98 Windstar Intermittent Hesitation / Bucking ?

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I have a 98 Windstar 3.0 with 128k miles on it, and over the past few months it has had a problem with hesitation and bucking. The problem is intermittent.
My mechanic has diagnosed a few things so far and a lot of money has been spent on 'repairs' but nothing he suggests fixes the problem. I've now taken it back from my mechanic and his "shotgunning" repair technique in order to try and diagnose the problem myself.
So far I've replaced the Throttle Position Sensor and the Camshaft Position Sensor, and while that seemed to fix it for a day or two, the problem came back. (It's intermittent nature means it can run fine for days).
There seems to be no rhyme or reason to the hesitation, it can happen when you're pulling away gently, or when you're cruising at highway speeds, or during a shift, pretty much any time. Turning overdrive off, or keeping the transmission in 1st or 2nd makes no difference. Also you can drive it really hard sometimes without the problem manifesting at all. Often I'll drive it hard 10 miles to work and it's fine, then it will act up on the way home with about 3 miles to go.
As good as I can tell it feels like the engine is cutting power to the injectors or plugs all at the same time. And the loss of power can last from a fraction of a second to a whole second before it recovers. Sometimes you just feel a bit of spluttering as though only a single injector or plug isn't firing. I thought it might be a transmission problem, but I'd imagine that the revs would climb while the transmission slips, and that's not happening.
I've also had poor idling on occasion, but it's very uncommon, so may not be related.
Any ideas what to try next?
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IA

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You may have to replace the computer.
"Instant Arcade" <www.InstantArcade.com> wrote in message

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Instant Arcade <www.InstantArcade.com> wrote:

Actually... if it's a HIGH idle, it may be related.
Sometimes ymptomatic of a bad wiring harness ground or engine to body ground.
Try doing a tug test on all engine wiring harness while it's idling, may point out a connector that's making bad contact
Are you sure there's no codes stored?
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Backyard Mechanic wrote:

my mechanic said I needed a new MAF at great expense. Didn't improve the problem though. I've had no other codes since.
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Instant Arcade wrote:

No codes? It sounds like the IAC valve is dirty/worn.
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Sharon K. Cooke wrote:

Doesn't the IAC only deal with engine idle?
My problem is while accelerating, when the engine is under load. Just after shifting is most common, also when pulling away from a stop it happens a lot too. Less common but still occurring is when pulling up a hill or accelerating on the highway. Maybe twice in the past 6 months I've had a rough idle for a few seconds, but that's all.
The oddest part about the problem is that it will not occur for a while, even if you really thrash the engine. But other times you can be really gentle with it and it will hesitate every time you try to speed up.
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Instant Arcade wrote:

The IAC valve has SOME engine control up to about 2K RPM. It really does sound as if yours is sticking. The IACV is also one of the things that OBD-II won't detect with a direct code.
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Sharon K. Cooke wrote:

All this talk about a sticky or dirty IAC. What do most DIYers clean an IAC with? What is the best brand fuel additive to clean and keep it from getting messed up? Thanks
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TP wrote:

See: http://www.lincolnsonline.com/tech/00017.html
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Fuel additive won't help the IAC. At this point there is no fuel. The IAC meters air only. It picks up carbon from crankcase vapors and errant exhaust gases in the intake manifold. You "can" clean it by removing it and dousing the pintle and seat with carb cleaner a few times. Cleaning a sticky IAC is a 33/33/33 proposition. Sometimes it works great, sometimes only for a little while, sometimes it doesn't work at all. It's usually best to bite the bullet and replace it with a new one, unless you have time to kill cleaning it and then replacing it later. The sticking is usually caused by wear in the pintle shaft and bushing.
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Sharon K. Cooke wrote:

Here's a link to a short 5 sec video of the hesitation in action. This is a pretty mild version, it can be a lot rougher than this (i.e. longer duration and bigger dropout gaps).
Thanks for the help so far, I'll go ahead and clean the IAC anyway.
http://s15.photobucket.com/albums/a388/InstantArcade/windstar/?action=view&current=hesitation.flv
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Instant Arcade wrote:

http://s15.photobucket.com/albums/a388/InstantArcade/windstar/?action=view&current=hesitation.flv

Cleaned out the IAC, problem still remains.
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Don't waste any more money on a fix, its not worth it. Just live with it
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Backyard Mechanic wrote:

I don't suppose any of this could be caused by a faulty fuel pump or filter? I'd expect in that case there would be a more gradual drop in fuel pressure rather than an instant and complete loss of power, but I thought I'd ask just to make sure.
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try checking fuel pump the foot valve might be sticking
Instant Arcade wrote:

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Number 3 plug not firing all the time.
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Instant Arcade wrote:

the MAF...? At your mileage it needs it anyway. Is there a certain way to drive it to make it do it for a longer period of time to set a code? If you can get it to do it long enough you will get a missfire code that will point you to the right cylinder at least.
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ShoeSalesman wrote:

It has new plugs, new wires and a brand new MAF, much to the distress of my wallet. No replacement fuel filter yet, but I wouldn't expect such harsh symptoms from a dirty filter.
I just changed out the coil pack myself about an hour ago (for $64), and drove back to work. Okay so far, but then it can run fine for a while.
If it's still good after a week, I'll concede that the coil pack was the culprit. I'm not holding out much hope though, so I still preemptively have replacement VSS and CPK sensors on my "to buy" list.
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Instant Arcade wrote:

Still broken :(
I also put on a new DPFE last night. That was a horrible job (I just hope I don't have to change the EGR, it seems like you'd have to pull the motor to do that). Who's clever idea was it to put the DPFE on the rear of the motor facing the firewall? I mean, AT LEAST use attaching bolts that face the front of the engine bay where you can actually get to them. Oh well, there's another tool that's forever lost somewhere in the engine bay :( Now I'm all cut up and burned :(
Anyway, the new DPFE made no discernible difference either.
I have a replacement fuel filter to put on this weekend (that's one huge mutha), but I doubt it will make any difference.
Is it worth trying a new fuel pressure regulator? Would a faulty one starve fuel so rapidly that the vehicle splutters and bucks? Or am I barking up the wrong tree?
There's also the possibility of changing out the CKP and the VSS (although I have no idea where the VSS is on the 3.0 motor). I'd rather not spend more than I need to so if any knows if either of those wouldn't help then let me know. I'd expect a faulty CKP to throw a code at least.
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Instant Arcade wrote:

Here are a couple more videos, just idling and revving. Nothing really shows up here, apart from on the revving one, but I think I'm just hitting the rev limiter on that one (hard to tell with no tacho).
Idling http://s15.photobucket.com/albums/a388/InstantArcade/windstar/?action=view&current=IMAG0005.flv
Revving http://s15.photobucket.com/albums/a388/InstantArcade/windstar/?action=view&current 2867f5.flv
Since I can't seem to get it to occur in N or P I'm leaning back toward it being a transmission problem again. Is there ANY way of finding out for sure that it's the transmission? I still get the same symptoms with overdrive OFF, and also if I tell the transmission to maually stay in 1st or 2nd (with the shifter) it still does it. Does that mean it might be the transmission control solenoid (AXODE-AX4S; 4 spd; 17 bolt pan; on/off lock-up solenoid) ??
I'll try to get some better video/audio footage of the problem. The vehicle does shake/buck pretty hard when it's doing it.
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