I was looking at the diagram for the injection system and I must admit I am a little confused. From out discussion so far I am understand that you think that the coolant (engine temperature) sensor might be failing. One way to check if that is the case is to take a couple of measurements across the sensor and see if they are within spec per the table in the above PDF.

Now, looking at the diagram (http://www.overgaardweb.com//dl-files/073-013.pdf) I see that the sensor is marked B11/2 but I do not understand how that is controlling the EGR valve. The only two valves I see that could influence that are the two vacuum valves marked 79. They are also monitoring the coolant but since they are vacuum valves you cannot measure any resistance across the pins. Both vacuum valves are connected to the vacuum amplifier (90) which connects to the EGR and controls the open/close through the vacuum.

If the EGR is involved wouldn’t it be a better guess that the vacuum valves are faulty rather than the B11/2 sensor?