Ford 4x4 Engine (V6 2.9 Cologne) Surging but only on Coasting & Engine Breaking

Hi there,
Title says it all, engine is the 2.9 Cologne, the engine surges quite badly when coasting and on engine breaking, acceleration is fine.
This is a fault that has been getting worse over the last couple of
years, the local Ford dealership plugged it into their computer and couldn't find the fault.
Any clues?
All help appreciated.
--

Stoatgobbler

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misinterpret.
Damn, I had something similar a while back on one of my vehicals and it drove me crazy for a while. I even took it in to a local repair shop and they put it on the computer and couldn't get a diagnosis. Turned out to be the idle stepper motor/idle solenoid/idel speed control motor (different names for the same thing). It can be hard to diagnose this, since it often doesn't show up when it's sitting idling like it would do in a shop connected to a computer, but will show up when driving. Another possibility is a vacuum leak. I'd check to make sure everything's bolted down to proper torque specs and check for vacuum leaks as well. In my case, I tore apart the fuel enjection system and all sorts of wonderful things before it was finally fixed.
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Stoat wrote:

If you've ruled out vacuum leaks and such, and it's a rhythmic surging or loping (not erratic), it's almost certainly a dirty or broken idle air control valve (large cylindrical object bolted to the upper intake near the throttle plate). Take it off, blast it out with lots of throttle body cleaner, let it dry, and put it back on. If that doesn't help, it needs replacing.
-lee
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Many thanks for the replies.
Stoat

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Stoatgobbler

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