Better to REV engine before shutting it off

[TallPaul]

Since the PCV valve is shut (allowing only a very minimal flow) at idle, then if you shut the engine off at idle, you will trap all the nasty stuff in your crankcase. So why not REV it up and shut the key off while the REVs are up and the PCV valve is actively purging the crankcase?

 

[spasm3]

Will you burn the extra fuel in the cyl when you shut it down?

 

[TallPaul]

Originally Posted By: spasm3
Will you burn the extra fuel in the cyl when you shut it down?
No, but the crankshaft will turn a few more times and probably blow it out the tailpipe anyway.

 

[Chris142]

your thinking backwards.when you step on the gas vacuum drops and you lose the pcv.

 

[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--i.e., the added vapor will not cause the engine to run rough.

Wikipedia says:
Quote:
At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. With a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow.

 

[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.

 

[Zaedock]

Most PCV's are wide open at WOT or engine off (i.e. no manifold vacuum) and at minimum at idle. The little knob you see at the bottom of the valve is not the plunger. It is typically a needle valve surrounded by a spring. If you take it off at idle and put it back on, you'll hear the "thunk" of the valve shutting.

The other style of PCV is the metered orifice (like many of my Jeeps).

 

[Zaedock]

Originally Posted By: TallPaul
Since the PCV valve is shut (allowing only a very minimal flow) at idle, then if you shut the engine off at idle, you will trap all the nasty stuff in your crankcase. So why not REV it up and shut the key off while the REVs are up and the PCV valve is actively purging the crankcase?

I don't think there would be any real benefit as it wouldn't be enough to purge the entire crankcase. Besides, you're still going to get a bit of cylinder washdown and condensation once the engine cools.

 

[Artem]

Originally Posted By: TallPaul
Since the PCV valve is shut (allowing only a very minimal flow) at idle, then if you shut the engine off at idle, you will trap all the nasty stuff in your crankcase. So why not REV it up and shut the key off while the REVs are up and the PCV valve is actively purging the crankcase?

Honestly... Who cares? My 98 Camry still has the 15 year old OEM PCV valve @ 225,000 miles. The engine is still spinning happily even though the rest of the car is falling apart into pieces. I'll end up selling or trashing the car for a gazillion other reasons other then the engine going bad due to the PCV valve. Don't hurt me...

By the way, i plan to replace the PCV valve soon when i go back there (under the intake manifold) in order to reach the spark plugs which now have 120,000 miles on them.

 

[crazyoildude]

my grandfather used to do that....lol
that was in the 60's today just shut it off

 

[Highline9]

flood cyl walls with fuel not good. Carb does it more iI would think.

 

[bigjl]

Widely regarded as detrimental to the engine.

 

[NHGUY]

I'm thinking people claimed this worked because you were leaving some unburned fuel in the cylinder so when you started it next time it would fire over quicker.Not even thinking that the revolutions the engine does to eventually stop rotating will pump out any raw fuel anyway.Unless the engine is that tight and shuts off immediately when the key is turned.

 

[LineArrayNut]

When I was running quadrabogs on my buicks this was accepted practice. As, above, it was to aid starting. Now, what urinary events me off, is to keep all the tree huggers happy, the hyundai ECU revs the motor on cold start to "hasten the cat-con lightoff"! Who gives a scatological reference about the engine wear, let's all stomp the gas and keep the snail darter alive! lol word filter censors don't like biological functions...

 

[Kestas]

During the days of carburators, I was taught the exact opposite -- let the engine idle for a few moments before shutoff to burn off the fuel wash on the intake.

 

[bigjl]

From what I remember it was thought to get oil round the engine prior to shut off. As after you cut the ignition the engine will turn with no fuel being pumped but that oil pump being mechanical and linked to the engine revs would continue to pump oil for a second or so after shut off.

The practice as I remember it was to rev the engine then shut off.

 

[eljefino]

Originally Posted By: bigjl
Widely regarded as detrimental to the engine.


But a good show!

 

[Pop_Rivit]

Originally Posted By: TallPaul
Since the PCV valve is shut (allowing only a very minimal flow) at idle, then if you shut the engine off at idle, you will trap all the nasty stuff in your crankcase. So why not REV it up and shut the key off while the REVs are up and the PCV valve is actively purging the crankcase?

I'd be interested in seeing the research you've done to back up your "nasty stuff" assumptions. Can you point me to the research that concludes it's better to floor your engine and shut it off rather than allow it to idle for a moment and shut it off? Do you have a link to the research you (or someone else) has done? I'd also be interested in knowing how long the "floor it" part has to last before the engine can actually be turned off. 10 seconds? A minute? And how much longer can an engine last if subjected to your "floor it" procedure? The research should be able to at least provide answers so it can be discussed.

Please provide the actual data (versus the current conjecture) so we can all be just as enlightened.

I've seen the inside of more than a couple engines, all of which (to my knowledge) were never subjected to your "floor it and shut it off" procedure, and never seen the "nasty stuff" to which you refer, so I'm very interested in seeing your research that proves the contrary.

 

[TallPaul]

Originally Posted By: Pop_Rivit

I'd be interested in seeing the research you've done to back up your "nasty stuff" assumptions. Can you point me to the research that concludes it's better to floor your engine and shut it off rather than allow it to idle for a moment and shut it off? Do you have a link to the research you (or someone else) has done? I'd also be interested in knowing how long the "floor it" part has to last before the engine can actually be turned off. 10 seconds? A minute? And how much longer can an engine last if subjected to your "floor it" procedure? The research should be able to at least provide answers so it can be discussed.

Please provide the actual data (versus the current conjecture) so we can all be just as enlightened.

I've seen the inside of more than a couple engines, all of which (to my knowledge) were never subjected to your "floor it and shut it off" procedure, and never seen the "nasty stuff" to which you refer, so I'm very interested in seeing your research that proves the contrary.


I didn't actually say "floor it," rather I said REV it up and that is subject to evaluation. What I am after is to see if maybe that would be a better procedure and if so, at what RPM, perhaps 2000, I don't know.

As for the "nasty stuff" I thought it was pretty much common knowledge that engine blow by happens and is not good, so I gave it a cutesy label. Not nasty stuff coating the insides of the engine, but nasty stuff that soaks into the oil and degrades it.

 

[HM12460]

LOL, I don't know where you guys come up with some of this stuff! Wouldn't you rather be chasing the Mrs. around the living room?