Better to REV engine before shutting it off

[TFB1]

You'll create more blowby with that sudden throttle goose than will ever be withdrawn on the coast down... With the small CFM draw of the PCV, would probably take between 30 & 60 seconds to completely purge the engine of vapors(assuming no additional blowby was being created)... Theoretically if you're shutting down from operating at heavy load, would likely be better to let the engine idle for a minute to somewhat remove the extra vapor that was created during the severe operation...

If anyone is that anal, install a small vacuum pump that can run for a minute or so at shutdown to remove the vapors... I won't be using one...

 

[TallPaul]

Thought it would generate some interesting posts and truly I wondered about it.

 

[artificialist]

If you have a turbo engine, you would want to let it idle when you shut it off. If you shut the turbo engine off at high RPM, the loss of oil flow plus the high speed of the turbo would result in turbo bearing wear.

Also, you would want to idle the engine for 1 or 2 minutes if the turbo gets very hot.

 

[mechtech2]

Tall Paul -
You don't quite have this one right.
At idle, engine vacuum is very high. This pulls whatever gasses past the PCV valve into the intake.
And, at idle, gasses are minimized - in no condition are there less blowby gasses.

Revving the engine before shutdown is a waste, annoying, and possibly harmful in the long run.

 

[rslifkin]

On a fuel injected engine, it shouldn't hurt anything, but it's probably not doing much of anything beneficial either. On a carb, you'll wash some extra fuel down the cylinder walls. For a rotary, it should be revved to around 4k and shut down before the revs drop. IIRC, it's to get more oil onto the apex seals for the next startup.

As far as the PCV valve thing, I know my Jeep has plenty of PCV flow at idle, as pulling the crankcase breather off the airbox and holding your finger to it produces a notable vacuum (unless you also disconnect the PCV line).

 

[lexus114]

Originally Posted By: Highline9
flood cyl walls with fuel not good. Carb does it more iI would think.



This is what I was taught over the years. ^^

 

[DavidZ28]

I knew a guy who did this back in '82 with his brand new '82 Z28 305 carb (LG4). He told my why but I forgot, I was only 15 back then. He traded the car in '85 with only 46K on it for a new 'vette. Flood the cyl walls with fuel, yep not good.

 

[TallPaul]

Originally Posted By: DavidZ28
... Flood the cyl walls with fuel, yep not good.

Keep those cylinder walls nice and clean anyway.

 

[TallPaul]

Originally Posted By: OVERKILL
Originally Posted By: TallPaul
When the engine idles and manifold vacuum is high, the plunger on the PCV valve is pulled to a nearly closed position so that vapor flow is light and does not disturb the smoothness of the engine at idle. As the throttle is opened, manifold vacuum drops and the spring pushes the valve open allowing more vapor to flow when the engine can better accept it.


Depends on the PCV valve design. Most (all?) of the old style design were wide open when vacuum was applied to them. I've seen a few new ones that operate similar to how you've described however.


According to the PCV tech service bulletin at this site they all operate nearly closed at idle.

 

[morris]

did the factory say to do that? some times the factory DOES know what to do.

 

[TallPaul]

Originally Posted By: morris
did the factory say to do that? some times the factory DOES know what to do.
No, I saw a kid do it in high school once and he didn't have a muffler on his car but it sounded really cool.

 

[OVERKILL]

Originally Posted By: TallPaul
Originally Posted By: OVERKILL
Originally Posted By: TallPaul
When the engine idles and manifold vacuum is high, the plunger on the PCV valve is pulled to a nearly closed position so that vapor flow is light and does not disturb the smoothness of the engine at idle. As the throttle is opened, manifold vacuum drops and the spring pushes the valve open allowing more vapor to flow when the engine can better accept it.


Depends on the PCV valve design. Most (all?) of the old style design were wide open when vacuum was applied to them. I've seen a few new ones that operate similar to how you've described however.


According to the PCV tech service bulletin at this site they all operate nearly closed at idle.


I'm thinking specifically of the Ford ones with the screen underneath them, which I think of as the "old style"

When you pulled them out of the grommet they'd be flowing like crazy.

 

[morris]

" but it sounded really cool. " i think if you do or dont will have little to with how long the engine lasts.

 

[TallPaul]

Originally Posted By: morris
" but it sounded really cool. " i think if you do or dont will have little to with how long the engine lasts.
Might just as well have fun, but actually I am more inclined to rev it in gear for double the fun.

 

[TallPaul]

Originally Posted By: OVERKILL
I'm thinking specifically of the Ford ones with the screen underneath them, which I think of as the "old style"

When you pulled them out of the grommet they'd be flowing like crazy.
Well there probably were a limited amount that worked differently. It's not that I know much about it, but for what I have read and then I have to trust they are telling it to me straight. Not always a good idea in all areas of information.

 

[Vikas]

James Bond does it on his old silver Aston Martin (in Skyfall)

 

[addyguy]

My dad used to do this all the time on our '78 Malibu 305, said it was to get one good shot of oil around the engine before shut-down.

I used to do it on my cars, but stopped long ago....

 

[TallPaul]

Guess is works for a Mazda rotary:

 

[65convertible]

I used to rev the engine before shutdown on my '65 that I still have. But after some studying and a couple of lectures about not doing it I quite. But I will admit it sounded pretty good at the time. But I have since learned that it is not a good thing to do. I now just simply turn it off.

 

[TallPaul]

I was looking through my owner's manual the other day for my 2001 Ford Ranger and it says that revving the engine before shutting it off may waste fuel"