PCV valve when change it?

Or the two other explanations I gave explain the significant improvement. Or
there's some other explanation waiting to be had.

Sorry, but all the web sites that talk about this and my personal experience to
me represent a great deal of evidence that a plugged PCV valve will reduce fuel
efficiency.

 

This is true. In fact this excess pressure is what I blame for the rubber
plug at the exhaust camshaft end being pushed out on Integra DOHC engines.
It almost always happens in Winter when just one shortish run will leave
moisture in the valve which freezes... and allows excessive pressure build
up in the engine cavity on the next startup.

Hmmm, I've seen several different discussions on the operation of a PCV
valve. Some have it closed under heavy load and others at max
flow-through!! It may vary by actual design and mfr but this Toyota
document seems quite definitive about things:
http://www.autoshop101.com/forms/h63.pdf.

Certainly drivability is affected and the valve can fail in more than one
way: stuck in any position, i.e. not just closed. Under normal operation,
it basically works like a variable sized air leak past the throttle valve
into the manifold, which is taken into account in the ECU programming and
whatever its max flow characteristic should be, if it's not right you'll be
running "rich" most of the time. I think this may be the simplest
explanation of effect on mpg.

All the Hondas I've seen have it on the intake manifold plenum but even if
it was on only the one runner, it is still really part of the plenum and
would be affected by the other runners.

Rgds, George Macdonald

"Just because they're paranoid doesn't mean you're not psychotic" - Who, me??

 

This has been dicsussed before and I don't recall all the details but ISTR
that with a V8 which has eight evenly spaced combustions - actually kinda
rare - you will have approx. constant crankcase pressure. Even with a
4-cyl, the variable speed of the pistons in the cylinders over their
travel, due to crankpin offset, means that you do get some pulsing of
crankcase pressure. IOW the two that are going up while the others come
down do not offset each other perfectly.

Uhh - look again. The B18s I have seen all have the hose fitted to the air
intake plenum. The valve itself is stuck between two runners, up to
1994(?), but that's just a mount point.

Now for a point of controversy: why did Honda have such an elaborate
arrangement all those years, with a breather box stuck on the back of the
engine block (so damned difficult to get to and work on) and elaborate hose
routing?... only to latterly, abandon it all for a simple valve plugged
right into the cylinder head cover like everybody?? This must have been a
savings of ~$20. cost per car. It makes no sense!

Rgds, George Macdonald

"Just because they're paranoid doesn't mean you're not psychotic" - Who, me??

 

how would it be able to tell which cylinder to "lean out", if its sniffing a
mixture of all 4?

 

I concur that the open/shut operation (and of course location within the system)
appears to vary quite a lot depending on the make and model. This is based on my
reading on the web this weekend.

My remarks above are only for a 1991 Civic.

I am pondering especially your comment about the ECU and how it would respond
(or not respond) to a failed PCV valve. I don't have any objection to it but I
haven't put a lot of time into studying how specifically the ECU controls the
engine, either.

I read your other remarks. Thanks for sharing your thoughts.

 

Another take on that article, from a retired tech in the Toyota group:

"Ok.... let's look at the text:

'A plugged PCV valve will prevent the normal flow of crankcase vapors
into the intake tract and *can* result in a richer than normal
air/fuel mixture.'

This is true with a carbureted engine WITHOUT closed loop feedback
fuel mixture control. In this case, a clogged PCV is no longer a
metered air leak so the mixture would richen. However, if this same
example had a highly fuel contaminated oil supply, pinching off the
PCV could also LEAN the fuel mixture. All a matter of degree.

Where a feedback fuel system is in place (be it feedback carburetion
or feedback fuel injection), the ECU will maintain the optimum
air/fuel ratio regardless of the PCV being clogged, normal, or even
removed (an open vacuum leak)."



Which is what *I* thought. If the ECU started seeing incorrect voltage from
the O2 sensor, it would determine that the mixture was incorrect, and
lean/richen it, compensating for excess HCs in the crankcase.

My assertion: A plugged PCV valve wil NOT affect mileage in a controlled
engine. If Caroline has a mileage problem ,he's got deeper issues with his
motor.

Have already posted a correction of that mistake.

--
TeGGeR®

The Unofficial Honda FAQ
http://www3.telus.net/public/johnings/faq.html

How to find anything on the Internet or in Usenet Groups:
www.google.com
www.groups.google.com

 

But the purpose and ultimate function is IDENTICAL regardless of the method
chosen to make the PCV valve work: A PCV system ONLY makes certain that
excess crankcase vapors are sucked back into the intake manifold to keep
them out of the atmosphere.

The ECU will compensate. That is the purpose of the oxygen sensor. Excess
oxygen in the exhaust will lower the voltage produced by the sensor, and
insufficient oxygen will increase it, regardless of where the fuel or air
came from. ALL the exhaust passes in front of the same O2 sensor.

The ECU will lean or richen the mixture to make sure that it sees the
correct voltage from the sensor. That is called "feedback loop". The engine
will only run in feedback once it is fully warm, or once the ECU receives
appropriate readings from the various sensors that indicate that the engine
is fully warm.

Changing the PCV valve would have no effect on mileage on your Civic, since
you have a closed feedback system.

You have something else wrong with your car or are not measuring your
mileage correctly.

Does your state have emissions testing or facilities where you can get an
exhaust gas analysis? I'll bet you'll score high on CO and HCs even now.



--
TeGGeR®

The Unofficial Honda FAQ
http://www3.telus.net/public/johnings/faq.html

How to find anything on the Internet or in Usenet Groups:
www.google.com
www.groups.google.com

 

As you know, there are several inputs to the ECU. For this discussion, I
wouldn't focus on just the O2 sensor's input. I'd be considering the air flow
signal and the throttle position signal, right now, too.

Of course it's a closed loop system. What I think George is saying is that the
normal signals from either the air flow meter, the throttle position switch, or
both "assume" the PCV valve line flow is normal.

In support of this, from
http://www.thecarconnection.com/index.asp?n=158,277&sid=277&article=4681 :
"A defective or clogged PCV valve can make matters worse, sending more unburned
gasoline fumes into the cylinders, displacing the flow of fresh oxygen past the
valve and again causing the computer to run the engine rich."

http://www.interro.com/techgas.html also states that a malfunctioning PCV system
may cause the engine to run rich.

I agree the O2 sensor responds to rich combustion, but right now I wouldn't say
the ECU can take the O2 sensor signal and necessarily fully compensate.

I note the discussion of monitoring PCV valve line function starting in 2004.
Site http://www.asashop.org/autoinc/oct2000/obd_ii.htm talks about this as a
post-OBD II development. OBD II is "stricly emissions oriented." I expect any
OBD III requirements for monitoring the PCV valve line are also geared towards
emissions concerns. This seems to back up further my point that a malfunctioning
PCV valve may affect fuel mileage significantly.

This is your opinion, never having seen my car, disbelieving I am careful in my
mileage calculations, and not troubling yourself (it seems) to note all the
sites that say a malfunctioning PCV valve may cause a loss in fuel efficiency.
Google for {"pcv valve" fuel efficiency}and see. Or keep blowing off this point.
Other readers will check.

It's not a big deal to note when one's trip odometer goes over 400 miles on a
tank of gas and when it doesn't, Tegger. I reset the trip odometer at every
fill-up, and as I wrote, I almost always drive until the fuel gage needle is
touching or just about touching the lowest mark. I monitor my fuel mileage
precisely to tell me if anything might be amiss with my car's engine, so I am
careful about it.

I mentioned the odometer going over 460 once after changing the PCV valve. It
actually happened twice, IIRC (I remember the specific trips). That's on no more
than about 11 gallons of gas, and my recollection is it was closer to 10.7.

You proposed that replacing a malfunctioning PCV valve would compensate for the
following:

My mileage was pretty consistent for years before replacing the PCV valve. I
have always changed my oil religiously and use quality stuff--Pennzoil only. So
there seems to me to be no justification for severe bore wear or worn rings.
Seems like I'd have other symptoms for leaky injectors. I haven't checked the
breather hose for blockage recently but next time I have the valve cover off,
I'll take a look.

Also, you are ignoring the fact that getting 40+ mpg out of a 1991 Civic is
excellent mileage. This suggests to me that the car is operating well (and
arguably really well) for its age and miles (152k miles). According to
www.fueleconomy.gov, my Civic (manual, 5-speed, 1.5 liter) is supposed to get
only 31/35 city/highway.

My state does not require emissions testing.

I had an emissions test done 1.3 years ago in another state. I did not save the
record, but nothing exceeded the regulations. I don't think any of the counts
were on the high side, either, but that's just a crude recollection. I had the
old, probably plugged, PCV valve in place at the time the test was done.

Aside: You referred to me as a "he" in one of your posts. Maybe this was a typo.
For the record, I am a woman, as the screenname implies.

 

And a damn smart one, too! As a fellow chick, I'm impressed with your
knowledge. Thanks for the discussion on the subject. Sorry, Tegger, but
she's more compelling. JMO.

Now, how much does a PCV valve cost and how much should it run for
install?
My son is complaining of poor mileage and performance. If it's cheap
enough, it might be worth replacing to see if we get your results.

 

The ECU still measures all other parameters, like mass air flow, in the
case of Honda with throttle position sensor, MAP sensor and IAT sensor.
The ECU needs a reference point to start from before applying O2 sensor
corrections.... otherwise unintentional minor air leaks in the induction
system would have no effect either.

If it can affect "drivability" it can certainly affect mpg IMO.

Rgds, George Macdonald

"Just because they're paranoid doesn't mean you're not psychotic" - Who, me??

 

Thank you, though for the record (and because I try to be a lady, dammit ;-) ) I
will note that Tegger's posts and those of a few other regulars here are the
ones I *usually* ;-) give the most credibility.

It should cost you no more than about $25 and perhaps as little as $13 for an
OEM (original equipment manufacture) part purchased online. For non-OEM PCV
valves, you might try Autozone and pay only a few bucks. I'm not sure how
reliable the non-OEM PCV valves are. But it's certainly a simple part and what I
saw at Autozone recently looked no different from the one I bought from the
dealer last year for $19. Of course, looks can be deceiving...

E.g. http://www.hondaautomotiveparts.com/auto/jsp/mws/catdisplay.jsp . Put in
your car's year, make, etc. then select "Breather Chamber."

I'd guess install will run $15 to $50. A shop should be very willing to give an
estimate of this. Definitely under an hour and often under five minutes of
labor. As you may know, shops tend to have minimum charges. You might be able to
talk a shop into changing the PCV valve out at the same time as an oil change
and so save a little money in labor.

I trust you have seen the little test one can do with a pair of pliers on most
(all?) PCV valve systems? I finally heard the little click the other day on my
car's PCV valve when I did the test. Earlier I hadn't held the hose pinched long
enough for the valve to operate and so didn't get the click confirming correct
operation.

Give the year and model of your Honda and I can send you to a web site that
might help. (Or if you're a regular here, you might have already seen it.) The
PCV can be painful to replace on some models and a piece of cake on others. If
it's the piece of cake version, you might want to give it a whirl by yourself or
with your son.

Note: My 91 Civic's manual calls for inspection (read: clean thoroughly with
WD-40, or replace) of the PCV valve every 60k miles/4 years. So think in those
terms as to whether your son's car's PCV valve should be replaced.

 

I've got 175,000km on my Civic and the PCV valve is original (never replaced
or maintained). My car is getting amazing mileage. Even better then past
years. I think a contributing factor to the better mileage is my recent
switch to regular gas over premium. Regardless of gas type, the PCV hasn't
effected my mileage.

Pars
98 Civic Hatch

 

I agree with you, Caroline. But this one discussion about PCVs had me in
your corner.

Thanks. I thought I 'membered it was a cheapy.

Definitely a doable cost. He's got a new buddy who knows Honda Civics
(99 EX coupe). He's set to look at his car tonight to see if he can
determine if there's more of a problem than the fact it's a four banger
car with a 17 year old driving it.

 

Have you also considered other cheapies that can affect mileage and performance?
Like replacing the spark plugs (~$10), distributor cap (~$17), distributor rotor
(~$5), and air filter (~$10)? Also to be very thorough, possibly replace the
ignition wires (~$50) and fuel filter (~$20)?

All dollar figures above are for parts only. For labor: These are all DIY items
if one is a little handy. Maybe this new buddy can help the first time? Careful
with the distributor cap. It must be put on straight and carefully or things in
it will break. Otherwise, the labor shouldn't be too expensive at a shop. Couple
hours of labor, tops.

Good luck!

 

i do agree that gas is a VERY important issue on mileage, in France (where
the price of gas is scaring 4,77 euros per gallon... -+/- same in $...)
We have choice for unleaded 95 or 98 the 2° is a little more expensive +
3%... it is really more efficient and have a better millage...
29.94 and 26.97 for the 95 one (calculated on 12 000 milles) ouah hard to
converse milles kilometers gallons liters...here we speak in " liters per
100 kilometers"...
Car: Opel 2 litres injection.
So the choice is clearly obvious.

Philippe

 

Caroline how lucky you are i paid yesterday 40.40 euros for OEM (+/- same in
$) ....

expensive..
i should buy on line in USA ;-))

What is the best online dealer for you?

 

Whoa, I see this is over about $45 U.S.

At least you have socialized health care?

I've only ordered parts online maybe three-five times. The last time was several
years ago. All times I was pleased, but that's a very small sample.

I think I used the Majestic online company once and was pleased with the cost,
delivery, and service. I do go to the Majestic site often for their great
drawings and quick price checks (to see if my local OEM Honda parts shop is
charging a fair price). See

http://www.hondaautomotiveparts.com/auto/jsp/mws/catdisplay.jsp (this is
Majestic's site)

Consider also:

http://www.slhonda.com/ (same drawings as Majestic)

http://www.manhonda.com/ap/

http://www.honda-auto-parts-wholesale.com/

Lots of other sites are on the net, but these are the ones I have bookmarked for
price comparisons.

I don't know if any of these companies will ship overseas.

Someone else here maybe will chime in with more recent experience?

 

Lol...
it should be also afficent for half price..
Aniway... my little support to Kyoto' agreement ;-))

ok
Thanks
Philippe

for responding please drop -discret- in my adress

 

I agree, a faulty PCV valve can probably affect mpg.

In my 91 CRX, the PCV is one of three air paths around the throttle plate
(from filtered ambient air to intake plenum):
1. idle adjust screw and passage
2. IACV idle air control valve
3. the PCV valve

If the PCV valve is stuck closed, too little air at idle.
If the PCV valve is stuck way open, too much air at idle.
(If you assume the PCV is always, but varying, metering some air.)
Thus it might affect mpg by altering the idle settings.

The ECU MIGHT compensate by controlling the IACV.
I doubt it is smart enough, but I lack definitive knowledge of ECU program.

Part of setting idle speed is disconnecting the IACV
and adjusting the idle speed screw to a certain RPM (500 on my car.)
Thus the idle setting screw must establish some baseline flow
around the throttle plate.
A faulty PCV changes that baseline air volume.
I would guess that the ECU program assumes a certain baseline,
and doesn't have a feedback loop on idle speed,
but just sets IACV (thus the idle speed)
according to some formula based mostly on the load,
that is whether the alternator and fans are running.

On the other hand, Caroline, you may have a statistical problem.
You shouldn't cite one tank of gas to make your case,
since the variability among tanks of gas is too great.
I keep records for my car too.
There are many factors affecting the mpg for one tank.
For example, how full the tank is filled.
If you cite the improved mpg over ten tankfuls,
maybe more readers would believe it.

 

Yes, this is what I'm thinking.

Aside: Do you have an L-shaped, plastic PCV valve or a metal, straight PCV valve
or something else? (1.6 liter enginer = probably L-shaped. 1.5 liter engine =
probably the second, from Chilton's) How often do you inspect your PCV valve?
Ever see any blockage in it?

I'd say this depends.

From my reading, it seems that nailing down under *exactly which conditions* a
car's ECU is in closed loop (and so has feedback, principally or exclusively
from the O2 sensor) and when it is in open loop is no easy trick. I suspect
there's a fair amount of variation from one car to another, and that this is
proprietary information that only the design engineers or a very seasoned
technician knows.

Generally, closed loop should kick in at steady state conditions (car is warmed
up and running at constant speed, etc.).

Open loop is necessary when, for example, the car is warming up and one wants
the engine to run a little (fuel) rich.

My reading also suggests that certain (all?) ECU designs switch to open loop
anytime the car even accelerates. This makes sense to me from the standpoint
that the limitations of the physical controls (they're not quick enough!) do not
permit the O2 sensor feedback to be useful when accelerating. But I'm still
studying this point.

What I'm pondering is whether a malfunctioning PCV valve principally messes up
closed loop operation, open loop operation, or both.

It makes sense that it would definitely mess up open loop operation.

The site George gave suggests that, yes, in feedback control, a malfunctioning
PCV valve may also cause rich combustion.

Bootch, this wasn't one tank of gas. Late last winter I replaced my thoroughly
clogged PCV valve (dunno if this was a fail open or fail shut condition--could
be either, depending). The mileage shot up over 10% immediately. That is, I was
getting more than 440 miles (460 miles twice) easily, vs. the previous highs of
about 400 miles (420 miles once?). The mpg continued to stay up for
approximately 12k miles over 8 months, or over 25 tanks of gas.

I almost always drive until the gage is touching the low mark, putting in 10-11
gallons until the tank is full. I do not use premium gasoline. I reset the trip
odometer at each fillup.

My mileage started declining a few months ago. A week ago I cleared my PCV valve
and PCV hose of waxy buildup (not as bad as last year, but definitely notable).
I was getting just under 40 mpg. This past week the car got 42 mpg. I am
absolutely convinced that on 91 Civics, a clogged PCV valve will have a notable
effect on fuel mileage.

I checked my breather hose the other day. It's quite clear, having only a film
of carbon dust in it.

Lastly, four citations to support what you say, George says, and I say:

1. (from George's web site)
"Because PCV operation is factored into the proper operation of the feedback
control system, problems with the PCV system may disrupt the normal air / fuel
ratio balance. A plugged PCV valve will prevent the normal flow of crankcase
vapors into the intake tract and can result in a richer than normal air/fuel
mixture." http://www.autoshop101.com/forms/h63.pdf

2.
"A defective or clogged PCV valve can make matters worse, sending more unburned
gasoline fumes into the cylinders, displacing the flow of fresh oxygen past the
valve and again causing the computer to run the engine rich."
http://www.thecarconnection.com/index.asp?n=158,277&sid=277&article=4681 :

3.
http://www.interro.com/techgas.html
"High Carbon Monoxide (CO) readings usually indicate a fuel mixture richer than
ideal (rich mixture - air fuel ratio below 14.7). In general CO is an indicator
of combustion efficiency. The amount of CO in a vehicle's exhaust is directly
related to its air-fuel ratio. High CO levels result from inadequate O2 supply
needed for complete combustion. This is caused by a too rich mixture - too much
fuel or not enough air (AFR readings below the optimal 14.7, Lambda below 1.0).
Circumstances that can lead to high CO emissions: ... *Non-functioning PCV valve
system "

4.
"A defective or clogged PCV valve can allow excessive, unburned gasoline fumes
into the cylinders, which displace the flow of fresh air past the valve. This
can result in the computer commanding an over rich mixture wasting fuel and
decreasing performance."
http://www.womanmotorist.com/index.php/news/main/2522/event=view