Why do you REV the engine before shutdown - Answer

[Jim Seamans]

I took the Yanmar diesel one day course at the Phila show and asked the question that was burning my mind.Larry the instructor from Mack-Boring stated, "After motoring in at slow speed and idling at the dock, carbon does/might/probably build up on the fuel injectors. So before shutting down, you will want to rapidly bump the throtle three times, a few hundred rpm to clear the injectors. If you don't, the carbon over the week or however long until the next outting, the carbon will crystallize and change the spray pattern into the cylinder and over time will cause problems."He didn't state how long the bump should be, but I expect fairly rapid.JimS/V Java

 

[Don]

RPM

I have a Yanmar 2GM20F and I was told be my service department to run the motor at 1800-2000 RPMS for about 5 min. after spending time at low RPM or idling. This has worked for me and its the same RPM range i need to re-charge my bats.

 

[Gord May]

Goose and Kill

After a "cooling off" period @ idle - I always “goose” my RPM at shutdown - a couple of slower throttle increase-decrease cycles (700 - 2000 - 700 - 2000 RPM over 6 - 10 seconds), then a quick goose to abut 2000 RPM & kill then leave throttle in minimum position. I’m told it burns the last injected fuel, so it doesn’t sit on the piston head.

 

[Bob]

For Real?

This is vewry enlightening news if the concensus is to "goose" the engine before shut down. I have never heard of this and allow my Universal M18 engine to idle at dock to cool down before shutdown. With my old Honda outboard (7.5), I used to disconnect the fuel line and let the gas burn til the engine died. That kept the carb. and needle valve from varnishing. The same logic can be used in the diesel with clearing an injector. But, what about an injector cleaner added to the fuel? Anyone else onboard with this?BobCatalina 30Atlanta

 

[Calif. Ted]

I start motoring a sufficient distance from

the harbor to allow the engine to reach operating temp well before it's time to reduce speed. I motor at a little above idle (diesels have very low oil press. at idle) thru marina to slip, considering it a cooling down period. I let it idle for a couple minutes while I tie up and then shut down.I worked with diesel powered fire apparatus for 30 years and it was always a practice to allow engines to cool by operating with a minimal load till they were back down towards lower end of operating range. Over the road use of diesels cooling systems must work to maintain a high enough temp and are stressed to keep the temp low when used as stationary engines. When used to pump water at or near capacity temperatures soared, this condition seems most like our use in sailboats with minimal airflows. When any engine is shut-off the block temp skyrockets because circulation of both oil & water stops and these cooling fluids now function as insulators. Really can't say if current thinking supports otherwise and how much of this is just old shadetree mechanics. Our unofficial motto was - 200 years of tradition unimpeded by progress. I'm afraid to use quotation marks.

 

[p323ms]

My Volvo MD11C manual

says to idle before shutdown. Their reasoning seems to be that temperatures are uneven after hard use and by idling the engine cools off and the temperature differences aren't as great. I don't shut down my engine when it's hot. But I guess a minute at high speed without a load wouldn't increase the engine temperature. I've noticed that my engine cools off rapidly when running without a load.

 

[Rodney]

Blipping is BS

OK, now that I have your attention, my Yanmar 4JH2E manual says:"Idle the engine before stopping: 1) To stop the engine, place the control lever to the Neutral position and operate the engine at low idling speed for about 5 minutes. !Caution: If the engine is stopped suddenly at a high temperature, the temperature of various parts will increase and engine troubles may occur." No mention of blipping or shutting down at high speed. The manual does talk about low speed operation: "When the engine is operated at a low idling speed (below 1000 rpm) for a long period of time (over 2 hours), excessive carbon and fuel residue tends to accumulate due to incomplete combustion. Carbon deposits on the injection holes of the fuel injection valves, exhaust valve, and the turbine blades of the turbocharger, etc. cause a drop in engine output, knocking, and other troubles. To prevent these problems, be sure to blow off the carbon accumulations by full speed operation. Operate the engine at over 2500 rpm for one minute in every 2 hours of continuous low idling operation."It would seem to me that blipping the throttle is not necessary unless your marina is so large that you must idle 2 hours to reach your slip. I have practiced this with 4 diesel engines over the years, 1 Volvo, 1 Universal, and 2 Yanmars and have never had a single problem with any of them. I have a friend who blipped his brand new Yanmar 2GM20F on every shutdown. That engine died with severe overheating damage after only about 600 hours. My friend swore up and down he never overheated the engine or heard the high temp alarm. He doesn't blip his Yanmar now either. IMHO, blipping is not necessary and may actually cause overheating damage. As Calif. Ted said, how much of this is shade tree mechanics vs real world manufacturer recommendations? Just my 2 cents.