Need a little help.  I purchased a 04 Ford Ranger FX4 with a 4.0L and 5sp manual transmission.  I purchased it as-is with the understanding that it had a few issues.  The vent only blew on defrost no matter what setting it was on.  Turns out there were 3 vacuum leaks which have been fixed....too easy.  

I'll try to explain the next issue as best as possible.  When parked, I can run the RPM's up to 4k and release the throttle.  The RPM's drop back to normal levels (800 to 1000 RPM).  However, I can be driving down the road at say...45mph...in 3rd gear at around 3000rpm's.  If I depress the clutch and coast to a stop the engine will only drop down to about 2500 RPM's and hold there until I come to a complete stop.  Once I stop it acts normal.  I could put the truck in first gear, accelerate and increase the RPM's to 3k again, depress the clutch, and again it will hold at about 2250-2500 RPM's until I come to a complete stop.  I cleaned the Idle Air Control and after no change I replaced it.  I'm still getting the same issue.  The key to the entire thing (I believe anyhow) is that the idle drops only when I come to a complete stop.  I thought it might have been a vacuum leak issue but after smoking all the lines I believe we found them all.  What are the possibilities?  Wheel speed sensor?  Any help would be much appreciated!  Thanks!

CMS

You did a great job explaining it, and it's doing exactly what it's supposed to do!!!!!

Many Ford vehicles actually open the throttle (or idle air bypass valve) to increase engine speed a bit, under lift throttle. It's annoying as hell, because it becomes difficult to use just the throttle to match speeds on the highway, in fast moving, dense traffic.

The reason for this is to reduce misfires and to keep the catalytic converter hot.

If you think of certain motorcycles without mufflers, you often hear them sound a bit like popcorn (pop, brrrrrr, pop, brrrrrrr, pop, pop, pop) under closed throttle deceleration from high speed.

Misfires occur under closed throttle deceleration because there is very little air/fuel in the combustion chamber. The engine is at high speed and the throttle is completely closed. Very high vacuum in the intake manifold. It's difficult to ignite this mix!

Other manufacturers completely cut fuel and ignition under lift throttle conditions. This results in the catalytic converter going cold quickly. Upon throttle application again, there is a short period of time of less than ideal catalytic converter operation and increased tailpipe emissions.

I'm sorry you are chasing this "problem" and you spent time/money on it.

As I mentioned, I HATE this method of operation. It's contrary to what a driver "asks" for and most good drivers would rather have the system where fuel and ignition are shut off 100% under lift throttle.

There is a possible solution. You could restrict the air passage for the idle air valve with some form of calibrated orifice. Enough to allow the engine to cold start, and run the air conditioning compressor without the RPM dropping. But not enough to allow 2500RPM operation. I make no promises in your case, but I believe it will work, if done with precision.

Thanks for the help!  If this is what it's supposed to do then I'll learn to live with it...however it does this sometimes and other times it's fine.  The reason it seems different to me is because I had a 94 Ranger with a 4.0L and manual transmission as well and it didn't do this.  It would drop to about 1500 RPM and then drop down to regular idle when I stop.  

Was the idle speed increased in newer year trucks?  It's just annoying because in a parking lot if I go 100 feet in 1st gear and then push in the clutch and coast to a stop sign it's screaming at 2500 RPM.  Can a dealership adjust this or is that not possible.  I can understand if it was 1500 RPM but it's never consistent.  Sometimes it sticks at 2000...sometimes 2500...and a few times even at 3000 RPM.  It just seems too inconsistent to be normal.  


CMS

Well, you could have a vacuum leak contributing to the problem. Clamp off the power brake booster hose with vice grips. See if that helps. Also, look and listen carefully for leaks. The computer will add additional air regardless of whether you have a leak or not. So, that's a possibility.

I see your point, the RPM's are quite high. Make sure the throttle plate is clean and closes fully.

My '99 Contour did the same thing.  In the contour there was another problem referred to as "moosing" when the idle would fluctuate while stopped and the RPMs would hang between shifts.  My solution was the same as above.  I got half a dozen copper, or brass, pipe end caps that would fit inside the hose leading to the IAC valve.





I drilled increasing hole sizes in the end caps and experimented with the hole size to see what would keep the engine running.  The first hole that worked well in the summer time didn't allow a cold start in the winter so I had to go one size larger then everything worked well until I sold the car.  I didn't have problems passing emissions tests either with the fix.





If you google "Ford Contour moosing" you should be able to find the hole sizes to try drilling in the end cap.