TECH: 1991 Ford Crown Victoria - 5.0 - Will Not Idle

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!
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snipped-for-privacy@yahoo.com wrote:

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
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That doesnt sound right... that from the book?
I thought it was .95 sitting on the stop, after idle screw was adjusted.
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Backyard Mechanic wrote:

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
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"Comboverfish" wrote

Pretty much all of them, actually. And not just that year or model, either.
http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/17/34/d3/0900823d801734d3.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.

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.
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MasterBlaster wrote:

http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/17/34/d3/0900823d801734d3.jsp
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.
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snipped-for-privacy@yahoo.com wrote:

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.
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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!
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snipped-for-privacy@yahoo.com wrote:

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
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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!!!
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snipped-for-privacy@yahoo.com wrote:

The problem with cleaning the IAC is you only have a 50-50 chance of long term success. Many of them stick again a few hours, days, weeks after cleaning. How pissed would you be if a shop charged you, say, $60 to diagnose and clean the IAC and it failed again a week or so later?
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Tom Adkins wrote:

What factors determine the success of cleaning an IAC?
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Fordfan wrote:

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 cause the pintle to stick. I've had enough of them stick again after a thorough cleaning that I don't bother any more. When they were over $100, it was worth a shot, but for $50-60 it's not worth my time or the customers money for me to take a chance on having to do the job twice. On the car in question, my take is that the IAC has been in service for 15 years and probably 100K plus miles. It's done it's job, reached the end of it's service life, and now it's time to replace it.
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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 you.
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.
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