A few month ago my om603 with #14 head cracked. I sought out a new head but found and entire engine. The new engine is very smooth and quite compared to the last. I installed a new 80c thermostat and the now the temp gauge reads 95c to 97c once warmed up. I have a temp gun and the hottest part I can find on the head or around the thermostat is 87c.

So what gives? This also happened when I used a different 80c thermostat on my previous engine so I used a 71c thermostat. With the 71c Thermostat the engine took longer to warm up but would stay at 82c on the gauge.

The reading with the temp gun off the location of the temp sender for the gauge is 60-65c. I know the coolant inside is hotter but thats still a big jump from the gauge to what the temp gun reads.

Should I trust the gauge or what the temp gun is reading? I don't want to crack the head on this engine but I also don't want it to run to cool with a 71c thermostat. The temp stays consistent with the 80c thermo and the rest of the cooling system has been gone through.

Also one thing to note is I used the cluster from the car I pulled the engine from. So both times I tried to use a thermostat higher than 71c the gauge read hotter than I could find on the engine with a temp gun.