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Re: ISV and stalling (a bit long)

Ed Kellock wrote:
> I read about Doug (Antilles Engineering) and his stalling 90 200tqw.
> I've been trying to track down a heat-induced low-boost problem for a
> while (significant low-boost, not a relative reduction to combat
> detonation).  So at lunch I disconnect my ISV to see what the effect
> would be.
> (BTW, This is on my 91 200qw)
> With ISV, on a cold start, or even a partially warm start sometimes, I have
> an idle which will fall off and then catch and work its way back up to
> normal.  It does this a few times until the car has run a minute or two.  If I
> try to drive away during this period, when I apply the accelerator, the
> engine will bog, just like a too lean carburetor on a cold morning.
> Also, since I've owned the car, whenever boost does start to really build,
> I have always experienced a slight power "bubble".  It will start to build
> and pull strongly, then taper off slightly but noticeably then maintain a
> consistent pull from there on.  At first I thought it was normal, but as time
> wore on I began to wonder about that too.
> When I disconnected the ISV wires this is what I found:
> It started just fine and held a steady idle.  I engaged the AC and the idle
> dropped to around 600, but remained steady.  I pulled away smoothly and
> when I slowed for a speed bump, clutch in, engine stalled.  So far pretty
> much what I would expect without the ISV operating.
> On the way back to work, boost was strong; I did not experience the low
> boost situation.  I did experience a healthy stumble when _opening_ the
> throttle, regardless of engine speed or initial throttle position.  Basically,
> whenever I started to _open_ the throttle from any position, the first bit of
> movement seemed to cause the engine to bog just a bit and then it would
> be okay.  I also noticed some quick stumbling (kind of like a misfire, but
> not) when producing high boost levels.  I have experienced something like
> this once before, but worse and only once.  However the engine did not
> stall on deceleration.
> So...  disconnecting the ISV did seem to alter the behavior, but in an
> expected manner.  I'm trying to figure out if this test has given me any
> new insight into my problem.  Without the ISV operating, the car seemed
> to run worse overall, but not just at idle.  When I would start the car
> without the ISV plugged in, I would _not_ see the check engine light come
> on, but once I began driving it would light.  Each time I started the car, it
> would not be lit, then while driving it would come on.  Is it telling me about
> the ISV, even though it doesn't light at idle?  Just for verification, the ISV
> on the 3B engine is at the top rear of the intake manifold, level with the top
> of the engine, pointed east/west or transversely, perpendicular to the
> engine.  The connector is on the driver's side of the valve.... Right?
> Could the ISV be trying to compensate for a vacuum leak even when the
> engine is not at idle?
> Thanks in advance for any input.  The nice thing is, even though I know
> it's not running exactly right, the car is still driveable and enjoyable.  But I
> just know it can be much better.
> Ed Kellock .............. Greenville, SC  USA
> 91 200qw ................ 87 Coupe GT

Your comments point in the direction of two areas IMHO.
One insifficient air at the intake ( replace air filter )
and lean condition in a portion of the plunger travel (fuel regulator)
possibly from a minor binding in the plunger.
When replacing the filter ( use only O.E.M. !!!) manually check the fuel
control plate and plunger for smooth operation up and down.
Next stop may be fuel pressure and flow analysis !!!! starting with fuel
filter replacement.
Good luck