The actual temperature scale needs to be calibrated/adjusted based on the emissivity of the item you're looking at. When you're looking at something like an engine bay which has many different materials, results will be somewhat inaccurate.
For example - put a 100°F piece of rubber and a 100°F piece of polished stainless next to each other, and the IR camera will show them at different temps. I don't have a good sense for how much of a difference it makes (1°F?? 20°F??) but it's something to keep in mind if you're comparing temps of adjacent parts which are different materials or finishes.
