Have a 1.6d in a Jetta, and have put about 10k miles on this engine since swapping it in to replace the previous one that was tired and ate a head gasket. The water pump was on this engine when I got it.
Just recently its been overheating, and the issue seems to be progressively getting more severe.
I installed a new 180 thermostat, and tested the old one - which seemed to open right at spec. The coolant flow through the vent line seems good when the coolant is cold. Since this issue developed, the vent line flow has been dropping off as the coolant temperature climbs. When the engine is hot (say 180-200) the vent line flow is just dribbling now at idle. It increases when the engine is revved, but is not what it used to be. Before this issue the vent line flow was same hot or cold. I checked the orifice in the vent line and it is perfectly clear. I suspected then maybe a water pump issue and checked to confirm the impeller is tight on its shaft, and it seems to be.
I was thinking maybe head gasket issue, but the engine cranks evenly, and a nitrile glove on the reservoir doesn't inflate. Also the vent line flow isn't aerated when the engine temp is climbing past the thermostat opening temp. After refilling the system, and the system has purged the initial air out, the vent line flow is liquid only.
I don't know how to upload pics to this forum, but the pump has a stamped steel impeller that looks just like the images of RockAuto's FVP 1801070. Same as this link:
https://www.pelicanparts.com/More_Info/1801070.htm?pn=180-1070-INT The impeller blades all look intact, but don't seem to "seal up" much against the housing like the images of the cast impellers do. However, the impeller didn't change its shape just recently, and this pump cooled the engine without any issue for 10k miles before this. Pump bearing is snug and smooth, and seal is holding fine. Judging by the stainless M6 fasteners, its been replaced.
The heater performance has been erratic as well lately. Heater core is new and I resealed the doors in the heater box at that same time. Sure seems like water pump to me.
Anyone have ideas or suggestions??