Correction: I think the cloud chamber experiments start by creating a vacuum
in the chamber with some water present then when ambient air is admitted
(rapidly)
the cloud forms. At any rate, the rapid changes from near-ambient to
high-vacuum
and vice versa can cause the suspended carbon to be slammed throughout the
intake manifold.
bs
********************************************
Bob Spidell San Jose, CA bspidell@comcast.net
'67 Austin-Healey 3000 '56 Austin-Healey 100M
********************************************
----- Original Message -----
From: "Bob Spidell" <bspidell@comcast.net>
To: "Jim Juhas" <james.f.juhas@snet.net>
Cc: <shop-talk@autox.team.net>
Sent: Sunday, March 13, 2005 9:34 AM
Subject: Re: Carbon Buildup, was Re: question on C (neither L nor B)
>
> Jim,
>
>
> I have the same problem with my '96 Ranger ... have to clean around the
>throttle
> plates every 15K miles or so to keep the throttle from getting sticky.
>
> I'm not a physicist, but from what I know about airflow it's possible to
>explain the
> buildup around the throttle: when open, air is of course flowing rapidly past
>the
> throttle in a "forward" direction. Closing the throttle--especially when
>done quickly--
> rapidly creates a significant vacuum in the intake manifold. For a moment,
>the flow
> can actually reverse, blowing the crankcase gases and other contaminants
>throughout
> the manifold (remember the "cloud chamber" experiments from high school
>science,
> where rapidly lowering the pressure of a vessel created a fog throughout the
>vessel from
> the dispersed water droplets in otherwise clear air?). In other words,
>closing the throttle
> creates a backflow which can send the crankcase gases hurtling toward the
>throttle.
> Last time I cleaned the throttle I noticed quite a bit of sticky buildup
>throughout the
> intake manifold--in a truck with 90K miles that is otherwise running fine.
>
> Another source of the carbon crud could be the fuel vapor recovery system in
>modern
> cars. I have had to replace a vacuum flow sensor--which I "autopsied" and
>found
> a layer of (apparently) carbon dust covering the sensor (which looke like
>some sort
> of thermistor). A reader of "Auto Restorer" magazine wrote in the most
>recent issue
> of putting a small fuel filter upstream of these sensors to prevent the
>carbon dust from
> the fuel vapor recovery canister from contaminating these sensors (resulting
>in a "check
> engine" light). I may do this myself.
>
> I'm not aware of these types of problems (throttle buildup, etc.) on older
>cars which
> didn't use so many vacuum systems.
>
>
> bs
> ********************************************
> Bob Spidell San Jose, CA bspidell@comcast.net
> '67 Austin-Healey 3000 '56 Austin-Healey 100M
> ********************************************
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