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Discussion Starter · #1 ·
We all know these baby Cummins shake at shutdown, about like a horse when you take the saddle off him! I think I discovered exactly what is going on.

Maybe a dozen times when I've been in a hurry I've let out the clutch in gear too soon on shutdown. What I've found is the truck lurches backwards for a second as engine dies. You know what that means: the engine is reversing rotation at shutdown!!!

Of course I have no idea why it does this, but it certainly does it every time I shutdown and let the clutch out too soon. I don't remember any other engine that did that in over 40 years of driving.
 

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I dont see how that is possible. The rotating mass would have to stop and be acted upon by an outside force to change direction. In that short of a time span, the stress would be phenominal to do that.

The only thing that can do that is the starter which is not engaging in reverse polarity.

I think as the pressure plate comes down on the flywheel, the energy is spent to forward motion which is very brief as the fuel is off. After the initial small foward lurch, the backlash is what you may be feeling.

my 2cents
 

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It is possible that compression remaining in one or more cylinders as the rotating assembly comes to a halt, causes the crank to move backwards a bit, to relieve that compression. But we are talking about a fraction of a revolution.
 

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It is possible that compression remaining in one or more cylinders as the rotating assembly comes to a halt, causes the crank to move backwards a bit, to relieve that compression. But we are talking about a fraction of a revolution.
That's exactly it. Being a 4cyl it can do almost half a turn backwards before the next cylinder starts compression.
Lower compression engines and engines with more cylinders don't do it as badly. Throttled engines (petrol) don't have a full cylinder full of air to use as the spring.
 

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Had a gasser that did this all the time

It is called "run on " and yes it can happen with a really advanced engine. The sucker will rattle and pop and then weeze with scads of black smoke pouring out just before it dies. Lots of piston slapping going on . Hot plugs or carbon build up would help the backwards running engine run backwards too.

In a diesel ? Why not ?
 

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I've had my tractor engine do that a time or two, but when cranking cold, and it started up in the wrong direction. :eek: - it's nasty when a diesel with an oil bath air filter runs backwards - it make's a real mess.
 

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toyota actually had reverse rotation protection on their 2H diesel. it must happen for them to have bothered.
 

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I've seen an old Detroit actually run backwards. It's not a good thing..

It wasn't a compression kickback, though. Driver stalled a dump truck on a hill, didn't get it out of gear / clutch disengaged before it rolled backwards, and Bingo!
 

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I did one thataway once too on CAT 650 loader: brakes wouldn't hold while going up this ramp, you had to let it coast down in reverse against the engine. (old POS - They didnt' tell me this prior to the incident)

Needless to say once I let off the brakes and the transmission re-engaged (some sort of clutched auto) it ran backwards for a bit.
 

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In a diesel ? Why not ?
Unless it is a 2-cycle like the old Detroits, it would be injecting fuel about 20 degrees ATDC instead of 20 BTDC. I am surprised any 4 cycle diesel would run backwards at all. Gasoline engines won't run at 20 degrees ATDC.
 

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Discussion Starter · #11 ·
What I'm talking about occurs like on the very last stroke of the engine. I'd agree that it's a result of the high diesel compression, causing it to kick back on the last stroke. I guess the way I wrote that it could be misunderstood: NOT saying that as soon as the key is turned off the engine runs backward hah! I'm thinking that it only contributes to shutdown shake, the last gasp before it goes silent.

For all I know this might contribute to the instant starting of diesels? The cylinder was all ready to go one more stroke, kicks back and then sits there primed so to speak until the engine is turned over again: BAM starts NOW!

Maybe somebody else that's bored with their normal shutdown routine could be ready to let their clutch out just before their engine stops and see if theirs also lurches backwards right at the last instant. Either way it explains that hard shudder right when these 4BT's shut down.
 

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my 2003 VW Golf 1.9 turbo diesel had a throttle plate that would close as soon as the key was shut off and the engine stopped very calmly. This valve was incorporated into the egr valve wich i removed... After that was removed it had a nice shutdown shake to it.
 

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The Hyster forklifts I worked with once upon a time used a throttle valve to shut down, and they were smooth on shutdown. Of course, could have been the 10K weight they had, too.. Maybe incorporate a nice big-bore throttle body into the intake plumbing for regular shutdown and not just the "Uh, oh she's gone runaway" use, if the shutdown shakes are of concern.

Mine occasionally gives a nice lurch on shutdown, and I figured it was doing something along these lines. I think it may have to do about exactly where in the cycle things are when you shut down. Did a cylinder JUST fire, etc.

Now, if I had a sufficiently high speed video camera and a piece of tape, I'd put the tape on the crank pulley, set the camera to look at it, then do a few startup / shutdown events. Might not even need to be that high a speed camera, thinking about it..
 

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Discussion Starter · #14 ·
Hmmm, interesting on the throttle valve. Turns out this whole subject of shutdown may be a bigger deal than first apprears?

Makes one wonder exactly what happens during those last few rev's. At a 900 rpm idle that's 15 r's per second. It takes about 1/2 - 3/4 second [7.5 - 12 revolutions of crank] for the engine to shutdown from a closed fuel solenoid. The turbo is still spinning fairly slowly, and some residual fuel is in the lines, but the pump isn't receiving any more that it can pressurize. Pump is gear driven so whatever is in there on IP side of solenoid gets pumped. That implies that a slight vacuum is created between IP pump pistons and fuel shutoff solenoid after the last pump shot to the injectors? Lift pump is direct drive so it builds and holds pressure on the other side of fuel solenoid until engine stops rotating completely. The inertia in flywheel keeps crank rotating untill compression pressures without combustion finally become a greater force.

At that final moment then the flywheel inertia has become less than compression pressure force in cylinder. It's easy to see that the compression force which exceeds flywheel's latent force could cause a reverse rotation right at the last cylinder cycle.

These stinky old diesels are fascinating! Stinky? Well, we're the only guys shopping the cologne aisle at JC Penney looking for 'Eau du Diesel' or diesel scented deodorant....:rasta:
 
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