Stiff Plunger in Fuel Distributor
rob hod
rob3 at hod3.fsnet.co.uk
Wed Nov 27 22:16:10 EST 2002
Hi Jesse,
Well I think you're well into the 1% of the global population who
understand the basics of the K-jet fuel distributor. However please forget
about system pressure as far as resistance to the movement of the air plate
is concerned. You are correct to say that system pressure is present in the
fuel distributor. It is of course regulated by the (usually?) integral
pressure regulator. It is the motive force for all movement of fuel to the
injectors and is present in full to the chambers of the lower half of each
port (below the diaphragm ) and to the CSV. Also central to the metering
system are the pressure regulating valves that ensure that the quantity of
metered fuel supplied to the injectors is governed solely by the position of
the plunger rather than any significant pressure diferential between top and
bottom chambers. So give or take a few psi system pressure is present right
through to the injectors. The crucial point to note is that it is control
pressure only which is used to oppose the upward movement of the plunger and
hence air plate. When I say 'oppose the upward movement' it may be easier to
visualize it as having a certain 'weight' depending on CP.
If this were not the case then the scenario you are thinking about would
indeed be a problem. If the pressures on either side of the diapraghm were
significantly different it would be easy to imagine a situation where it
would be difficult to stop the flow of fuel to the injectors, - bit like the
difference between holding your finger in a hole in a dam, compared to the
difficulty in stopping the flow once you've taken it out!
From what Tigran was saying it sounds very much like his fule dist has a
problem, maybe related to the DPR. There should be no significant resistance
to the downward movement of the air plate, and smooth steady resistance on
the way up, the strength of which is related to the control pressure. Anyone
who wants to see it demonstrated can get a pressure gauge, plumb it into the
feed to the warm-up reg and jumper the FPR. Then with an electrical supply
to the bi-metallic strip in the warm-up reg, play with the air plate and
feel it get harder to lift as the warm up reg gradually increases control
pressure.
Okay, hope this helps get you in the picture, if not let me know, we'll
get this straight in the end!
rob
----- Original Message ----- > Message: 6
> To: quattro at audifans.com
> Subject: Re: Stiff Plunger in Fuel Distributor
> Reply-To: awdaudi at excite.com
> From: "Jesse A Clendenning" <awdaudi at excite.com>
> Cc:
> Date: Wed, 27 Nov 2002 10:46:29 -0500 (EST)
>
>
>
> --
> [ Picked text/plain from multipart/alternative ]
> Rob, I agree with everything you've said. The only difference being that
I=
> do not have a DPR either as my car is a turbo. We've both spoke of
Control=
> Pressure as being the resistance to upward movement on the plunger. The
re=
> ference that Tom Chudzinski (thanks, excellent read) gave me, and the
Bosch=
> fuel injection manuals that I own, part of the Fuel distributor is
subject=
> ed to System Pressure (supplied by fuel pump). This is where I was
speculat=
> ing that a failing fuel pump pushing much less pressure then normal could
c=
> ause a stiff plunger in the fuel distributor. The Warm up Regulator would
s=
> ection off the amount of pressure that it was set for ~3.5 bar. What was
le=
> ft would be in the other portion of the fuel distributor leaving a very
unb=
> alanced Air Flow Meter. Let me know what you think. I think I
underst=
> and what you are saying, but I don't think systempressure comes into it
*di=
> rectly* . Its probably best to regard theresistance to upwards movement
as=
> being related to control pressure. Lowercontrol pressure =3D less
resistan=
> ce to upwards movement =3D higher airplateposistion for given airflow =3D
h=
> igher plunger in dist =3D more fuel =3D richermixture. Now as far as I
k=
> now control pressure is controlled by warm up reg andDPR only and remember
=
> that control pressure is derived by a calibratedorifice from system
pressur=
> e=2E There is thus a fixed supply of fuel andpressure to the control
system=
> . This is how the warm up reg works - whencold it bleeds off fuel in the
co=
> ntrol pressure system at a higher rate,leading to lower control pressure.
H=
> aving never had a car with a DPR (UKdweller) I can;t vouch for it but I
gue=
> ss the DPR does a similar thing toinfluence control pressure and hence
mixt=
> ure. Now concerning the downward movement of the air plate, I think
ther=
> eshould not be any significant amount. If released the plate should
movesma=
> rtly back to rest with no hesitation, regardless of system or
controlpressu=
> re. HTHrob> Message: 6> To: quattro at audifans.com> Subject:
Stif=
> f Plunger in Fuel Distributor> Reply-To: awdaudi at excite.com> From:
"J=
> esse A Clendenning" <awdaudi at excite.com>> Cc:> Date: Tue, 26
No=
> v 2002 17:58:27 -0500 (EST)>>>> --> [ Picked text/plain
from=
> multipart/alternative ]>> As I am still at work and do not have
this=
> problem - as of yet, I wasthink=3D> ing... I've read a couple of posts
=
> about stiff control plungers in thefuel=3D> distributor and have
experi=
> enced it once myself on an old 5000s. Mostpeop=3D> le assume that it is
=
> a result of the fuel pump running in an"overpressuriz=3D> ation mode."
A=
> fter thinking about this I believe that maybe this could bej=3D> ust
the=
> opposite. The difference between the pressure on top of thedistrib=3D>
=
> utor and on the bottom of the distributor are what causes the pressure
ont=
> =3D> he plunger, correct? If there was excessive pressure on the bottom
=
> of thef=3D> uel distributor, wouldn't the plunger be easier to lift?
Now=
> if thepressur=3D> e were low from a failing fuel pump - then pressure
w=
> ould be as high aspos=3D> sible on the top side of the fuel
distributor,=
> and greatly reduced onbotto=3D> m side - making a stiff plunger. In
th=
> e Bentley it describes cold control=3D> pressure (on top of fuel
distrib=
> utor) as being less than warm, making thep=3D> lunger move easier.
Does=
> this seem to coincide with your theories? or amI=3D> flawed in my
view=
> s? I want to understand this better. Thanks, JesseClende=3D>
nning>&g=
> t; _______________________________________________> Join Excite! -
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