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Discussion Starter #1
2005 Vulcan FFV will not return to normal idle speed when AC is on -- 2500+ RPMs and is difficult to stop at traffic lights or stop signs. No Codes! As soon as the AC is switched off, engine returns to a normal smooth idle and I never see the problem at all if the AC is not turned on. I was able to research this problem on the internet and I found a similar problem for a Taurus with a Duratec engine. That fault was caused by low alternator output when the AC was on, but my alternator is outputting a constant 14+ volts, even with the AC on. I have changed the IAC which has had no effect on the problem. I suspect the PCM but I am reluctant to spend that kind of money without confirmation of my diagnosis. I am also reluctant to take it to a dealer for diagnosis because I am afraid of what that bill might come to. Does anybody have any insight into this problem?
 

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^+1. Motorcraft only iac recommended. If the replacement iac is aftermarket, may try to clean/oil the old Motorcraft iac if you still have it. If not, buy a new MC or grab an MC from a pick-n-pull jy for your year/engine and clean/oil/install. May clean up and attempt to return the aftermarket iac. I have successfully.
 

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Discussion Starter #4
The fault only occurs if the AC is on. When AC is off there is no problem -- normal idle speed and idles smoothly. When the AC is on and the idle is high, the problem immediately goes away when the AC is turned off. Cold idle is fine if AC is off.
 

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Discussion Starter #6
The replacement is aftermarket -- not sure about the first one. The problem does occur when defroster is on. Basically, whenever the compressor is on, I have the problem. The only time I don't have the problem is when the AC control is in the vent position or is off.
 

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Your grounds are suspect. There is a braided ground strap between the engine and cowl area in back of passenger side engine bay. Remove ground screw, clean ground terminals and sand body side at ground screw hole. Put a dab of grease over the area after it's back together to prevent further corrosion.
 

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Your grounds are suspect. There is a braided ground strap between the engine and cowl area in back of passenger side engine bay. Remove ground screw, clean ground terminals and sand body side at ground screw hole. Put a dab of grease over the area after it's back together to prevent further corrosion.
Pic of the poor job ford did for this. Big eyelet is major current to the body, smaller is PCM ground. Bolted over paint and subject to rust. Simple fix, does not effect PCM memory. Little awkward to reach. Fixed an idle speed issue. I consider this a routine maintenance item.


I do this to all my cars, along with replace the OE pressure cap. Just did that on my most recent used Taurus. High fail rate on that. Ford new design is robust.


-chart-
 

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Discussion Starter #9
Thanks for the info. I'll work on it when I get a better weather forecast. It looks like I'll have to remove a lot of the cowling in order to get access to the ground location. I sure hope this fixes the problem.
 

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You should be able to reach that ground without removing anything. The tab it bolts to also can be bent to get a better position with a socket.

Agree, I used a stick of wood and hammer to bend it down a bit, then easy to remove. Household ammonia is the best for removal of the green corrosion. Ammonia is the best when used in a vented area since it is a gas dissolved in water so when it evaporates no residue.


-chart-
 

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Discussion Starter #12
Thanks for the tips on the ground point. There was no corrosion, so I cleaned everything up and put it back together. However, it made no difference on the fast idle problem. Anybody have any additional ideas.
 

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Discussion Starter #13
BTW, apparently there is at least one scenario where the PCM would keep the IAC from properly modulating the idle speed. See the discussion here: https://www.cargurus.com/Cars/Discussion-t673_ds599579 This is why I suspect my problem could be in the PCM. This sounds exactly like my problem except my alternator output is constant at just a hair over 14 Volts. How about it, does anyone know how the PCM logic works? Could this be a PCM problem?
 

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well a few thoughts:


look closely for any vacuum hoses that looked cracked or deteriorated and
replace as needed


look down the throttle body and see if there is build up of crud by the butterfly.
if so. get some proper cleaner and clean the throttle body out


if you got the plastic intake manifold, look close for any cracks or distortion.



bob
 

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Discussion Starter #15
I don't think that these suggestions apply to my particular problem. My high idle only occurs when the AC is on; otherwise the idle is normal and very smooth. Actually, the engine runs smoothly even when the high idle situation occurs. If the problem was a vacuum leak or a cruddy throttle body or intake, the problem would be occurring constantly, not just when the AC was on. Also, something I haven't mentioned before, if I disconnect the IAC electrical connector. the engine immediately dies. This indicates to me that the IAC is not receiving the proper signal from the PCM when the AC is on -- another indication, to me, that the problem is somewhere in the electronic engine controls. I have to believe that someone out there has experienced something like this. Please, I need help with this!
 

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I don't think that these suggestions apply to my particular problem. My high idle only occurs when the AC is on; otherwise the idle is normal and very smooth. Actually, the engine runs smoothly even when the high idle situation occurs. If the problem was a vacuum leak or a cruddy throttle body or intake, the problem would be occurring constantly, not just when the AC was on. Also, something I haven't mentioned before, if I disconnect the IAC electrical connector. the engine immediately dies. This indicates to me that the IAC is not receiving the proper signal from the PCM when the AC is on -- another indication, to me, that the problem is somewhere in the electronic engine controls. I have to believe that someone out there has experienced something like this. Please, I need help with this!
Well if its one thing i have learned over the years working on stuff is once you dismiss something as not possible, it is.

All i suggested were things that i have had issues with on my bulls in the past. IAC controls air into the intake. Vacuum leak
can affect air into the intake. Butterfly valve affects air into the intake. cracked intake manifold can affect air into the intake.
Im not saying that any of the above are the issues, but there all easier things to check and fix if there a problem.
(with maybe the exception of cracked plastic intake).

what i would do also is find somebody with a scan tool with live data that you can read the fuel trims on and see what there at
at idle. That should give you some good clues. could even be weirdness with the TPS

I would also as good PM take off ALL underhood ground straps, clean them up, and re install them. I had one weird issue once
because the block to firewall ground strap was disintegrating.


bob
 

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Could also be a vacuum leak in the damper system of the HVAC system. When you switch to AC the process also uses vacuum to change dampers position like when you put it on MAX. That setting closes outside air and recirculates inside through the AC coil, this is done with vacuum. Maybe that leak is getting to the engine during idle when AC is on.
 

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Discussion Starter #19 (Edited)
Problem Solved --- I Hope

Halleluiah, I think I may finally have a solution to my problem. After rereading the entire thread, I decided to take some advice I had previously disregarded. I cleaned up the original IAC and reinstalled it. Voila, the problem seems to be fixed.

I say "seems to be" because originally the problem I had with the car was when this original IAC was still installed. When driving home one night in October last year, the engine would not return to idle when I was approaching a red traffic light with my foot off the accelerator pedal. I replaced the original Motorcraft IAC with a BWD from Advance Auto Parts, and this cleared the problem -- for a while at least. Several months later, the problem detailed in this thread arose, and I have been fighting it for about three months now. The original was pretty cruddy, so I sprayed it with throttle body cleaner and it seems to be doing okay right now, but I plan to replace it with a new Motorcraft IAC before too long. I suspect it was sticking when the first problem occurred, and it is now fourteen years old and I don't know how long I can trust it. I will leave this thread active for a few weeks longer just in case the problem shows up again.

As far as what was going on with the BWD IAC, I can only surmise that it didn't like all the signals the PCM was outputting when the AC was on. BTW, I was able to get a refund for the BWD IAC from AAPs. It wasn't their first choice but after I explained what was going on and how it was different from the OEM part, they finally relented.

My sincere thanks to everyone who offered advice. Automender12345 and sheila nailed it the first time around saying I needed a Motorcraft IAC, and soundu reminded me not to dismiss possible solutions before trying them out. In the future, I'll try to accept all advice with more of an open mind.

Does anyone know if The Club has a discount code for RockAuto? I think I remember seeing something about a discount code but can't seem to find it right now.
 
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