The correct answer is D) high exhaust temperatures. Clogged or partially obstructed exhaust ports on a diesel engine can cause high exhaust temperatures. This is because the restricted exhaust flow creates backpressure in the engine, which forces the hot exhaust gases to remain in the engine for a longer period before being expelled. This elevated backpressure and extended residence time of the exhaust gases leads to higher overall exhaust temperatures. The other answer choices are incorrect because: A) overspeeding would not occur due to a clogged exhaust, B) the engine would not fail to shut down, and C) engine performance would be impacted by the increased backpressure and elevated temperatures.