I thought I had done this already with my 300w external bolt-on block heater, but I couldn't find the data anywhere on my computer.
Moot point. I've since added an 800w inline coolant heater, and haven't collected any info from it yet.
So, what I'm going to do is plug the car in & check the temperature every 15 minutes for an hour or so.
The results will be skewed somewhat by the fact that the coolant is not heated uniformly, and I need to run the engine briefly to get an accurate temperature reading.
I know the coolant temp isn't uniform because when I read the temp on the
ScanGauge before I start the engine, it gives me a value that's significantly lower than the value I see within a few seconds of starting the engine.
So the water pump has to circulate things briefly to mix it up and get an accurate (or "average") reading from the coolant temp sensor.
I can say with some confidence it's not the heat of combustion responsible for the temp jump, because it's quite large (say 50 degrees F) and shows up on the
SG within 10 seconds.
The best way to run the test might be to take off the belt & spin the water pump manually/electrically, but I can't think of anything offhand that I can use to do that easily.
Anyway, I'll report back later with some DATA! ...