ModMech
Platinum Member
Cool down is almost completely worthless unless the engine is turbocharged and even then would only be necessary if you were *JUST* working the engine HARD.
While an engine is in use, it is almost always operating below the peak design temperature and certainly is if you lifted the deck and drove it carefully into your shop for parking. Additional "cool-down" time is not warranted under such conditions and in fact may cause more harm than you may suspect because the cylinder temperatures drop quickly when an engine is under low or light loads. This can cause fuel to burn incompletely adding soot load and unburned fuel to the oil, neither are good.
With a turbocharged engine things are a little different, it is *most* important to allow the engine to return completely to idle for a few seconds before shutting down. Failure to do so can result in excessive bearing wear of the turbo due to lack of lubrication while it is spinning down.
As an aside, I did a crude test one day out of idle curiosity. After replacing the oil cooler on a CAT 3116 in a C5500 I started the engine to check for leaks before I re-installed the air filter assembly to the turbo (it is a PITA to remove/install and in the way of EVERYTHING on the RH side). We allowed the engien to idle for a few minutes and finding no leaks I shut the engine off. From IDLE the turbo continued to spin for nearly a minute! I brought the techs over, started the engine again then shut it down. They were AMAZED that a turbo spun for that long w/o oil. The lesson is, NEVER EVER "goose" a turbocharged egine before shutting it off, unless you like replacing turbos that is.
While an engine is in use, it is almost always operating below the peak design temperature and certainly is if you lifted the deck and drove it carefully into your shop for parking. Additional "cool-down" time is not warranted under such conditions and in fact may cause more harm than you may suspect because the cylinder temperatures drop quickly when an engine is under low or light loads. This can cause fuel to burn incompletely adding soot load and unburned fuel to the oil, neither are good.
With a turbocharged engine things are a little different, it is *most* important to allow the engine to return completely to idle for a few seconds before shutting down. Failure to do so can result in excessive bearing wear of the turbo due to lack of lubrication while it is spinning down.
As an aside, I did a crude test one day out of idle curiosity. After replacing the oil cooler on a CAT 3116 in a C5500 I started the engine to check for leaks before I re-installed the air filter assembly to the turbo (it is a PITA to remove/install and in the way of EVERYTHING on the RH side). We allowed the engien to idle for a few minutes and finding no leaks I shut the engine off. From IDLE the turbo continued to spin for nearly a minute! I brought the techs over, started the engine again then shut it down. They were AMAZED that a turbo spun for that long w/o oil. The lesson is, NEVER EVER "goose" a turbocharged egine before shutting it off, unless you like replacing turbos that is.
