Idle spikes after running at freeway speeds

[Rice01]

When I get off the HWY on the way to and from work any time I engage the clutch the RPM's shoot about 2grand higher then wherever it was in gear with no throttle applied. I have modified the exhaust by removing one cat from the pair but rewelded a bung for the down pipe O2 sensor. leaving the one cat gave it the mello tone I wanted and there are no emissions laws in my county so im good to go on that before the sierra club chimes in. back to my question, I was told these v6's have an emission part called the teapot or something similar that increases the rpm during deceleration to blah blah smog stuff, when the problem started it would only spike for a moment so I thought this to be normal but over time it has taken longer and longer to kick off. not to mention I sound like an ass clown every time i'm at a stoplight. any help is appreciated. could the o2 sensor reading wrong cause this?

 

[RonD]

No, can't see O2 sensor issues in that symptom.

Yes it is "normal" for RPMs to stay the same for a second or two when clutch is pushed in, this is for shifting gears and yes, lower emissions.
But not normal for RPMs to increase.

Found this on another site:

If your engine surges/has a high idle/rpm's shoot up when you press the clutch (my ranger had a 5 speed), check the canister purge solenoid. It's located on the driver's side fender by the engine compartment fuses. Mine started doing it about a year ago, and I finally got tired of it. I traced down every vacuum line in the engine compartment, and by process of elimination, found the purge solenoid bad.
It has a little cap that you can turn, (it's snapped on). Remove the cap, and there is a solenoid vent with a foam filter around it. If the vent becomes plugged, it will cause the surging. (Just for reference, mine had 3 vacuum lines, one going to the intake manifold, one going to the charcoal canister, and a small one, which supplies vacuum power). My Ranger had 170,000 miles on it when it started doing it.

Also found a TSB that mentioned TPS(throttle position sensor) wiring as a possible cause.
If you tap the gas pedal a couple of times do the RPMs drop?

Also check the throttle plate return spring make sure it is working.

And just to take it off the possible cause list, disconnect the cruise control cable from the throttle for a few days, and see if problem reoccurs.


I wouldn't think it would be an IAC(idle air control) valve issue, the IAC valve fully opened could get RPMs up to about 1,800rpms, and by your description RPMs can climb higher than that based on current RPMs when clutch pedal is pushed in.