Can someone elaborate on an "engine cooling to much at the bottom of a hill"? (FAQ)

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07-09-2005, 04:48 PM

Dave88LX

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Can someone elaborate on an "engine cooling to much at the bottom of a hill"? (FAQ)

I was reading this here:
https://www.dieseltruckresource.com/...n&onlynewfaq=1

I don't quite understand this here:

Quote:

Another situation to watch is when a hill is being crested. The engine must be cooled by shifting down and backing out of the throttle because the engine will cool down fast. It is not good to let an engine cool down too much at the bottom of the hill before starting up again. When the engine is not being operated in the most efficient range, fuel is being wasted and engine damage will occur if the proper procedure is not followed.

Downshifting is going from 5th-4th, 4th-3rd, 4th-2nd etc, right? So when you get up over a hill, you're supposed to downshift and let up off the fuel? What is meant by letting an engine cool too much at the bottom of a hill? What IS the most efficient range?

Thanks,
Dave

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07-09-2005, 07:03 PM

wannadiesel

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What he's talking about there is downshifting and backing out of the gas before you crest the hill. If you are pulling hard on a long hill in 5th and EGT's are high, you don't want to come over the top and start downhill immediately with very high EGT, since rapid cooling can cause cracks in the manifold. You want to downshift to 4th before you hit the top of the hill and let the EGT drop to a more reasonable (under 1,000 degrees) level before you start down the other side.

When you hit the bottom and EGT is 200 degrees, don't hammer it to start up the next hill, roll into it. Rapid temperature changes are what you are trying to avoid. An exhaust brake is a big help in both situations.

Efficient range - The best rule to drive by is: Cruise slow, work fast. When the truck is loafing along on flat ground, high gear is fine. when you come to a hill and the engine starts to bog down, it's best to downshift instead of just mashing the gas to use the torque to keep the truck moving. The engine and drivetrain will last longer if you don't have your foot planted in the firewall going uphill. Basically, if you need to have the truck floored to maintain speed, downshift. Light throttle at higher RPM is preferable to heavy throttle at low RPM.