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
********************************************
Subject: Re: Carbon Buildup, was Re: question on C (neither L nor B)
> Well, that's what I always thought, but it seems that the crud I got from
>around the
> throttle body was upstream of where the pcv and egr gases come in. Does it
>blow
> backwards up the intake? Build up around the valves I can understand as
>coming from
> that pollution stuff, but the thing I had with the Neon was right where the
>throttle
> plate seats against the throttle body.
>
> Bob Spidell wrote:
>
>> Jim,
>>
>> Just got done pondering this question myself.
>>
>> Have concluded the "carbon"--which is downstream of the throttle plate--must
>> be coming from various vacuum inputs to the intake manifold, probably 90% or
>> more from the PCV valve (whose job it is to allow crankcase blowby to be
>sucked
>> into the intake manifold ... hence the carbon buildup).
>>
>> bs
>> ********************************************
>> Bob Spidell San Jose, CA bspidell@comcast.net
>> '67 Austin-Healey 3000 '56 Austin-Healey 100M
>> ********************************************
>>
>> Subject: Carbon Buildup, was Re: question on C (neither L nor B)
>>
>> >
>> > Where does the intake side carbon build up come from? I recently
>addressed a
>> > carbon build up problem on our 1998 Dodge Neon R/T with the 2.0 DOHC
>engine.
>> > The throttle plate was sticking after being parked for a while. I cleaned
>a ton
>> > of black soot from the throttle body.
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