The unfixable

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JRibbens

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I own a 2001 Dodge Ram 1500 with the 5.9 Laramie. One day I was washing my truck, while doing the undercarriage my truck suddenly revved up to 2,500 RPM. Naturally I suspected a vacuume leak. I smoked the system found no vacuume leaks. The next step I did was replaced the TPS sensor. No cigar. I replaced the IAC with no success. I replaced the maps sensor... still nothing. Removed and cleaned the throttle body checking the cable I found and fixed nothing. Frusturated I figured my computer was shot. My first replacement came and got new issues with shifting, it still revved to 2,500 RPM. Sent the computer back and got another. The shifting problem went away but it still revved up. Frustrated I brought it to my technician. He figured it must have an intake gasket leak. He replaced it. It fixed nothing. He then proceeded to once again replace all my new sensors again. He checked voltage on the wires to the sensors(I never was able to verify what was found). Needless to say he didn't fix it. So he has decided the computer is bad. I told the shop owner I'm taking the truck and I'll pay for the parts but not the labor. This has been three months now. I need a solution or direction to go in.
 

SportRam00

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Any check engine lights? Since you were washing underneath the truck, could it be possible you knocked a transmission sensor connector loose?
 
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JRibbens

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I have. I check engine lights and am not sure what sensor on the transmission would cause this
 

SportRam00

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True... I was just thinking maybe you knocked speed sensor loose while you were under there... But why would that affect the engine RPM.

This is quite an interesting problem. It would take a sizable vacuum leak to allow enough air through to cause the engine to rev that high. Something has had to have been knocked loose while you were washing your truck. I believe there are some vacuum lines on top of the transfer case that control the CAD. Maybe check there as well.
 
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JRibbens

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I agree I thought it would've been a loose vacuum line as well but when I smoke the system I would've seen it. I will get under there and check again
 

Casper

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Just reaching for straws here, but could you have shorted an oxygen sensor on the exhaust?

I don't know why that would make it rev even with a continuous ground, but its something to check.
 

Yeret

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Sounds like a large vacuum leak or IAC stuck open to me. I used to have this same issue occasionally and replacing the IAC fixed it.

Were you messing around anywhere near the front of the transmission? There's a lever on the tranny connected to the throttle lever via cable. I've never heard of it happening, but I suppose it could be possible that hitting this lever could yank the cable through the adjuster and pull/hold open the throttle blades which will obviously skyrocket your idle.

Also, 2WD or 4WD?
 
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dudeman2009

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What were you washing with is another good question. A garden hose, pressure washer, broom?

None of the sensors underneath would cause high idle like that. Hitting the vacuum line on the front 4x4 actuator might cause that issue, so might the vacuum line on top of the transfer case.

Yeret has a point. If you were using a brush/broom or pressure washer, its not impossible for the throttle cable on the transmission to have noved /bound up and caused high idle.

The only other things that cross my mind is the body harness, the pressurized water might have caused and issue with one of the body connectors. You can try unplugging them all and plugging them back in to see what happens.
 
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JRibbens

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I have rechecked the entire vaccume system. No leaks found. The wash was an automated car wash. I spent another three hour disconnecting/reconnecting cables. I am sooo lost... I thought maybe bad voltage but the alternator tested fine.

This is a 2wd
 

dudeman2009

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Maybe we need to start over from the beginning. There are only a couple things that can cause high idle.

Massive vacuum leak (as small ones are compensated for by the IAC valve). IAC valve failure, Throttle position.

When it gets right down to it, even if the computer desires to rev up to 2500rpm, it cannot as it's limited by the amount of air it can move. In this case, the engine is nothing more than a huge fan. No sensor reading can change the throttle position to cause it to rev higher than a certain amount.

Try actuating the throttle body by hand, all the way open to all the way closed, force it if you have to. If you're unsure, remove the air filter housing and visually inspect the throttle plate.

Second, check all the large vacuum lines by pinching them off and seeing if that fixes the issue. Brake booster, PCV hose, accessory vacuum lines.

The last thing that I can think of that has the ability to control air into the engine is the IAC valve, try unplugging that and see what happens. I cant remember if it will hold its last position or revert to the shut position, Dapeper or Yeret might know more.

Aside from that, the only other place air could be leaking from is between the throttle body and the intake, intake and heads, intake and plenum pan.
 

Yeret

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When I was fiddling with my idle last week, I noticed that the IAC pintle was open when the engine was off and cold. When I started the engine, the pintle moved to a more closed position but as the engine warmed up, it slowly opened back up. So, it seems to sit at it's "last known" position but closes when the engine is cold-started and then opens back up again. Ignition on, engine off seems to have no affect on it's movement as far as I could tell.

And I'll second what dudeman said. There are no sensors on the engine that's going to cause the engine to idle at 2,500 RPM. Idle is a combination of throttle blade opening and the IAC. If either opens too far, the idle will obviously rise as well.

Since the IAC is controlled by the computer, it's possible that a fault in the computer could cause it to basically just say **** it and hold the IAC fully open irregardless of the fact that the idle is sky high.
 
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