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
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
Pretty much all of them, actually.
And not just that year or model, either.
- 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.
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
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
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!
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!
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!!!
The problem with cleaning the IAC is you only have a 50-50 chance of long
success. Many of them stick again a few hours, days, weeks after cleaning. How
would you be if a shop charged you, say, $60 to diagnose and clean the IAC and
failed again a week or so later?
Not sure, although I would assume wear on the pintle shaft along with wear and
unreachable debris in the shaft bore. Also, the pintle and seat wear and can
pintle to stick. I've had enough of them stick again after a thorough cleaning
don't bother any more. When they were over $100, it was worth a shot, but for
it's not worth my time or the customers money for me to take a chance on having
the job twice.
On the car in question, my take is that the IAC has been in service for 15
probably 100K plus miles. It's done it's job, reached the end of it's service
and now it's time to replace it.
Glad you fixed it!
I initially assumed that you had a Federal emissions 5.0 just based on
the overwhelming likelyhood. You next post referenced a MAP (actually
used for baro reading) and a MAF sensor, so it's clear now that you
have a California emissions car. I figured the idle issue was just the
IAC. Like other people said, it may need to be replaced vs cleaned in
the near future. It depends on how much your time/money is worth to
As my TPS spec was called out in error, I will say that the minimum
allowable spec at idle is actually *lower* on the Cali engine - .2
volt. The TPS is not adjustable according to Ford but there is some
wiggle room around the fastener holes. Chances are it is just plain
good or bad and no amount of adjustment will compensate adequately. In
fairness, I did pull up "quick specs" that are very generic but backed
up the other posters' statements of expected idle voltage in the .7 to
1.2 range. It's still a good idea and simple to check your TPS for
idle and WOT voltage, and for dropouts during a slow sweep, if you have
any driveability concerns in the future.
Toyota MDT in MO
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