PCV valve when change it?

Hi,
We have a Prelude 3° generation 2.0i 4 ws 1991 75 000 kilometers
(young lady)
I have heard quite a lot of time on this NG user speaking of PCV valve
Positive Crankcase Ventilation (Many Thanks to Clarence ;-))
In France nobody speak of it on NG
it is called RGC...(recyclage des gaz de carter)

* Is it useful to change it?
* Where is it exactly on a Prelude?
* Does it improve mileage or some other issue?

Many Thanks for your help.
Philippe

for responding please drop -discret- in my adress

 

Change it.

The PCV valve routes fumes from the engine crankcase to the engine air intake.
The fumes have perfectly good fuel in them, so the effect is to (1) improve fuel
mileage; and (2) reduce bad emissions going out the tail pipe.

A 1990 Prelude's PCV valve location and description:

http://www.honda.co.uk/owner/PreludeManual/62sf100/6-208.pdf

If your 1991 Prelude is the same, then it looks like it's pretty easy to get to.

I replaced my 1991 Civic's for the first time last spring at about 140k miles.
It was full of black, carbon, waxy buildup. My miles per gallon went up
significantly, like 15%.

I checked the new PCV valve for the second time in a year yesterday, as my
mileage seems a little low lately. It again had a lot of the carbon waxy buildup
in it. I'm now thinking of cleaning it every three months and replacing it every
60k miles or four years, whichever comes first.

I'm thinking of replacing the PCV valve hose, too. It's full of junk though it
doesn't appear clogged. I'm wondering if I can buy a length of high temp. hose
at Home Depot that will work just fine. Anyone?

Also, when I do the hose pinch test on my PCV valve, the clicking, if it's
there, is barely audible. Then again when I hold the PCV valve in my hand and
shake it, it's not very loud, either.

 

autozone, kragen, or pep boys would carry comething more suitable.it has to
be fuel and oil resistant as well as heat resistant.

its a simple valve... spray it out with carb cleaner, shake, spray again.

 

Ok i have take it off for check..
Yes it was the same.
The highest part brake down in the inspection :-(( so I WILL change it for
sure....
By the way, it doesn't seem to be dirty... but the little ball seems to be
steined (? good word ? rust? excuse me i'm French)...
i will change for OEM on Monday..
I am very amazed to see how much in Us people seems to be very aware and
concerned by the inspection and the replacement of this valve... -as often
as spark plug-
In Europe it seems to be ignored.

PS whe had a 1991 Civic hatchback (it was a great fun and a little jewel...)

Many Thanks Caroline (nice name my daughter has the same ))

 

The PCV valve has nothing to do with tailpipe emissions. It is there to
prevent crankcase fumes from being emitted to the atmosphere directly. Cars
up to about 1960/65 had crankcase breathers/draft tubes that allowed
crankcase vapors to be dumped straight to the atmosphere.

The volume of air flow through the PCV valve is tiny compared to the volume
of air flowing through the intake manifold, so any mileage increase due to
the amount of fuel in blowby gases is similarly tiny.

Unless your engine is severely worn and has very low compression, I cannot
see that the PCV valve would have any measurable effect on gas mileage.

This is how Honda can put the PCV valve directly into the intake runner for
a single cylinder and not affect the engine's behavior.

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Point taken.

It's an emissions control device, but not the emissions going out the tailpipe.

I am doubtful your explanation is complete.

I think my theory of why a malfunctioning PCV valve reduces fuel efficiency is
wrong, but the outcome (better fuel mileage) is still correct.

The following sites (among many others) back up my claim that a properly
functioning PCV valve is necessary to ensure maximum fuel efficiency:

http://www.is-it-a-lemon.com/fuel/pcv_valve.htm

http://www.courier-tribune.com/guide/gasmileage.html

http://www.courier-tribune.com/guide/gasmileage.html

So about the theory behind this--
I am wondering if, instead of the crankcase fumes providing significant fuel to
the intake air, a fouled PCV valve actually causes combustion to be impeded, so
this is what causes the lower fuel mileage. E.g. as the sites above suggest, a
plugged PCV valve increases crankcase pressure, with a corresponding deleterious
effect on combustion. Maintenance of proper crankcase pressure is what ensures
optimal fuel efficiency?

The increase in fuel mileage was remarkable when I replaced my completely
clogged and fouled PCV valve. The car went from getting 390-420 miles per tank
of gas to consistently over 440 miles per tank of gas. This on a 12 year old
car. The car made it without difficulty to 460+ miles one day. I did nothing
else to the car--no new plugs; air filter; nada. Added nothing special to the
fuel. Continued to buy the same cheap gas I've always bought. So my personal
experience backs up what these (and many other web sites) say.

 

Stained and rusted?

Sometimes in English we say, "fouled." But "stained and rusted" makes the point,
too.

Either way, the little ball likely isn't going to work properly if it has crud
on it, as I'm sure you know.

No problem. Do see the discussion Tegger has started. My theory about why a
fouled PCV valve will worsen fuel mileage is being discussed there, though I
with other authorities stand by the contention that a fouled PCV valve will
lower efficiency.

Yesterday I thoroughly cleaned the PCV valve hose, too. (Mike, I am doubtful
Autozone has hose any better than Home Depot, but I'll ask. I don't want to pay
the $13 (Majestic's online price) for the part when I can get hose for probably
under a buck at Home Depot. Call me cheap. Or I dunno; maybe the cleaning I did
of the old hose yesterday is perfectly fine.)

 

"Caroline" <> spake unto the masses in

What you are suggesting is roughly a 10% decrease in fuel mileage due to a
plugged PCV valve. I wonder about that. That's pretty substantial for
something that handles much less than 10% of the engine's total air
consumption.

There is no "excess pressure" unless the breather hose and PCV both get
plugged. If the PCV valve gets plugged, blowby gases (and lots of oil) get
pumped back through the breather hose and make a big mess in the intake
tract.

The crankcase is under some degree of positive and negative pressure at
various times during its operation. When the pressure is over ambient, air
flows through the PCV valve. When it's under ambient, it flows through the
breather pipe. A plugged PCV valve means that air flows through the
breather pipe at all times, back and forth.

I'm going to have to try to verify these claims of increased mileage. Maybe
it's so, but I can't see it.



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With only a plugged PCV valve, my take is that the pressure in the crankcase and
breather hose is most certainly going to be higher than if the PCV valve were
properly operating. Why? Because there are fewer paths for the crankcase fumes
to take to escape.

Whether with a plugged PCV valve it's excessive or not is the issue. I see
warnings on the net that a plugged PCV valve by itself may cause oil leaks, for
example.

Chilton's description talks more in terms of the pressure in the intake
manifold. At low throttle, there tends to be a higher vacuum in the intake than
at high throttle. At low throttle, crankcase fume flow is through the open PCV
valve to the air intake.

At high throttle, the PCV valve by design shuts. At high throttle, crankcase
fume flow is through the breather hose.

The system is designed for two avenues, if you will, of crankcase fume escape.
If one is gone, then the other will have more pressure in it than is usual. As
I say above.

I have three proposals for why the fuel efficiency is lower with a plugged PCV
valve:

1. The pistons have to work against the higher pressure in the crankcase.

2. Because of the higher crankcase pressure, the engine oil becomes more
saturated more quickly with contaminants. As a result, lubrication is worse.

3. Fuel in the crankcase fumes doesn't flow as readily into the air intake.


Site http://www.et-performance.com/tech_info.html has a little on 1.

Post 2 at site http://www.automotiveforums.com/t193624.html talks about 2.

I am still pondering your claim that the crankcase fumes that pass through the
PCV valve go to *only one* cylinder's air intake. I'm not sure this is so on all
cars. The descriptions I've seen talk pretty consistently about the fumes going
to "the intake manifold," not one cylinder's intake runner. I'd be more inclined
to think the vacuum that causes the PCV valve to open is due to direct
connection to the manifold and not any single runner. For smooth operation. I
mean, one wants the crankcase pressure to depend on not a single cylinder but
all cylinders.

On my 91 Civic the PCV valve seems to T-off...

 

Harley Davidson engines are oddities. BOTH pistons go down at once.
http://auto.howstuffworks.com/question325.htm
This would lead to pressurization issues that do NOT exist in Honda fours
or sixes.

Don't forget that in a four, when two pistons are going down, two are also
going UP. I submit that piston movement has no effect on crankcase pressure
in engines that have more than three cylinders.

Clearly, the poster in your link fitted a PCV valve that was inadequate for
the blowby generated by his 350. There is no indication what sort of shape
his 350 was in, which is critically important.

Ever put your hand over the carb air horn in a 350, even at idle? Go do
that then do the same thing to a 1.6L four. NO comparison. The 350 will try
to suck your fingers off. The amount of blowby generated by a 5.7L is
almost FOUR TIMES that of the 1.6L four. You'd need a PCV valve with four
times the flow rate. Bet this guy never even thought of that.

I once had a 1975 Corolla with tons of blowby. The PCV valve was
overwhelmed, and oil used to fill the air cleaner all the time. It was
being pumped in through the breather. My teenage solution--a quarter
covering the breather screen--led to oil leakage through the oil seals.

It isn't. But it IS on the B18 engines (such as my '91 Integra's) and some
other Hondas I'm sure. Honda could not do this if there were a substantial
difference in fuel content in crankcase gases.

A large difference in fuel content would result in a rough-running engine,
as one cylinder would have a very different mixture in it than the other
three.





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I just learned something: PCV valves have a flow rate from 5 cubic feet per
minute to 12 cfm.

A 1600cc four at 3,000 rpm will process 42 cfm at wide-open-throttle.
Assuming that 10% of that leaks off as blowby (and that's quite high),
that's 4 cfm at wide open throttle, and far less than that at smaller
throttle openings.

Since very little of an engine's operating time is spent at WOT, and that
lots of it is spent at closed-throttle or close to it, the PCV valve is
processing very little air. Sometimes none at all.

I'm guessing that the PCV valve's portion of air is around two or three
percent of the total air consumed by the engine.

Another point: Yes, some blowby gases contain fuel, but that's only from
the compression stroke. There are also blowby gases from the EXHAUST stroke
(the same thing that turns your oil black). Remember how an EGR valve
works? I submit that any fuel economy gains from the portion of blowby that
contain fuel are offset 100% by the lack of oxygen in the gases from the
exhaust stroke.

I think your claims of 10% mileage increase are erroneous and based on
faulty methodology.

You're claiming 440 miles on a "tank" as opposed to 390 before changing the
PCV valve. Did you run the car until the engine died each time, or did you
guess based on gas gauge position?

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You seem to be assuming that the percent that PCV flow is to overall air intake
flow should equal something like the % increase in fuel mileage. This seems to
me to be quite a leap.

I think the starting point of this analysis should be how much fuel is in blowby
gases. Compare this to the fuel injector rate.

But that's just a starting point.

I don't think we're on the same page at all.

When I hear the word "blowby," I think of gases that get past the rings during
the expansion stroke. Hence what gets into the crankcase is mostly exhaust
gases.

Why are you talking about the lack of oxygen during the exhaust stroke? The
point is any unburned hydrocarbons in vapor form in the crankcase go (via a
correctly operating PCV valve) into the air intake manifold where there's plenty
of oxygen and then ultimately to the cylinders for combustion.

You can google for "blowby" and you'll see the emphasis is by far on combustion
gases.

I record the gallons at each fill-up and the miles driven since the last
fill-up. I top off the tank every time. I generally drive to within a gallon of
completely empty. This was not a one-time deal. From late winter 2003 through
about late Fall 2003 I saw this much improved mileage. The weather cooled and it
wasn't quite as good, but still better than what I'd been seeing for years. What
I'd been seeing for years was consistently under 40 MPG. It suddenly leapt up to
42 MPG and sometimes higher.

It turns out I did an oil change the day after I changed out the PCV valve late
last winter. That might be related, though I'm thinking not by much. I have
always changed my oil regularly and never saw a leap in fuel mileage like this.
Still, starting with a brand new PCV valve and fresh oil gives the whole
operation a fresh start, engineering-wise.

I think the most significant factor is likely the fact that the old PCV valve
was chock full of waxy build-up. It was plugged, in my estimation.

Anyway, no problem if you're unconvinced, though if I were you, I'd at least be
troubled at the number of automotive web sites that talk about malfunctioning
PCV valves reducing fuel efficiency.

Maybe you should plug your PCV valve port for a tank of gas and report back on
your mileage. ;-)

I would welcome an authoritative analysis of why a correctly operating PCV valve
increases fuel efficiency. Otherwise, I think the three causes I gave are quite
plausible.

 

Blowby is ANYTHING that gets past the rings.

And I got the strokes mixed up. For "exhaust", read POWER stroke. That's
when exhaust gas is blown into the crankcase, and why your oil turns black.

Because that lack of oxygen will cause an EGR-like effect, impeding
combustion and thus mileage. The oxygen got burned up during combustion,
remember?

There is NOT that much fuel in there. Again, if there were, Honda would be
unable to put the PCV in the runner of a single cylinder, as it would have
the same effect as compression markedly different than the other cylinders.

Then the emphasis is wrong. Remember, on the compression stroke, youve got
only the blowby generated by the compression itself. On the power stroke,
youv'e got the pressure generated by combustion, MANY times higher.

Then you must have written record. Please state the exact amount of fuel
consumed during the times you write about, and the exact mileage covered
during that time.

I can give you very precise figures on my car, day-by-day if you wish.

I do not think so.


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Why? You can't burn fuel that is not there. If it IS there, it is subject
to combustion, assuming correct mixture.

If you introduce 10% more air/fuel mixture to the engine, and that mixture
did NOT come from the injectors, then it has to do something, right? How
can 10% more air/fuel have 2% effect?

However...another fact that came to light last night is this: In any car
with a properly functioning engine management system, the oxygen sensor
would detect any excess/inadequate fuel and alter injector duration
accordingly. So whether or not some of the fuel ended up in the crankcase
or not, the ECU will compensate.

Therefore, whether you have fuel entering through the crankcase or through
the injectors, the oxygen sensor will detect all of it and adjust the
mixture so it's correct again. End result: Fuel mileage remains exactly as
it was before.

Yes, with NO FUEL IN IT. You're now contradicting yourself.


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Hey, I just realized something.

Under normal conditions, crankcase gases are routed through the PCV valve
and into the engine.

If the PCV valve is plugged, the crankcase vapors are routed through the
breather tube in the intake tract, and...into the engine!

Same place regardless if the PCV is working or not!

The PCV will not affect mileage whether working or not.


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I saved the gas receipt, wrote the miles I'd put on the car since the last
fill-up, took it home and punched the numbers into my calculator. I am going
from memory. It was striking to get 460 miles from a tank of gas (with room to
spare). It was striking to go easily over 440 miles per tank. That's not
something I'd forget when for years I'd rarely get over 400 miles per tank.

I am not requiring you to believe me, Tegger.

I don't know what motivation you think there is to lie about this, particularly
when so many authorities say a plugged or fouled PCV valve will reduce fuel
efficiency.

When you plug your PCV port, I'd love to have the before and after numbers. ;-)

 

Exhaust gases have fuel in them, Tegger. Don't think in terms of strictly
gasoline being in the exhaust gases. The typical description of what goes from
the crankcase through the PCV valve is something like "unburned hydrocarbons"
that subsequently will combust (in full or in part).

Google.

I'd treat your other points but I don't think it's worth my while, especially
with a mistake like yours above.

 

Ya and I guess the oil will not foul any more quickly with a plugged PCV valve.

Not.

IMO the breather hose is not per se a substitute for the PCV valve. For one
thing, the PCV valve regulates, and flow is one-directional; how far open it is
may vary. The breather hose is fully open constantly.

Why don't you plug your PCV valve line and get back to the group?

 

Let me be a simple and clear as I can be:

1) The amount of fuel/fuel byproducts in exhaust gases is infinitesimally
tiny compared to that dispensed by the injector's pulse.

2) The amount of gas passed by the PCV valve even at full flow is a tiny
fraction of the engine's air consumption.

If your engine is depositing such enormous amounts of fuel into the
crankcase that changing your PCV valve results in a 10% increase in
mileage, then you have some serious issues with your engine.

You have some or all of the following:
severe bore wear, excessive blowby and poor compression
sludge
leaky injectors
breather hose blocked or vented to the atmosphere



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A correction of my one mistake here:

The hose routing fooled me when I looked at it the other day. The PCV valve
in my B18A1 engine does in fact dump into the the intake plenum, not into
one of the runners. The valve itself is BETWEEN the #3 and #4 cylinder
intake runners, but no gas transfer takes place there.


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