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'02 QX4 - RPMs Skyrocket in Park or Neutral

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3.5L V6 with 4WD


I just got down replacing my exhaust from the headers back and replaced both downstream oxygen sensors when I had it off the ground with the wheels off I started it up to hear the new exhaust, it started doing this and grinded loudly when put back in park, I shut it off fast with the RPMs really high (maybe 5k not redlined) 

After putting the wheels back on and putting it on the ground it no longer makes the grinding sound. The RPMs still take off when I shift into park or neutral. Really hope I didn't strip some splines or something but that's outside of my knowledge. The throttle was sticking in drive but I fixed that with some WD40 on the throttle cable. TPS sensor? IAC sensor? MAF sensor? Transmission problem? ECU not liking the new O2 sensors? What's going on??? 



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The grinding sound was likely because the drivetrain started to spin up when the engine took off. Even if it was in neutral, there's a little drag in the trans that can get the drivetrain spinning, and then the parking pawl has a bad time when you throw it in park with the cog it engages still spinning. If park still holds the truck from rolling, you didn't kill it. I've barked mine a few times without issue.


I doubt the trans has anything to do with it. I doubt the TPS could do it, either, though if your scanner does live data, it wouldn't hurt press/release the pedal slowly (key on, engine off) and make sure the throttle % isn't bouncing all over the place. The worst the oxygen sensors or MAF could do is throw the mixture off, and that shouldn't make it rev to five grand. There's not much an engine can do to run away like that if the throttle is shut unless it's getting air from somewhere else. Could be the idle control valve stuck wide open (or held wide open by the computer for some reason), though I'd be surprised if the idle control could pass enough air to rev it that high. (If it was an earlier rig, I'd ask whether you'd taken the EGR valve off, but it doesn't look like the VQ has one.) Recheck anything you messed with that wasn't the exhaust, see if it's got any codes. You say the tach shoots up if you shift into park or neutral--does that mean it runs normally in gear? (I'm asking if you've tried it, not saying you should--I don't advise neutral-dropping it from five grand to see if it launches itself through the neighbor's shed).

Did you weld the exhaust with the battery connected? I've never had welding cause an electrical problem on a vehicle I've worked on, but I've heard that computers don't like high-frequency TIG.

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