2.5L throttle issue

[LilBlueTruck]

I have a 2000 2.5L 4x2 with a 5-speed. Since last spring, when letting off of the throttle, the truck would idle erratically. For example, when putting the truck into neutral at a stop-sign, the throttle would surge up to 2,000 - 3,000 rpms for several seconds before dropping back down to the normal 800. When letting of the throttle in gear, you can feel the truck continue to move forwards, not slowing as much as expected. This problem has been continuing for some time, but it is not consistent. Back in June, I took a trip to PA (nearly 600 miles round-trip) and I was worried that the problem would be an annoyance the whole way; it ran fine with no throttle surge when getting off of the highway or driving around town. It seems to be more of an issue during local driving. I did unplug the IAC back awhile ago to see what happens and, after the engine heated up, it ran smoothly with it unplugged. This would suggest to me that the IAC is causing the surging, but what is causing it to react this way? About a year ago, I did a lot of work to the truck including new timing-belt / cooling system flush / new thermostat / housing with temp-sensors / hoses / O2 sensors / IAC / TPS / plugs / serp-belt. I tried searching the forums here and noticed somebody writing about the DPFE sensor. I figure that I should get a little more input before I spend money replacing parts. There has been no "Check Engine" light, by the way. Any help will be appreciated. Thank you.

 

[RonD]

The throttle plate return spring is what closes the throttle when you take your foot off the gas pedal.
If this is getting weak or throttle shaft is getting sticky then you would have those symptoms.
The air flowing past the throttle plate when it is open will hold it open if it wasn't for the spring.

The IAC is a controlled vacuum leak and is only used at idle, if engine is warm and you unplug it the idle should drop down to about 600 rpm, when plugged back in idle should go back up to 750-800 rpm range.
If idle doesn't drop there may be a set screw on the throttle plate that has been adjusted too high, this wouldn't cause the surging so is not the issue.
Or throttle/cruse control cable may be too tight.

I would retest IAC when engine is cold, unplug it and idle should drop from 1,000 rpm, then plug it back in, idle should surge then go back to 1,000 rpm

When first started an IAC equipped engine should surge to 1,500+ rpm then drop to cold or warm idle level that's the computer testing the IAC valve motor.

EGR system could cause surging if vacuum line was broken, the EGR valve's vacuum line is only active during above idle rpms, if it was broken then when activated that would create a vacuum leak which would cause engine to surge.
The DPFE is the valve that controls that vacuum line.
Also EGR valve's diaphragm could be broken so a vacuum leak when active.

 

[Road Angels]

Unplug the vacuum line going to the egr valve and plug it if the problem goes away then its in that system, you could have carbon build up in the egr valves seat, it can come and go, in that case plugging the line wont help, cause the carbon is keeping the valve from closing correctly, valve will have to be cleaned or replaced

 

[LilBlueTruck]

It seems that if the problem were mechanical, it would affect the drivability of the truck on a regular basis, not come and go. Vacuum-leaks don't come and go; they hang around until you find them and fix them. I drove the truck to and from work today and it ran fine. This leads me to believe that the erratic idle is being caused by the IAC, especially since I unplugged it when the engine was warm and it ran fine. The IAC-valve itself should be fine since it is new, but it is receiving some input from elsewhere even when the engine is warm telling it to open when not necessary, causing the engine to race. I will probably have to take a look at the EGR system - this has not been touched except when I unbolted it to remove the upper intake-mani for changing the plugs. By the way, the truck has about 180,000 miles on it, which may or may not be a vital bit of information.

 

[98singlecab]

my 98 is having the same issue. idles high when coasting to a light but drops back down when i stop the truck completely.
i've changed out the
tps
iac
pcv
cleaned the maf
throttle plate.

i'm going to to unplug my iac this morning to check. and see how she runs today