Hello All,

 

I came across the problem described bellow which I would like to have a opinion from those who might have a similar experience or experience with engine diagnostic. 

 

The engine temperature started dropping slightly bellow 90 degrees on gauge cluster. It's happening only during longer deceleration for example if I'm driving down the hill on motorway. Once I start using throttle pedal to accelerate or maintain speed temperature starting climb on 90 and stay there. Same happening if I stop and let engine idle. I did live data streaming vie EOBD tool and coolant temperature is between 75 to 80 degrees while temperature on gauge showing 90. I don't have any error codes stored in ECU.

 

My question is where the failure could be? Thermostat, coolant sensor or both? 

 

Thanks for your thoughts
 

Edited 12 March, 20197 yr by Nosnx

The software is designed to display 90C over a wide range of actual  temperatures and only shows other temperatures when there is a need to worry.

It looks like the thermostat may be stuck open as the temperature display falls when the engine is not producing much heat (idle and downhill). I would say the temp sender is working correctly and you need to replace the thermostat.

Another symptom to confirm this is if the engine takes a long time to reach normal temperature.

 

Is 5 miles journey to get engine warm up to 90 in these cold days the long time? In warmer days it's even quicker. 

5 miles to get to 90 degrees on clear A roads is good in these temps. My MkIII takes about 6-8 miles on free-moving A roads to warm to 80 degrees. My old diesel was barely warm over the same distance. 

More likely to be thermostat IMO. As above, the temperature gauge is weighted towards 90 so take that reading with a pinch of salt. If you warm the car up from cold at idle speed, watch the temp on your OBD tool while feeling the radiator hose. If the radiator hose gets warm as the car warms up steadily its opening too early or may be partially stuck open.

 

Use an OEM thermostat

Yeah...looks like piggy thermostat replacement is waiting for me