[Shotimes] engine stumbling

[Zach Leahy]

Ok, Fuel pressure regulator needs to be thought of for a moment.

The fuel pressure regulator is driven by the manifold vaccum.  More
vaccum and the pressure drops, less pressure and it increases.

At Idle you are getting about 24 in Hg and that coincides well with
the pressure you have listed at idle.

When you blip the throttle, the mnifold vacuum goes down to near 0
inches Hg.  Pressure goes up accordingly. Just like yours did.

When you slowly open the throttle the vaccum level is still pretty
high, probably near 20 inches Hg.  In reality pressure should be
somwhere between your two pressures.  your's is not.

But testing fuel pressure is only one deal.  The pump must be able to
handle the volume of WOT driving (which you say seems to be ok).  That
is the highest fuel consumption period, so the highest amount of fuel
goes into the rail and out the injector, instead of returning to the
tank.  If the volume capacity of the pump were to dip, then the
problem would be seen as the car leaning out at WOT and high RPM. 
However, it seems to me there are two failure modes for a pump. 
Volume failure, as described here, or pressure failure.  Pressure
failure may be due to faulty pump valve, or a few other things.  If
the pump stops turning, it fails on both accounts.  :)

Now, here's a noteworthy thing, if the  volume capacity begins  o drop
past a particular threshold, then the pressure will begin to drop. 
That threshold is based on the consumption of fuel by the engine.  The
FPR can only block so much flow and get a pressure on it, before it is
blocking almost all of the flow and the pressure begins to dip.

So all to say, your numbers seem to indicate that your pump is having
trouble supplying enough volume, and therfore the pressure is
dropping.  It would be interesting to see how the pressure dips/rises
as you drive the car on the highway, but then you would need an in car
Fuel Pressure gauge which is mucho dinero.

Z

On 8/4/05, Co-Op <Co-Op@schramminc.com> wrote:
> The plug wells were dry when I swapped plugs.
> 
> When I checked fuel pressure, I noted that when I slowly increased the
> throttle, pressure went down while RPMs went up.  I understand the concept
> that high pressure goes with low flow rates, and low pressure/high flow.
> So, maybe the pressure that it drops to is not sufficient at high RPMs.  The
> pressures that I read were with cool running gear.  I suppose that if a
> failing pump were to run for a long time it could heat up and slow down.
> 
> Mike
> 
> -----Original Message-----
> From: Al & Angela [mailto:aa4jc1@netzero.com]
> Sent: Thursday, August 04, 2005 7:37 AM
> To: shotimes@autox.team.net
> Subject: Re: [Shotimes] engine stumbling
> 
> 
> Lee did you check the plug wires to make sure there wasn't anything in the
> bores (water or oil).
> 
> I have a 190 Walbro and it is fantastic (no problems).
> 
> Take care
> 
> Al
> 
> 
> ****ORIG MSG****
> From: "Leigh Smith" <leighsm@comcast.net>
> To: "Co-Op" <Co-Op@SchrammInc.com>, <shotimes@autox.team.net>
> Subject: Re: [Shotimes] engine stumbling
> Date: Wed, 3 Aug 2005 18:49:39 -0400
> 
> I think not wires, they would miss most when things are lean, like warm
> idle, or especially 2200-3000 rpm, light throttle. Not hard running. Save
> your money.
> I sauspect fuel pump. That describes the exact symptoms I had when mine died
> at about 180k.
> Three years doesn't sound like long for a fuel pump. Both my stock ones
> lasted 10 yrs.
> Anyone else with a Walbro pump have any comments on how long they last
> before I put the one on my bench into my car?
> Lee
> ****END****
> 
> --
> No virus found in this outgoing message.
> Checked by AVG Anti-Virus.
> Version: 7.0.338 / Virus Database: 267.10.0/63 - Release Date: 8/3/2005
> _______________________________________________
> Shotimes mailing list
> Shotimes@autox.team.net
> http://www.team.net/mailman/listinfo/shotimes
> _______________________________________________
> Shotimes mailing list
> Shotimes@autox.team.net
> http://www.team.net/mailman/listinfo/shotimes