Mk1 Punto 8v Coolant Temp Fluctuation

[dukeboys]

For some time now Ive noticed my motors temp gauge has been playing me up. It used to sit solidly at 4-5mm under the halfway mark whether on the move or stuck in traffic.

From cold she warms up fine (takes the usual 5-10 minutes) and the gauge reads a nice reading just under halfway for a while but then it will rise to halfway if I idle in traffic.

Initially I thought that the gauge was only rising to halfway when I came to a halt in traffic (then the gauge would drop once on the move again) but now on a decent run out yesterday I noticed the gauge would slowly fluctuate between what I class as its normal reading and the higher halfway mark reading during the course of a steady 55mph cruise. It shifts very slowly between those readings - up down up down. Weird. In traffic it goes up quite quickly though.

Initially my thoughts were that as the temp seemed to rising in traffic only it pointed to a worn water pump but now I am beginning to think perhaps it is the thermostat that has developed a fault hence the fluctuation at a steady speed.

I wouldnt have thought the coolant temperature sending unit was faulty - thats effectively under the control of the temperature of the coolant that is being allowed to flow past it by the stat and its best to rule the stat out first before buying a new CTS. the radiator and existing stat, rad fan switch and CTS are all 2.5 years old and the rad is in very good condition having covered less than 15000 miles. The electric fan kicks in fine and I have no leaks in the system. Fairly sure all airlocks have been removed and the interior heater works fine. The heater matrix is brand new and the system has regularly been cleaned and flushed and refilled with 50/50 mix. This slight fluctuation problem existed before I renewed the matrix BTW. History on when if ever the water pump was changed is unknown with the car on 130,000 miles it could be the original but will be costly mistake to simply go ahead and have it replaced if its something else.

Any pointers or am I on the right track with changing the stat for a new one? Wish it was 11 months and 27 days old as I could have tried to claim a new one under FIATS 12 months parts guarantee !! No such luck.

Anyone else had similar? My gauge never goes into the upper red sector or above halfway but I would like to return it back to its previous state of health.

I am wondering whether my stat whilst opening correctly as the motor warms up is shifting irratically between fully open and partially closed ie; its backing off a bit reducing flow slightly to the rad? It would explain the fluctuation or at least offer one possible explanation.

 

[s130]

Could be a thermostat problem. In the case you explain I would be thinking a stat that is partially stuck open.

You need to run a simple test.

Leave car overnight to fully cool. Locate the top and bottom radiator hoses. Next bit is boring but you have to be patient and be present.

Start car and leave to idle. Return to engine bay and place one hand (grasp) on the top radiator hose and another on the bottom radiator hose. What you are looking for is the top radiator hose slowly getting hooter and hooter as the engine warms. Meanwhile the bottom radiator hose should remain fairly cool. As time progresses the top radiator hose will become very hot but the bottom hose will only have warmed up a lot less. Then at some point the thrmostat will open and the bottom radiator hose will get very hot very quickly.

If however the bottom rad hose generally increases in temperature with (or slightly lagging) the top rad hose then the thermostat is partially or fully stuck open.