A few note about the cooling system flow:
Both heater cores don’t go through the thermostat, so normally if you turn on the 2nd heater fan blower at the full & HOT position, the needle should slightly go down (It's a good way, in heavy traffic, to lower the engine temperature)
The cooling fan thermo sensor is located on the front radiator and has nothing to do with the real engine temperature, it just go on when the front radiator is to hot.
The front radiator stays cold!!! until thermostat opens (but the engine as been at operating temperature for some time) The fuel injection system depends on and varies with engine temperature so it is crucial that you have a fully functioning thermostat.
Note: a thermostat can open partially and not necessarily to the full open position (it gives what the engine needs and no more), as soon as the engine gets enough cool coolant the thermostate will close back (and maybe not all the way).
VW is now at the letter “F” (part # generation) for the standard 87deg thermostat, but I called VW Canada yesterday and they explained to me that the “F” thermostat was a bit higher than 87 deg and the older model were a bit lower than 87deg!!!!!!, they say that the van was running to cold with the subsequent model (A to E!!!!!).
Note: I live in Montreal, Canada. The
temperature these days is around the freezing point; the front
radiator is always cool, even if the engine idles for a long time. That's
with the new thermostat. With the
old one the front radiator was warm but not hot at all.
Also, VW confirmed that the new thermostat and the new temp
gauge sensor (the one in front of the
thermostat housing) give a higher reading (for the sensor, about 15% more)
and higher temperature (for
the thermostat, about 20% more heat). The gauge should normally (by VW
spec) read ľ or a bit below
I find it completely stupid that the gauge is not in the middle when at normal temperature, it is logical to have the same proportion each side of the needle for an accurate reading and safety (more time to stop the car in case of overheating). My little Honda gauge is always in the middle like it’s frozen there, I once had an overheating problem due to a lost of pressure and in went all the way near the red section giving me some time to stop the car.
Note: the VW car (not van) I worked on had a 3/4 reading in the front at normal operating temperature. I just change my thermostat and sensor and I had the 3/4 reading in the front, I didn’t like that at all and went back with the old on. I like to see my needle in the middle, period.
Spécial thanks to Malcolm Stebbins (re-edited English)