We have had the same problem during really dry conditions. Several years ago it was 103 degrees and very dry. I was cutting wheat, and I kept smelling something burning. Got out checked the machine and found nothing. Got back in the cab, made another round same thing. Got out, opened the engine box and some of the dry wheat chaff had settled on the manifold and started smoldering, then fell down and ignited dust on the floor of the engine box. One puff from the fire extinguisher solved the problem. I took an old broom and swept the dust and chaff out of the engine box, and did not have any more problems. I was running the A_C in the cab on max, and I think that may have been making the engine run slightly hotter than normal although the heat gauge was still showing the temp well within the normal range. It was so dry, and windy, that that same day my neighbors Gleaner set a 25 acre field of wheat on fire when the strawchopper bearing went out and started making sparks. He barely managed to save the combine and the grain truck that was also parked in the field, but he lost all the wheat, in that field, and an adjacent 50 acre field before he and the fire department got it put out.