fredsmythson@yahoo.com

I have a 1991 Ford LTD Crown Victoria with a 5.0 V8 that will not idle.
It will only run if I press on the accelerator right after it starts to
keep it at a higher RPM. Seems to run fine at a higher RPM. When I let
up on the accelerator, it goes to a very slow idle and runs rough, like
it's missing, and then it stalls. There's no "check engine" warning
light. I'd like to get it to idle temporarily so I can move it to a
loaction where I can work on it. Will turning the throttle plate
adjusting screw raise the idle a bit? What might be the source of the
problem - spark plugs, bypass air value, EGR valve??? Thanks!

Comboverfish

fredsmythson@yahoo.com wrote:
> I have a 1991 Ford LTD Crown Victoria with a 5.0 V8 that will not idle.
> It will only run if I press on the accelerator right after it starts to
> keep it at a higher RPM. Seems to run fine at a higher RPM. When I let
> up on the accelerator, it goes to a very slow idle and runs rough, like
> it's missing, and then it stalls. There's no "check engine" warning
> light. I'd like to get it to idle temporarily so I can move it to a
> loaction where I can work on it. Will turning the throttle plate
> adjusting screw raise the idle a bit? What might be the source of the
> problem - spark plugs, bypass air value, EGR valve??? Thanks!

The IAC valve (or Bypass Air Valve) is most suspect on Fords with poor
idle quality. If you had to shotgun one part, that would be it. Don't
randomly mess with the throttle blade stop screw. There is a procedure
for setting it, but if it hasn't been touched, then just leave it alone
for now.

Also check your TPS voltage reading. It's best to check it at the ECM,
so you will need a wiring diagram. Look for .35 to .5 volts at 0%
throttle. If there are any devices that open the throttle when the
engine isn't running, you will need to be aware of them. Ideally, you
would check TPS voltage *with* the engine running to compensate for
engine vacuum on the throttle blade, dashpots, etc - but in your case
that isn't possible.

Look/listen for gross vacuum leaks as well. Check around major
sources, like the PCV hose, broken PCV valve, intake gaskets, brake
booster and hose, etc.

Toyota MDT in MO

Backyard Mechanic

fredsmythson@yahoo.com wrote:

> I have a 1991 Ford LTD Crown Victoria with a 5.0 V8 that will not idle.
> It will only run if I press on the accelerator right after it starts to
> keep it at a higher RPM. Seems to run fine at a higher RPM. When I let
> up on the accelerator, it goes to a very slow idle and runs rough, like
> it's missing, and then it stalls. There's no "check engine" warning
> light. I'd like to get it to idle temporarily so I can move it to a
> loaction where I can work on it. Will turning the throttle plate
> adjusting screw raise the idle a bit? What might be the source of the
> problem - spark plugs, bypass air value, EGR valve??? Thanks!
>

Do not adjust that screw... if you already have, come back on here to get
directions on how to adjust it back..

Look at your vaccuum lines as they distribute at the firewall.

If you have a distribution log, it may have a smaller capped nipple on
it.

create a vacuum leak by removing one of the caps... that will get you
enough air that the car should idle.


Another thing that will cause this, but also causes some hesitation at
lights, etc.. is a dirty MAF (mass air fuel sensor)

To see if this affects problem, unplug the harness to it before starting
engine. If it runs BETTER, the MAF is dirty.

But replace the Idle Speed Control valve first... believe me, it's not
worth cleaning it... usually the idle 'stuck' point just changes.


--
Yeh, I'm a Krusty old Geezer, putting up with my 'smartass' is the price
you pay..DEAL with it!

Backyard Mechanic

"Comboverfish" <comboverfish@yahoo.com> wrote:

> Look for .35 to .5 volts at 0%
> throttle.

That doesnt sound right... that from the book?

I thought it was .95 sitting on the stop, after idle screw was adjusted.

--
Yeh, I'm a Krusty old Geezer, putting up with my 'smartass' is the price
you pay..DEAL with it!

fredsmythson@yahoo.com

I did not touch the throttle blade stop screw. I disconnected the MAF
sensor, but it still would not idle and the "check engine" light came
on. I also tried to create a vacuum leak - difficult to find accessible
vacuum hoses on this model - I disconnected the air hose from the MAP
sensor, but it barely ran, so I connected it again. Then I disconnected
the air hose from the air/smog pump. This actually helped at first,
because it idled for about two minutes, then stalled again and would
not idle after that. Would it be worth trying to clean the air bypass
valve first, since it looks like the new ones cost about $70 for this
engine? Thanks for your help!

Backyard Mechanic

fredsmythson@yahoo.com wrote:

> I did not touch the throttle blade stop screw. I disconnected the MAF
> sensor, but it still would not idle and the "check engine" light came
> on. I also tried to create a vacuum leak - difficult to find accessible
> vacuum hoses on this model - I disconnected the air hose from the MAP
> sensor, but it barely ran, so I connected it again. Then I disconnected
> the air hose from the air/smog pump. This actually helped at first,
> because it idled for about two minutes, then stalled again and would
> not idle after that. Would it be worth trying to clean the air bypass
> valve first, since it looks like the new ones cost about $70 for this
> engine? Thanks for your help!
>
>

Yeah, you can try. It should change the symptoms, like I said.. but it,
for instance, might idle okay cold, but not when it warms. Or might fix it
altogether.. for a while

--
Yeh, I'm a Krusty old Geezer, putting up with my 'smartass' is the price
you pay..DEAL with it!

Comboverfish

Backyard Mechanic wrote:
> "Comboverfish" <comboverfish@yahoo.com> wrote:
>
> > Look for .35 to .5 volts at 0%
> > throttle.
>
> That doesnt sound right... that from the book?
>
> I thought it was .95 sitting on the stop, after idle screw was adjusted.
>

And which 1991 5.0 Federal emissions Crown Vic TPS is set to .95 volts?
I'd try around .4 volts myself, as previously suggested.

Toyota MDT in MO

MasterBlaster

"Comboverfish" wrote

> Backyard Mechanic wrote:
> > "Comboverfish" <comboverfish@yahoo.com> wrote:
> >
> > > Look for .35 to .5 volts at 0%
> > > throttle.
> >
> > That doesnt sound right... that from the book?
> >
> > I thought it was .95 sitting on the stop, after idle screw was adjusted.
> >
>
> And which 1991 5.0 Federal emissions Crown Vic TPS is set to .95 volts?

Pretty much all of them, actually.
And not just that year or model, either.

http://www.autozone.com/servlet/UiBr...3d801734d3.jsp

- The TP sensor receives a 5 volt reference signal and a ground circuit
- from the PCM. A return signal circuit is connected to a wiper that runs
- on a resistor internally on the sensor. The further the throttle is opened,
- the wiper moves along the resistor, at wide open throttle, the wiper
- essentially creates a loop between the reference signal and the signal
- return returning the full or nearly full 5 volt signal back to the PCM.
- At idle the signal return should be approximately 0.9 volts.

> I'd try around .4 volts myself, as previously suggested.

You go right ahead and use any voltage you like. I'll stick with .95v at idle,
and around 5.0v at WOT, just like the manual for my 5.0 TBird says.

Comboverfish

MasterBlaster wrote:
> "Comboverfish" wrote
>
> > Backyard Mechanic wrote:
> > > "Comboverfish" <comboverfish@yahoo.com> wrote:
> > >
> > > > Look for .35 to .5 volts at 0%
> > > > throttle.
> > >
> > > That doesnt sound right... that from the book?
> > >
> > > I thought it was .95 sitting on the stop, after idle screw was adjusted.
> > >
> >
> > And which 1991 5.0 Federal emissions Crown Vic TPS is set to .95 volts?
>
> Pretty much all of them, actually.
> And not just that year or model, either.
>
> http://www.autozone.com/servlet/UiBr...3d801734d3.jsp
>
> - The TP sensor receives a 5 volt reference signal and a ground circuit
> - from the PCM. A return signal circuit is connected to a wiper that runs
> - on a resistor internally on the sensor. The further the throttle is opened,
> - the wiper moves along the resistor, at wide open throttle, the wiper
> - essentially creates a loop between the reference signal and the signal
> - return returning the full or nearly full 5 volt signal back to the PCM.
> - At idle the signal return should be approximately 0.9 volts.
>
> > I'd try around .4 volts myself, as previously suggested.
>
> You go right ahead and use any voltage you like. I'll stick with .95v at idle,
> and around 5.0v at WOT, just like the manual for my 5.0 TBird says.

I certainly won't use Autozone generic info for my source. MOD shows
what I said previously. Ford is very good for changing specs and
emissions components a zillion times in one model year. If MOD is
wrong, then I'm wrong. The point here is that I don't generalize Ford
specs because they change so much.

Toyota MDT in MO

fredsmythson@yahoo.com

I took off the bypass air value and cleaned it with spray throttle body
cleaner. Reinstalled, and now it idles perfectly!... Now I wonder what
the local auto shop would have charged for this repair? "There's
something wrong with the johnson rod... that will cost $400..." Thanks
for everyone's help!!!