Volkswagen Repair: volks jetta overheating, balance shaft, car overheats

Question
hi rockey;;;;
every time i fill my resovoir[1991 jetta]the car overheats and boils all the water out.my repair manual says if the thermostat is stuck closed the top hose would be cold.the top hose is very hot and the bottom hose is cold.i think the thermo is stuck closed.what do you think rockey?
an answer would be greatly appriciated.
thanks  rob

Answer
Hi Rob;
Pull the little hose off the top of the reservoir, and hold it over the open reservoir with the engine fairly cold.  Use your other hand to raise the rpm of the engine with the throttle, and just see how much flow there is from the little hose.  Use 2500-3000 rpm as your benchmark, or target rpm number.  If there is flow, the water pump is OK, if there is no flow, or little flow, a trickle, your water pump is no good.  I've found that the stainless steel thermostats are robust, never say die units, but they too fail occasionally.  The weakest link in any Volkswagen cooling system, in my humble opinion is the water pump.  If it turns out to be the water pump, pay very close attention to the orientation of the balance shaft, because it drives the distributor, and change the timing belt, and tensioner at the same time you do the water pump.  Finally, but most importantly, use a vacuum refill system to recharge the cooling system with coolant.  Don't just fill it up.  Use an evacuation system to get all of the air pockets out of the cylinder block, and the cylinder head.  Otherwise you will have an overheating problem that continues, and will be worse.  Hope this is of some small help to you, Rob.  Good luck.