Cooling issue

[phorbiuz]

More specifically for a 2.0 pinto engine, but I guess this is quite generic.

All was good, running normally, until one day the temp gauge dropped from the middle of the temp reading to virtually zero. Classic symptoms of a stuck open thermostat or a faulty gauge in my mind. Heaters warm but no longer hot. The top hose is hot (but not too hot to hold) with the bottom hose much more cool.

Easiest and first thing I tested was to ground the temp sender wire and the gauge went to max, so wiring and gauge OK in my book. I changed the temp sender for a spare with no effect. The temp sender on the pinto screws directly into the block.

Next thing was to change the stat so down to my local factors and bought two new ones rated at 92 degrees (nice to have a spare), then change the stat and flushed the coolant while I was there. No difference in temp reading. Just in case it was a duff stat I changed it again for the second of the two I'd bought. Still the exact same problem though. I have removed this 3rd thermostat to test and it starts opening in the 80's, fully open by the early 90's. Opened and closed several times no problem. I don't actually have a proper manual thermometer so used a cheap Laser infra red one.

Next I thought I'd fit an auxiliary mechanical temp gauge which plumbs in-line to the top hose to back up the original gauge reading. While there I also replaced the heater matrix top and bottom hose as they looked a bit aged. The reading on the new gauge is about 60 degrees which confirms it is running too cool. I don't think the dash gauge starts to pick up until about 70 degrees.

From there I removed the viscous fan and it eventually got hotter but it does take some time. At start up the fan can be stopped easily with a hand suggesting it is not stuck and running too quick. I even have at present half of the radiator covered to reduce air flow to no effect.

My limited mechanical skills still say to me thermostat as surely if the coolant was too cool it would shut, warming the engine more, and the temp sender reading would increase, then open at the correct temp. Continually opening/shutting and maintaining the correct temp.

Simply though, it never gets to the correct temp, there is no coolant loss, and there are no other issues.

Anyone got any suggestions please?

[tractorman]

My first thought would be the weather when the readings changed - if it's really cold, the engine will take a long time to get to temperature. OK, I'm now talking diesel engines, but the tractors are often stone cold in winter and it took about eight miles before the Golf was warm yesterday (and I had kept it in 4th to get the DPF warmed up!) and the heater was going flat out most of the time (I don't usually have it more than "half way" as it usually gets rather too hot). The air temp ranged from -5 to about 1.5 deg C: not very cold, but not a warm summer's day either.

I would suggest that, while the weather is cold, you don't worry too much about it (wear gloves and coat to keep warm - like I didn't yesterday!) and start to get worried if it's cold in June! Other factors that may affect it are mixture and timing - though I suspect they aren't the problem!