about ready to send the truck to the junk yard. recheck for vac leaks, put on a new MAF sencer . . . no change. if i disconnect the ICV it still idles at about 800. I removed the egr vac line & found no vac on the line at idle? should there be. removing the icv at idle with no change in idle speed tells me there must be unmetered air intering the system. i removed the power brake vac line but found no problem. removed the vac line to the heat/ac control . . no change. with the icv disconnected sometimes it will idle at 1000 then all of a sudden it will go to 800

 

About the only thing left is a sucked in intake gasket.

 

Good trouble shooting & feedback. With the IAC disconnected & the idle sometimes dropping from 1000 to 800 is a good clue, but I don't have a good explanation or suspect for it right now. It sounds like something switching off, like the AC cycling off as I ask earlier, but you said it does it with the AC turned off.

There shouldn't be any EGR feedback at idle, so No vacuum on the EGR diaphragm vacuum line at idle is right/as it should be/ok.

The other suspect might be the vapor recovery vacuum switching valve leaking, or not seating/sealing well. So maybe try disconnecting it at the intake manafold connection point & plugging the vapor line & its intake manafold vacuum connection port, drive it & see if the misbehaving idle stops acting out. Could do the same for the EGR vacuum line & do a test drive. OR, when its in one of its high idle fits, pull & plug the EGR valve & vapor recovery vacuum lines, one at a time & see if you get any response.

Once the engine idles down, the problem seems to be mostly gone away, so it may be more difficult to locate.

Then Murphys Law may be messing with you & you might have more than one problem. With the IAC Disconnected, the engine idle should drop & the engine might idle rough or stall, yet it seems to go right on idling at 1000 rpm & when it does drop, its only to 800, so yes I agree it sounds like you have an unmetered vacuum leak somewhere & I agree with slc10844 that intake manafold gaskets belong on your suspect list.

So maybe break out your torque wrench & try re-torquing the intake manafold fastners to spec in a crisscross fashion & see if you find any loose fastners.

Was the PCV valve rubber plumbing parts ok, no dryrot, splits, or soft mushy connections??? Same for the DPFE hose connections below/underneath the EGR valve, where they connect to the differential pressure orifice tube ports. It gets plenty hot down there & those hoses tend to get baked out & become hard/split/dryrot.

How did the under hood vacuum reservoir & its vacuum hose trouble shoot go. The clue you posted about hearing the blend door pop closed is a good clue that you have a vacuum leak some where, so keep looking & testing as has been suggested by disconnecting & plugging vacuum lines, to isolate various vacuum operated systems & see how it goes, or disconnect lines & apply a vacuum with your mighty vac, or the like, to see if those vacuum operated systems will hold vacuum ok & not quickly leak off.

More thoughts for consideration, keep us posted on your trouble shoot.

 

problem solved. recap, CEL on with PO171, when i come to a stop (stick shift) put in the clutch, engine would race to 2500 rpm then idle down. The egr valve was it. I removed it, flooded it with wd40, worked the valve vac. over and over, put it back on, cleared the code and its been working ok for 3 day and 200 miles.

 

OK, good feedback, can't argue with success, but its strange that a sticking open EGR valve would cause an increase in engine RPM????!!!!

 

I replaced the idle solenoid , now my truck idles perfect, same problem you had.