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Discussion Starter #1
I have a 2003 Ford Taurus Sedan with 3.0L OHV and standard transmission. The vehicle has about 120K.

The problem is a severe miss, jerk, stumble that occurs at 3000-3500 RPM. Over that RPM level the engine runs fine below that RPM level the engine run fine. No starting problem and no codes. Noticeable upon acceleration and deacceleration

I have replaced...

Plugs
Plug wires
Fuel filter
Air filter.


Additionally...

Fuel pump
Strainers
Cam positioning sensor for similar problem described in TSB-05-22-12 regarding buck, miss, jerk.

The fuel pressure on the rail is erratic when this occurs but it does not seem to be an electrical condition (monitored safety switch voltage). The test for the fuel pressure regulater indicates proper operation ( pressure rises when vacuum disconnected).

Any ideas or suggestions?
 

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Just speculation -- anybody know when the variable intake manifold runner changes the length of the manifold? Could this be the runner control sticking, then releasing?

For that matter, does the 03 have this? (I was surprised, and pleased, to find it on our 04 Vulcan).
 

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Discussion Starter #3
Not sure what the variable intake manifold runner looks like. Could you describe it?

One other point the fuel pressure regulator does not look like any of the photos in my manual. I typically see them as round with a vacuum line going to the top. Mine is square and in addition to the vacuum line it has an electrical connection
 

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QUOTE (occupa @ Apr 4 2009, 01:58 PM) index.php?act=findpost&pid=715205
Not sure what the variable intake manifold runner looks like. Could you describe it?[/b]
The official Ford name is "intake manifold runner control," and it's the 3rd and 4th pictures in my thread here: http://taurusclub.com/forum/index.php?show...c=66168&hl=

The photos ended up rotated, so you have to tilt your head to the right to figure out what you're looking at.

It makes the intake length longer at lower speeds for more available torque (power), shorter for high speeds where a short runner would limit power.

Maybe coincidentally, a Google search on variable intake runner turns up a lot of posts and articles that peg 3000 rpm as a critical changeover point.

However, I think but am not sure -- if the IMRC is bad, you get a Check Engine / Service Engine Soon light.
 

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Discussion Starter #5
Unfortunately, my son is out with his friend and so is the car.

If I do have it, how can I isolate it to determine if it defective.

I have voltmeter and etc.

Thanks for your help
 

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Discussion Starter #6
The car is back and it does not have the intake manifold runner control.

Any other suggestions?
 

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When you replaced the air filter, did you clean the throttle body? Possibly clogs in the EGR passages?

Does the tach reflect the stumble (needle jerking up and down)? Not a Taurus, but I had a late 80s Chevy with a wire that flapped in the wind over 60 mph causing an intermittent open -- less likely on the Gen4 because the wiring is stuffed into cable trays and/or protected by plastic armoring.

Re-reading your OP, the erratic pressure during the stumbling suggest some sort of electrical problem. How about both ends of the major cables to the battery?

Does the stumble/buck happen in all gears (that is, running out to 3k rpm in 1, 2, D and OD)?
 

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Discussion Starter #9
Regarding the coil pack. am I looking for visual cracks or arcing?

No I did not clean the throttle body and I'll look for cloggs.

I'll check the battey cable. Yes the tach is erratic also

Yes, it does happen in all gears and I think I elimnated the transmission by leaving it in first and just running the revs. I happened predictably at 3500 RPM and I believe it certainly was increasing RPM, not sure about decreasing RPM. Interesting the miss completely disappears over 3500 and under 3500 RPM
 

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You're looking for visual cracks or blemishes like the one on the coil pack in the pic below...
[attachment=29087:IgnitionCoilCrack.jpg]
...the underside of the coil pack is a spot that usually forms such cracks...which requires removal for inspection.
 

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QUOTE (occupa @ Apr 6 2009, 07:22 PM) index.php?act=findpost&pid=715810
.... Yes the tach is erratic also

Yes, it does happen in all gears .... happened predictably at 3500 RPM and I believe it certainly was increasing RPM, not sure about decreasing RPM. Interesting the miss completely disappears over 3500 and under 3500 RPM[/b]
A real puzzle.

If the throttle body and EGR check out...

(By the way checking the EGR should be as easy as pulling a vacuum on the EGR at idle. If the engine stumbles or stalls, the EGR and its passages are, if not pristine, at least open and working.)

...Try unplugging and checking the resistance of the throttle position sensor (TPS) while slowly increasing throttle. Is there a point where resistance bounces all over the place?

The only things left (that you haven't already changed) are the crankshaft position sensor and the power control module (PCM).

Unfortunately, I don't have the DVD or the paper powertrain control / emissions diagnosis manual so I can't help on how to diagnose the last two.
 

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Discussion Starter #12
Just getting back to this, I did see a crack on the ignition coil pack and replace it.

However, the problem still exists.

I'll check the TPS to check the resistance.

Thanks.
 
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