5.0L high idle problem

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I have been fighting a high idle problem on my 91 5.0 L H.O. engine. I unplugged the IAC, no change in idle. I removed IAC and cleaned it with carb cleaner, re-installed it. No change, still idles at 2000 rpm.
I unplugged the TPS , no change ! Shouldn't that make a difference? Does this simply mean I need a new TPS ? The engine seems to rev normally when I flick the throttle even with TPS unplugged.
Today is Sunday and the parts places are closed. Otherwise I'd just buy one and try it. Since I can't , I thought I come on line and ask first.
What do you think?
Dan
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I have been fighting a high idle problem on my 91 5.0 L H.O. engine. I unplugged the IAC, no change in idle. I removed IAC and cleaned it with carb cleaner, re-installed it. No change, still idles at 2000 rpm. I unplugged the TPS , no change ! Shouldn't that make a difference? Does this simply mean I need a new TPS ? The engine seems to rev normally when I flick the throttle even with TPS unplugged.
Today is Sunday and the parts places are closed. Otherwise I'd just buy one and try it. Since I can't , I thought I come on line and ask first.
What do you think?
I,ve been looking for a vacuum leak also. I took a propane torch, opened the valve and used the gas, putting it by everything I could find. No increase in idle.
Here is what I just got done doing.
I hooked an ohm meter up to the TPS sensor, not the wiring harness, the TPS sensor itself. On one end and the center I hav .87 ohms at idle (Engine off during this) increasing to 4.1K ohms at WOT. As I increase throttle the ohms go up . I hooked it from the center to the other end terminal. 3.6K reading at idle and .37 ohms at WOT. I then hooked it to both end terminals and got 4.34K ohms no matter where the throttle was.
Would this simply test rule out the TPS?
How can I check the IAC to make sure it is working? Can I apply 12v and see if it clicks?
Dan
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Dan wrote:

sensor itself. On one end and the center I hav .87 ohms at idle (Engine off during this) increasing to 4.1K ohms at WOT. As I increase throttle the ohms go up . I hooked it from the center to the other end terminal. 3.6K reading at idle and .37 ohms at WOT. I then hooked it to both end terminals and got 4.34K ohms no matter where the throttle was.

Think so. Looks like a perfectly working potmeter to me.
Two of the terminals are connected to the ends of a strip of some resistive materials, and the third one to a slider that goes up and down that strip with throttle movement. Your measurements seem to confirm it works. Soren
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On Sun, 11 Jan 2004 17:46:30 -0500, "Dan"

sensor itself. On one end and the center I hav .87 ohms at idle (Engine off during this) increasing to 4.1K ohms at WOT. As I increase throttle the ohms go up . I hooked it from the center to the other end terminal. 3.6K reading at idle and .37 ohms at WOT. I then hooked it to both end terminals and got 4.34K ohms no matter where the throttle was.

First, you should not be using an ohm meter on the TPS unless the harness is disconnected - I hope this is what you did. Also, you should use an analog meter and move the throttle very slowly watching for any blip in needle movement. The TPS may have a bad spot somewhere. To check the TPS with the ignition on, use a digital volt meter and probe the wires with a probe made for this or a couple of straight pins thru the wire. One wire is ground, one is approximately 5 VDC. The third is the sensing lead. It should read somewhere between about .35 VDC and slightly les than 1.0 VDC. You do not want it 1.0 VDC or greater because this is where the ECM "sees" that you are off idle and the fuel and timing curves begin to kick in. After you have check the TPS thru it's range with the ignition on and engine off, start the engine and be sure the idle signal voltage is less than 1.0 VDC.
If that is good, check the IAC for somewhere between 9 and 11 ohms resistance. Over that, it is open - under and it is shorted. If the ohms are correct. the engine should respond when it is disconnected. If it does not, that is not automatic reason for replacement if it has passed the ohms test and you have very high idle. I would suspect a large vacuum leak like a badt upper-to-lower intake gasket for which the 5.0L was occasionally known. A partially open EGR or a leaky brake booster diaphram can cause this. It is not unheardof for an intake gasket to be bad under the intake but, you would normally have this condition along with exhaust smoke or high oil consumption. Be sure to check your throttle valves for being fully closed. It is possible that a cable is hanging internally. Disconnect the linkage at the TB to be sure it can fully close by itself.
Lastly, if you have not already done so, you need to get the ECM scanned for fault codes and post them back here for possible pointer on where any problems may be found.
If you have an Autozone nearby, they may be able to extract the codes for you. Many of the stores cannot do EEC-IV which is what you have. Look at the site below for instructions on how to get the stored codes if you do not have access to a scanner:
http://dalidesign.com/hbook/eectest.html
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Have you checked to make sure things like the brake booster is not leaking vacuum? You most likely need a new IAC, not a new TPS.
I have been fighting a high idle problem on my 91 5.0 L H.O. engine. I unplugged the IAC, no change in idle. I removed IAC and cleaned it with carb cleaner, re-installed it. No change, still idles at 2000 rpm. I unplugged the TPS , no change ! Shouldn't that make a difference? Does this simply mean I need a new TPS ? The engine seems to rev normally when I flick the throttle even with TPS unplugged.
Today is Sunday and the parts places are closed. Otherwise I'd just buy one and try it. Since I can't , I thought I come on line and ask first.
What do you think?
Dan
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How would I check the brake booster? Simply pull the vacuum line and plug it? I'll go out in the garage and try that right now. Dan

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I Found the problem!
I checked the PB booster. It was fine. I read in another post about the bellows going bad in the IAC. I didn't remember seeing bellows, I saw springs. hhhmmmmm. I took it out again. My bellows are shot, it just looked like a spring. So I duct taped the opening for the bellows side and poked a 3/16" hole in it. Put it back on and it idled slow but steady at 600 rpm. So I enlarged hole a little, idle went up. I then did a temporary, possibly permanent fix. I took and cut the side out of an empty quart of oil. Made a restrictor plate/gasket. I used 1/4" hole for the bellows side and a full size hole for the other side. Results. It idles at 900 rpm and 700 in gear. WAY better. I drove it around, all is well.
Tomorrow I can shop for the new IAC valve. But since they are about $75 and this is working, I may hold off and see how this works for awhile.
Cheers. Dan

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an empty quart of oil. Made a restrictor plate/gasket. I used 1/4" hole for the bellows side and a full size hole for the other side. Results. It idles at 900 rpm and 700 in gear. WAY better. I drove it around, all is well.

Dan
Good job, you figured out the cause of high idle. I have been following this thread with interest. I just have a quick question for you, what is the difference between the following:
IAC, IAV Valve and Idle Solenoid ?
Are they three different structures, or are they the same?
I am trying to figure out why my 4.0L SOHC engine tends to stall when I start it cold and try to move right away.
Thanks
Matt
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Hi I'm new to this as well. I have been researching many, many newsgroups and forums on the net. From what I gathered, they are all the same, just different names for the same part.
Sorry I don't know the cause of yours. Keep trying and keep asking. It worked for me.
Dan

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Exactly what is a "bellows" in fererence to an IAC.
I just can't picture this.
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The covering over the spring that works against the pintle, it's inside the valve. The bellow brakes apart and jams the pintle open.

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I have been fighting a high idle problem on my 91 5.0 L H.O. engine. I unplugged the IAC, no change in idle. I removed IAC and cleaned it with carb cleaner, re-installed it. No change, still idles at 2000 rpm. I unplugged the TPS , no change ! Shouldn't that make a difference? Does this simply mean I need a new TPS ? The engine seems to rev normally when I flick the throttle even with TPS unplugged.
Today is Sunday and the parts places are closed. Otherwise I'd just buy one and try it. Since I can't , I thought I come on line and ask first.
What do you think?
Dan ======================Dan, if you dont find a leak, the throttle isnt sticking, just go get an IAC valve. It is a very common problem and will very likely fix it. Your only other choice is to buy a book and a scan tool to diagnos it right. Or take it somewhere and pay someone $75 an hour to do it for you. I would take my chances on the IAC for sure. GL
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Dan, Same problem, same engine. Did exactly what you said you did, made a restricter plate out of plastic with a 1/4 inch hole. Rpm's now 800 idle , 1000 drive. i was getting ready to take it into the shop before I found this thread. figure you saved me a couple hundred bucks at least. I wonder if I can drive like this without buying a new IAV. The car is rusting out (91 t-bird). I don't want to put anymore money into it than neccesary. Thanks again!!!!

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mudbone opined in

How about giving the IAC a good cleaning? They often stick... failing that go get one out of a wreck and clean it.
This is a COMMON problem but not a Universal one.
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Hey Backyard, I think I will go to a junk yard and find another IAC valve. In the meantime, being my idle seems ok now, would it have an adverse effect on the engine if I left that restrictor plate with the 1/4 inch hole in it on my IAC now? What do you think? thanks!
On Sun, 06 Feb 2005 18:05:07 GMT, Backyard Mechanic

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mudbone opined in

If you used reg bottle plastic, it may not take the heat.
And there may be somewhat an adverse effect starting hot in hot weather.
Suggest you separate the IAC motor from the valve and see if it wants to cycle when you turn ign to ON, there COULD be a bad drive circuit in the ECM.. but not likely.
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On Sun, 06 Feb 2005 22:12:11 GMT, Backyard

Good thinking. Thats what I needed to know. I will seperate it from the throttle body and watch the bellows while I have someone turn the key on. I should see some movement right? If I do, that should mean that the ecm is ok and maybe the bellows are bad in it. Do you know if you can apply 12 volts to the terminals to see if it moves? For when I go to the junk yard. I think your right that I can't leave it like this when the weather gets warmer. thanks again for the advice!!
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mudbone

Donnt think so.. you want to see if the control is "intelligent"
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If you don't mind me saying so, this seems like an aweful lot of trouble to jury-rig something. Seems it would be easier to fix it the right way, and perhaps even cheaper. I've successfully cleaned IAC's every time I've tried.
CJB

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CJb, Your probably right, money is a little tight right now for me. Can you tell me the best way to clean it? I see it has those bellows in there. What should i spray in there? Thanks!!
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