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From: Jonathan Ward on 28 Apr 2010 12:29
Okay, so here we are a month later with one symptom left and a
somewhat different set of conditions:

- No cold starting problem anymore.
- The "service engine soon" light no longer comes on (other than in
test mode when the key is turned on).

This is really puzzling because I didn't change anything.

The idle speed is still very high, and that's the only problem I can
observe. But it's not consistent. Sometimes it's fine. A couple of
things I've observed:

- Unplugging the connector from the idle air control valve has no
effect.

- Unplugging the connector from the throttle position sensor will make
the idle speed increase...but not always. Manually manipulating the
TPS has no effect on idle speed.

In the last year, I've replaced basically everything related to the
throttle body injection (including injectors), including related
emission control electromechanical parts. I'm kind of down to the
point where there's nothing left to change.

Any ideas of what to check would be greatly appreciated. The
Chilton's manual on this vehicle is essentially useless.

Many thanks!

-Jonathan Ward



On Mar 30, 8:42 am, Jonathan Ward <bigjon...(a)gmail.com> wrote:
> I have a 1991 GMC s15 jimmy 4x4 with the 4.3L 'Z' engine.  It has 197K
> miles and has been in the family since new.  A couple of weeks ago,
> the idle started to run very high and the "service engine soon" light
> came on.  The truck also became very hard or impossible to start when
> cold, but starts okay even if it has only been sitting in the sun for
> a couple of hours and isn't at operating temp.
>
> Many parts have been previously replaced for preventive maintenance,
> including:
>
> - Everything electrical/mechanical on the throttle body injection,
> including new injectors, throttle position sensor, idle air control.
>
> - All ignition parts are upgraded quality and in new condition.
>
> - Just replaced the gas tank, fuel pump, sender, and fuel filter.
> (The old pump had failed but the replacement didn't make the vehicle
> run any better than it had been.)
>
> I asking here because I'm out of ideas.  Could a plugged catalytic
> converter do this?  (It's still on the original one.)  There's no ODB
> port to connect to or anything, so I'm stuck with old-school
> troubleshooting.
>
> Any thoughts?  Many thanks in advance.
>
> -Jonathan Ward

From: Steve W. on 28 Apr 2010 13:55
Jonathan Ward wrote:
> Okay, so here we are a month later with one symptom left and a
> somewhat different set of conditions:
>
> - No cold starting problem anymore.
> - The "service engine soon" light no longer comes on (other than in
> test mode when the key is turned on).
>
> This is really puzzling because I didn't change anything.
>
> The idle speed is still very high, and that's the only problem I can
> observe. But it's not consistent. Sometimes it's fine. A couple of
> things I've observed:
>
> - Unplugging the connector from the idle air control valve has no
> effect.
>
> - Unplugging the connector from the throttle position sensor will make
> the idle speed increase...but not always. Manually manipulating the
> TPS has no effect on idle speed.
>
> In the last year, I've replaced basically everything related to the
> throttle body injection (including injectors), including related
> emission control electromechanical parts. I'm kind of down to the
> point where there's nothing left to change.
>
> Any ideas of what to check would be greatly appreciated. The
> Chilton's manual on this vehicle is essentially useless.
>
> Many thanks!
>
> -Jonathan Ward


Vacuum leak downstream of the sensors could cause it. Disconnect the
vacuum lines from the intake and block the fittings with caps. See if
there is any change.
Spray around the intake with some ether and listen for a change in
engine speed.
Toss a vacuum gauge on it and see what you have.

--
Steve W.
(\___/)
(='.'=)
(")_(")
From: Jonathan Ward on 28 Apr 2010 15:39
Steve,

Vacuum at the manifold is ~19 inches, very steady. No evidence of any
leaks downstream. I couldn't get the reading to change by manipulating
vacuum hoses or spraying penetrating oil on potential leak points.

While I was checking it, I disconnected the wires to the throttle
position sensor and set them aside. After rising significantly, after
about a minute the idle speed dropped to an acceptable level. Plug
the TPS back in and the idle roars up again. I replaced the TPS less
than two years ago, but I suppose one could certainly go bad. Seems
like the lazy man's way is to keep replacing parts until it works, but
I'm not sure what else to try.

-Jonathan


On Apr 28, 1:55 pm, "Steve W." <csr...(a)NOTyahoo.com> wrote:
> Jonathan Ward wrote:
> > Okay, so here we are a month later with one symptom left and a
> > somewhat different set of conditions:
>
> > - No cold starting problem anymore.
> > - The "service engine soon" light no longer comes on (other than in
> > test mode when the key is turned on).
>
> > This is really puzzling because I didn't change anything.
>
> > The idle speed is still very high, and that's the only problem I can
> > observe.  But it's not consistent.  Sometimes it's fine.  A couple of
> > things I've observed:
>
> > - Unplugging the connector from the idle air control valve has no
> > effect.
>
> > - Unplugging the connector from the throttle position sensor will make
> > the idle speed increase...but not always.  Manually manipulating the
> > TPS has no effect on idle speed.
>
> > In the last year, I've replaced basically everything related to the
> > throttle body injection (including injectors), including related
> > emission control electromechanical parts.  I'm kind of down to the
> > point where there's nothing left to change.
>
> > Any ideas of what to check would be greatly appreciated.  The
> > Chilton's manual on this vehicle is essentially useless.
>
> > Many thanks!
>
> > -Jonathan Ward
>
> Vacuum leak downstream of the sensors could cause it. Disconnect the
> vacuum lines from the intake and block the fittings with caps. See if
> there is any change.
> Spray around the intake with some ether and listen for a change in
> engine speed.
> Toss a vacuum gauge on it and see what you have.
>
> --
> Steve W.
> (\___/)
> (='.'=)
> (")_(")

From: Steve Austin on 28 Apr 2010 18:23
On 4/28/2010 3:39 PM, Jonathan Ward wrote:

> Vacuum at the manifold is ~19 inches, very steady. No evidence of any
> leaks downstream. I couldn't get the reading to change by manipulating
> vacuum hoses or spraying penetrating oil on potential leak points.
>
> While I was checking it, I disconnected the wires to the throttle
> position sensor and set them aside. After rising significantly, after
> about a minute the idle speed dropped to an acceptable level. Plug
> the TPS back in and the idle roars up again. I replaced the TPS less
> than two years ago, but I suppose one could certainly go bad. Seems
> like the lazy man's way is to keep replacing parts until it works, but
> I'm not sure what else to try.
>
What happens if you disconnect both the tps and the iac at the same time?
From: Steve W. on 28 Apr 2010 19:05
Jonathan Ward wrote:
> Steve,
>
> Vacuum at the manifold is ~19 inches, very steady. No evidence of any
> leaks downstream. I couldn't get the reading to change by manipulating
> vacuum hoses or spraying penetrating oil on potential leak points.
>
> While I was checking it, I disconnected the wires to the throttle
> position sensor and set them aside. After rising significantly, after
> about a minute the idle speed dropped to an acceptable level. Plug
> the TPS back in and the idle roars up again. I replaced the TPS less
> than two years ago, but I suppose one could certainly go bad. Seems
> like the lazy man's way is to keep replacing parts until it works, but
> I'm not sure what else to try.
>
> -Jonathan

Just for fun have you disconnected the battery and let it set for an
hour or so? I have seen more than one vehicle have a glitch in the ECM
which was cured doing this.
Also check the ground to the engine and to the battery. A voltage
problem could be making the ECM think something is wrong when it isn't.

I would also try to get a scan tool with live data readout and see what
the TPS and IAC are actually doing.
It could be the IAC isn't closing enough to drop the idle.


--
Steve W.
(\___/)
(='.'=)
(")_(")
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