Mysterious Engine Revving

[gorilla_skater]

My 98 XJ was running fine all weekend during the Fall Crawl, but I had a problem on the way home. I stopped for gas and when I started it after filling, it revved up to 3000 rpm and wouldn't kick down. I was checking the throttle cable and other visual stuff, when Lou from Elias 4WD Center said hi to Laura and asked how it was going and she told him we were having a problem. He suspected either a vacuum leak or maybe the TPS. He tapped the TPS housing, but it wouldn't kick down. One of the vacuum line elbows was dry rotted but nothing to cause a massive vacuum leak. I was afraid to put it into drive at 3000 rpm, but Lou suggested that I shift it into drive right after I start it before the rpms get high. It worked and then seemed to run normally. So I pulled it away from the pump, stopped and restarted it, and it started fine. I just tried it again now and everything is working fine still.

Soooo... I don't think it's a vacuum problem. Should I just change the TPS? Any other ideas on what it could be?

BTW - I said it in another thread, but Lou from Elias is a class act. He and his wife/girlfriend? stopped to help us and then they even aired up my tires and offered to follow us home for a while to make sure we were OK.

 

[beaker]

More likely to be a sticking idle air control motor, not throttle position sensor.  Though another sensor could be tricking the computer into reving it high too.  You could try one of those OBD2 readers on it if it acts up again.

 

[MasterDrazhar]

Idle/air control......I'll agree with beaker.......remove & clean, its on the front of
the throttle body, 2 bolts hold it in place.....small 2 wire harness goes to it.....
theres an O ring in there, so be gentle removing it.....carb cleaner works to
clean it.

Give it a shot, could be a $2.65 fix. :wink:

That's what was happening on mine Rey, cleaned the plunger on the air/idle motor and the rest of the throttlebody.

I also have an OBDII scanner if you want to check for other sensor issues.  Is your engine light on?

.

 

[gorilla_skater]

Beaker, Detour, MasterDrazher, It's unanimous. The Engine light did not come on and it's working fine right now, so I'll give the IAC a cleaning and see if it happens again.

Thanks guys.

 

[gorilla_skater]

It happened again today, but it didn't go to 3K only 2k. No engine light. So I cleaned the throttle body and the IAC plunger. Buttoned everything up started it and now it's back to 3k. When I was cleaning the IAC plunger, I was pushing it in and out to get to all of the plunger. Could I have F'd up the IAC when cleaning it?  :question:

 

[gorilla_skater]

Just read this on another site... Doh!

"You can take the IAC out of a 4.0 and clean it. Do not force it open and shut by hand. It is not a plain solenoid, but a stepper motor, and can be damaged by forcing. Clean it with carburetor cleaner or other such solvents, and check for wear on the pintle at the end."

 

[massxj]

oops

 

[Rubi04TJ]

I have one I belive, I will bring it with me to teh rally for you.

 

[gorilla_skater]

I have one I belive, I will bring it with me to teh rally for you.

Thanks Zach, but I'm not sure if I'll need it. I decide to give it another go and this time I removed the throttle body and I cleaned the housing where the IAC seats with a toothbrush. There was tons of crud in there. This time after I buttoned it up, it started perfectly.

If it happens again between now and the Rally, I may just take you up on it.

 

[beaker]

It sounds like the sticking may not be the plunger but what the plunger seats against.

 

[MasterDrazhar]

Rey,
  I took mine completely appart, did not know you shouldn't do that...   (*Took the entire plunger and shaft out of the stepper housing*)

  Mine works fine.

.

 

[gorilla_skater]

Rey,
   I took mine completely appart, did not know you shouldn't do that...   (*Took the entire plunger and shaft out of the stepper housing*)

  Mine works fine.

Yeah, that's definitely the deal with cleaing the TB... take it off. You can do a much more thorough cleaning job, plus it makes it easier to get to that bottom IAC torx screw.
Regarding the plunger, I unscrewed the whole thing and took it out too. So that bit about not manually pushing it, may be right but who knows. To me, it was the only way to get to the whole plunger to clean it. Running like a champ right now.