high idle - WHILE rolling

[9's]

1st off , new here and so much great info...lovin it.
I searched and read thread after thread. I have an idea of where to go with this but have also read stories of guys spending hundreds replacing sensors one at a time with no fix.

94 4.0L 5spd 2WD 89k.
Just bought truck(yes with the probhehe)

PROBLEM IS: The RPM's dont drop between shifts. They stay around 2200. Also seems to be tied into the truck moving. For example: if coming up to a light I can put it in nuetral and the truck will idle at about 2200....UNTILL stopped>...then it kicks down to about 1200(still high, yes I know)

The truck runs fine under load. No power loss, no hesitation, no sputtering. Pulls at a linear rate through all gears.
I have noticed that if let sit at idle for a few minutes the idle WILL kick down from 1200 to closer to spec- about 800rpm....but this problem occurs when Im moving and is pretty f'ning annoying

-I've pulled the IAC...cleaned it. Took the two pieces apart(motor and valve). ALso did the quick check of disconnecting it while idleing...yes the idle changed. pretty much died.
-Cleaned the MAF with QD elc.cleaner
-Took off the TPS and tried to clean but there not much ya can.
-Checked air filter-its new


I'm going to pull codes from it tonight but there is no CEL. looking for a possible stored code
To me it sounds like it has to do with the TPS- since this 2200 idle happens only while truck is rolling....soon as those tires stop....idle kicks down.

What other advice do you pro's have for a newb like me. I've tried to get it with just past threads...but mostly they are stationary idle problems, so I made my own

 

[Stranger ranger]

there is an array of things it could be. ie; vacuum leaks are the normal cause. Any coolant in the oil? check the plugs for abnormal residue on them. few other things, some pros should chime in any min. I've been fighting one for a while now.

 

[dogboy]

Hmm... mine does this too, but only most of the time, not all of the time. And when I come to a stop, the idle is high for a second or, two, then it drops to around 1000rpm. Pretty much the same as yours.

 

[rangerrager]

smoke test it the easiest way to find a vaccum leak and the only way i will do it at work.

 

[Yellowsplash]

One cause is not letting the idle lower itself before taking off. If you jump in the truck and just throw it in gear and go, it will do this. Try starting the truck and sitting for about a minute and then take off.

If this doesn’t help, I would look for vacuum leaks...specifically the intake manifold gasket.

 

[9's]

One cause is not letting the idle lower itself before taking off. If you jump in the truck and just throw it in gear and go, it will do this. Try starting the truck and sitting for about a minute and then take off.

I have been waiting untill its warmed up to drive it. It'll happen all the time, even after driving it for an hour.

I'll look into 'smoke testing' for a vacuum leak...never done it but cant be that hard. If not I'll spray around for one.
I'm also going to meter out the TPS to see if that is ok

 

[9's]

Hmm... mine does this too, but only most of the time, not all of the time. And when I come to a stop, the idle is high for a second or, two, then it drops to around 1000rpm. Pretty much the same as yours.

Anyluck with yours yet Dogboy?

Ya me neither lol.
Checked IAC, TPS, MAF(cleaned), PCV, checked for vacuum leaks....nothing.
This idle problem seems like its not the 'normal high idle' problem every 5th post is about.

This is tied into the truck moving. It drops by at least 1000-1100rpms when I stop.

5spd.

ehh- I'll keep pluggin at it

 

[Yellowsplash]

In standard transmission trucks, the IAC will actually add additional air into the engine soo the rpms wont drop as fast. You'll notive that they drop to around what rpm the next gear would be at. This results in smoother shifting. Try downshifting to 2nd and riding rpms down via engine braking till you reach around 1000 then press the clutch in as you come to a stop. If the idle stabilizes at around 1000 rpm then this is normal, if it rises back up to 2000 or more than you have other problems.

 

[BlackBII]

I may be wrong, but don't the newer trucks have a VSS, Vehicle Speed Sensor?

Which is there to keep the RPM's up ease the transition between gears....

 

[Forrest]

take a look at your idle control valve it is located on or next to throttle body.

 

[turbo cat]

this is normal like had been stated above for smooter shifting. Sometimes if you blip the throttle it will drop revs

 

[rickcdewitt]

And when I come to a stop, the idle is high for a second or, two, then it drops to around 1000rpm. .

that part is normal,the coast computer strategy floats the throttle a little for NOx emissions.it shouldn't stay up at 2000 though...

 

[srteach]

I had this problem. 2500 for about 30 seconds after forward movement stopped, then down to 1200. Found a small vac leak, partial fix. Checked the O2 sensors. They were very slow on the high/low switching speed (about 1-2Hz). Replaced them (all 3), and the problem disappeared. 160K on the original O2 sensors.

All this was after I found and fixed 3 burned exhaust valves out of 6. Gee, the engine has real power now.

 

[Stranger ranger]

where was your vac leak at?? and what gave you the idea of burnt valves?

 

[Hahnsb2]

My dads 2.3 did the same thing, turns out the TPS/throttle plate was adjusted wrong, backed off the screw until proper voltage spec was obtained and it ran perfect after that.