Years ago we had issues with big block van chassis ambulances over heating, and or vapor locking while on scene idling for extended periods of time. We experimented with a lot of ideas but the one that worked the best was having the engine hoods louvered. I do not think the louvers allowed the engine to run down the road at a lower temp, but it did keep the engines from over heating and vapor locking at long idle durations. Never had any issues with rain either, of course you must put the louvers in the right location to remove heat yet not direct water into the ignition system or air intake.