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New here and tried searching but came up with nothing similar.  My 06 LJ 4.0 (155000 miles) recently developed a habit of going to a fast idle/high rev of around 1500 RPM's or so.  If I blip the throttle it settles down until you take of again and it repeats.  I installed a Genright 31.5 gallon fuel tank and replaced the fuel pump assembly at the same time because it was a whole lot easier and I didn't want to drop the new tank anytime soon.  I had a code for EVAP system small leak which I attributed to the vent hose from the tank to the filler neck as the hose that came with the tank was rather short and had no clamps.  I installed it and put some zip ties on it until I could get a new section of line and clamp, which I did a week or so later.  I also had a code for O2 sensor which I forgot to write down but it was present long before these symptoms began.  I have searched for vacuum leaks but it's possible I might have missed one.  Anyone have thoughts on this?

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Bullseye240...I would clear the codes and see whether any come back, in particular EVAP or O2 sensor.  The idle issue could be an O2 sensor, but my first pick for this symptom would be the Throttle Position Sensor. 

Especially at this mileage, the TPS could very well be defective or malfunctioning.  Assuming that you have no vacuum leaks and that the EVAP and rollover valve hoses are routed correctly after the tank change and so forth, if anything mechanical is involved, I'd look to the TPS first.  This is a non-adjustable switch that functions like a voltage "rheostat" with direct sync to the throttle valve position.  The TPS wears out over time.  

If that does not do it, let us know...

Moses 

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Thanks Moses, I cleared the codes and they haven't returned yet so I'll check the TPS as soon as I find a bit warmer day.  No heat in the detached garage means no working out there on cold New England days.

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Appreciate the cold issue...I'm exercising our Border Collie/Aussie dog in 10-14 degree F weather each morning.  Yes, it's a "dry Nevada cold", still very cold.  Gets better when I toss  the Chuck-It ball for 45 minutes...Cold doesn't seem to bother Rusty!

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