1999 sunfire overheating

First The heading says 99 Taurus but the question says 99 f-150. Second, are you saying that the engine is overheating? The heater won't work if the level of coolant is low and that would explain overheating. The thermostat controls the temp of the heat, so if it is stuck closed then water can't circulate to the heater and the vehicle would overheat, but this is unlikely.

the other answers seem to concentrate on a non-working heater. Good luck with this problem.

have you bothered to check the thermal fan switch...

Lot's of these problems going around. First thing is to locate where the coolant is going. Is it leaking onto the ground or going through the engine and out the exhaust? As for no heat, Check to be sure you have enough antifreeze. Pure water can cause the engine to not warm up and result in no heat. Air in the system can also cause these problems and must be bled out through the air bleed screw on top of the engine. On most of these cars it is towards the passenger side front on a metal pipe. It's a funny looking fitting with a flat head screw on top and a hole on the side. With the system full and the car running, slowly open the screw until bubbles come out and continue to bleed until you get no more air out. Let the car run to reach normal running temp and try bleeding again and again until you can't get any air out (they're funny that way). If you can not locate coolant leaking onto the ground it's due to a blown head gasket or a cracked head.

Coolant leaking from a bad head gasket

If the temp is high at freeway speeds then the thermostat would be suspect. If the temp is high in slow traffic then suspect the cooling fans or its controls

sounds like your coolant level is low? this causes a air pocket in system and hot coolant does not reach the heater core in side of car to give you heat. This can also cause engine to run to hot.

high speed overheating are a good indication. something is wrong with radiator. check coolant level, make sure the fans are working, and check radiator for broken fins or dirt and debris in radiator fins. if air is not going through radiator, it will over heat. if all is fine. replace radiator cap to see if this fixes problem. but check radiator fins first for debris.

When running the engine TOUCH the bottom part of the radiator fins, if the temperature is not the same or cold, u got a blocked radiator FiNS, and u need to replace it! Check also ur Radiator fan, note down what temperature it started to work. If the Fan works at high temp, means ur TEMPERATURE SENSOR switch for rad fan is not working or it has a higher temp operating value! Hope that might solve ur temperament! j/k!

You may have a radiator leak. Or your coolant level is low. Using the heater is not a solution to an overheating vehicle. You can usually smell coolant and see the leak. Often times a gasket can leak mixing engine oil and coolant, decreasing effectiveness of both sytems. Check color of fluids make sure the cooloant is viscus and not sludgy.

Also check coolant fan electric connections and fuses. The fan really cools down the engine moreso than coolant. It should kick on when your engine reaches a certain temp. If it does not the vehicle will overheat.

The fans don't kick in until around 225 degrees.

As for the overheating, Check for coolant flow at the Purge line,this is a common problem on the Northstar.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.

Regarding the misfiring, have you changed your spark plug cables?

VERY IMPORTANT....only use AC Delco spark plugs and wires.

Spark plugs ....AC Delco part number 41950

Plug wires....AC Delco part number 748Q

Ignition problems are common if you use anything other than the above OEM parts.