Re: [Amc-list] Idle speed and PCV / Oil Filler Cap Questions ....

[Tom Jennings <tomj@xxxxxxx>]

Welcome back!


On Tue, 2007-07-24 at 09:04 -0400, greg_taylor@xxxxxxxxxxxxxx wrote:
> Hi all,
> I have the oil filler cap vent hooked directly to the PCV port on my 
> Holley carb (360-V8 in '89 GW) and a long tube with a vent filter 
> stuck into the stock PCV grommet at the back of the intake.  I've run the 
> Jeep like this for a long time (no PCV valve).  However, I 
> decided to squeeze off the hose from the oil fill cap to the PCV port on 
> the carb.  When I did this, the idle dropped by about 200-300 RPM 
> (it was idling too high).

The idea behind the PCV is to allow crankcase scavenging by sucking
crankcase air directly into the intake, but not be such a big "leak"
that it ruins the idle. The PCV has a weight and a spring, it needs to
be held vertical (all that I've seen). With high engine vacuum (idle)
the valve sucks mostly closed; at WOT the spring holds it open and then
it flows a lot of air. The weight should rattle freely. Sometimes they
buzz (open/close/open/close/...), that's usually normal.

Without looking at it, I would guess that if the current setup only
affects idle by 200 - 300 rpm on or off then at higher speeds it's not
doing a lot of scavenging. That's probably fine on a motor with good
rings, eg. there's not a lot of ring blowby to scavenge but as the motor
wears you might notice blowby gas coming out your vent filter.

> Does anyone know how critical the PCV valve is on the Jeep?

I don't know what the science of choosing PCV system "flow" is. My
hornet (258) is easy to tune with any PCV; my Rambler (232) was a PITA
to get the right valve. (Symptom was inability to set idle with 'idle
drop' method, there was no sharp edge). That's an IMPCO CA-125 propane
carb though.

> 
> Also, I'm considering that originally, the oil fill cap had "some vacuum" 
> to it, since it hooked to the air cleaner,

Actually that's just a source of clean air; it picks clean air out of
the big carb airbox. There's probably less than an inch of water of
vacuum there, and you're right, the PCV got manifold vac, so air goes IN
the filler neck and OUT the valve cover (six) manifold (eight).

I don't think it matters at all which way the air flows through the
crankcase; with the 8's it might even help to have it flow INTO the
manifold and out the filler. It's been a while since I've had a V8 but
there's a lot of moving oil and mist right under that manifold.


The PCV system does affect idle mixture and therefore setting even when
working right. A few hundred RPM sounds OK, maybe a bit much. 


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