first thing to check would be the thermostat in the engine. next check the radiator cap. also check that radiator hoses are good, and not soft.if they are soft they can colapse when hot, stopping the coolant from circulating check to see if there is coolant in the oil that would indicate a blown head gasket.
Tough to narrow it down with such little detail, but it sound like your car's thermostat may be malfunctioning and not controlling the temperature correctly.
Coolant is used to keep an engine in a vehicle at a temperature safe for operation. If no coolant is in the reservoir, an engine will not start.
It is located under the coolant reservoir. You can't just replace the sensor. The whole coolant reservoir must be replace. It's a simply 10 minute job.
The coolant level is set by the level in the coolant reservoir. There is a line for the level if the engine is cold and a line for if the engine is up to temperature. You should fill it to the corresponding line.
bottom of the reservoir (coolant recovery tank) on the engine side (below fill line)
NOTE: this answer is describing the location of the engine coolant temperature sensor - NOT the coolant sensor in the reservoir that warns when the level gets too low.The coolant temperature sensor is located directly adjacent to the thermostat housing. The thermostat housing is what the upper radiator hose (toward the right side of the engine compartment) connects to. You do not have to replace the reservoir to replace this part, but leaky coolant reservoirs are a common problem on this model.
Remove the coolant reservoir, flush it out, reinstall it and refill with proper coolant.
if the coolant in the overflow rises and overspills when you shut the car off or is boiling in the reservoir
Possibly a leak in the coolant reservoir bottle.
The coolant reservoir on the MK3 jetta is located against the firewall. It is a small round reservoir.
Yes- coolant reservoir is designed to accept excess coolant - As coolant heats up it will expand and flow to the coolant recovery tank (reservoir) and as it cools (contracts) it will be sucked back into the radiator
when the engine is running and coolant will become hot to its operating temperature,the cooling system is under pressure,coolant will overflow and goes to coolant reservoir.when the engine goes cold coolant will go back to the rediator.