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Old 07-24-2014, 05:35 PM   #1
krivs388
 
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Question Extremely weird idle. Suggestions?

Hey guys,

So I've got a 96 GT with the 4.6 and a 5 speed. I've owned it for about 3 years now and have dumped thousands into it, mostly mods and basic maintenance though. Ever since I've had it, it's had a weird idle and the revs take FOREVER to drop. Sometimes it idles perfectly when warmed up, other times it stays a few hundred RPM's higher than it should. It just seems like the computer is detecting the throttle hanging open longer than it should. I have checked the linkage and adjusted the set screw and what not, to no avail. The ECU isn't throwing any codes either, which is odd. I've also replaced the plugs, plug wires, coils, cat converter, ALL o2 sensors, TPS, IAC valve, MAF sensor, air charge sensor, throttle body, k&n cold air intake, and the PCV valve. After all of this, the revs still don't sound right. I am at a loss now, and before I spend a crazy amount of money on more parts, I figured I'd get some opinions. The motor also seems to bog down/lose power after about 4200-4500 rpm's. It pulls hard up until then, but goes absolutely flat right after. Maybe that is also a symptom of what ever is failing and causing the revs to drop slowly? Let me know what you think!
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Old 07-24-2014, 06:13 PM   #2
VTX1800N1
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EGR valve?
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Old 07-24-2014, 07:50 PM   #3
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I was thinking the same thing as well. I planned on changing the EGR next, then the ECU if that didn't fix it. I just wanted to see if someone else has had a similar issue. It's not only a pain having the motor not run like it should, but it's also wasting gas. It's also gradually burning out the clutch every time I go through the gears since the revs only drop when I go into a higher gear.
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Old 07-25-2014, 07:14 AM   #4
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If the EGR walve was bad, I would think you would have a severly rough idle. As the EGR would recirculate dead air into the engine. You could always test the EGR valve by taking off the small vacuum line and pulling a vacuum on the EGR valve. If the engine RPM drops and starts running rough, I would think your EGR valve is ok.

This may sound like a dumb question, but do you have any vacuum leaks? I'd expect it to be throwing a code if it was bad enough to keep the engine RPM up by 4-500, but possibly not if it clears out after you've warmed it up.

Also have you checked the fuel pressure while driving? Losing power under load would lead me to think you have fuel or spark problems, but since you've replaced the whole ignition system, that can probably be crossed off the list.
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Old 07-25-2014, 09:35 AM   #5
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Might try this before you spend a lot of money. The hanging rpms are common in the gt's of these year. Try this little iac restrictor:
http://www.ebay.com/itm/86-04-FORD-M...-/121156017287
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Old 07-25-2014, 11:40 AM   #6
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I may try that. I also forgot to mention, when the motor is cold and not up to normal operating temp, the rpm's drop perfectly between shifts. As soon as it gets warmed up, the revs go to **** again.
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Old 07-25-2014, 02:27 PM   #7
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Yes I've checked for vac leaks. That was one of the first things I checked, but all is well. I actually just purchased that IAC valve and EGR valve restrictor plate kit off eBay so we'll see how that goes. If that still doesn't help I may just replace the EGR just for shi*ts and giggles since I can grab one for less than $50. I run premium fuel in the car and I still only manage to get around 200-220 miles per tank of easy city and highway driving. I do know a failing EGR can cause poor gas mileage as well. So we'll see
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Old 07-25-2014, 02:27 PM
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