96 jetta overheats

I assume you mean radiator *fan* only turns on when overheated... you need a new thermoswitch for the fan to begin with, and I'd replace the thermostat on principle. Heater cores are known weak points on VWs although they were supposedly "fixed" by 96. I would have the system pressure tested to rule out a blown head gasket just because the blown heater core may be an indication that there's too much pressure building up in the cooling system.

good luck,

nate

If there's too much pressure in the system that'd mean the 'cap' is bad and needs to be replaced. It's supposed to let the system breath, while still keeping the system mostly sealed.

He didn't say which motor, so I'll got off what I know on the 2.0l. Since the post wasn't clear, I'll assume it's been overheating mostly since the heater core was replaced. Even so, it's worth checking...

Check to make sure the cooling system is 'burped'. The fan temp switch is separate from the temp sensor that feeds the dash and ECM. Since the systems been recently opened and refilled, if that wasn't done, the fan won't ever come on because the temp switch won't read air.

To burp the system, squeeze the top hose going to the head gooseneck. You should see the level in the coolant ball rise a little or bubbles come out, then drop below where it was when you release the hose. Do it a couple times. Run the engine to warm (no more than about 10-15 minutes) , turn it off and let it cool a bit. CAREFULLY open the cap. (beware of overflows if you open it too hot. Steam burns suck) Refill with the correct coolant. Repeat. Carry some premixed coolant in your trunk (boot) for awhile and top off the coolant as needed. It should eventually stabilize and you'll be done.

If the upper hose never gets HOT, then check the thermostat and water pump.

IIRC, on the 2.0l's if you're doing the water pump you may as well replace the thermostat and timing belt while you're there. Most seem to say replace the water pump and thermostat if you're doing the timing belt, so I'd do the same if I was in that far already unless the belt had been changed within the last 10k miles or so.

After all this if the fan motor isn't running, but it should be by temperature, check the voltages going to the fan motor. If it's got voltage, it's the fan motor. If not, check the switch and wiring.

Mark '95 Jetta GLS

Sounds like a water pump failure to me. Time for a new one.