other way u can to fix it first. Hope this helps.

yes it could be that

sounds like a fuel management problem. the catalytic converter is getting to hot due to tryin to burn off the excess fuel not burned. causing the muffler to overheat get it scaned at autozone for free. sounds like a o2 sensor runnin lean dumpin more fuel. or a bad mass airflow sensor good luck

Not sure if there are two thermostats, I know of one that I changed, it sits right above the starter and right below your air box. I had the problem my guage was reading hot on a cool night so I went and popped the hood and (after cooled) opened the radiator resivoir and the results were dry as a bone. I had steam coming from the cap. if you so this be sure to slowly release the pressure from the red cap that reads HOT WARNING DO NOT REMOVE. that is where you pour the coolant/water mix.

After I did that I took it to a shop and had it flushed, cleansed and refilled.....then I was runnin a beutiful bright green. Cost about 69.99 for flush then I found out my water pump pully was locked up and not rotating (not pumping coolant). So I fixed that last night and she's runnin pretty good after 190,000 miles.

Good Luck Mazda's