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Old 06-28-2015, 12:17 AM
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help! over-revving

My son-in-law has a grand cherokee 4.7 2002,he said he tried to start it and there was a loud backfire,he tried to start it again and it acts like the accelerator is stuck to the floor !! I tried unplugging the throttle position sensor and the idle air control and the coolant temp sensor and it still acts like you are flooring it!! I removed the air intake to view the thottle body butterfly and it is still in the closed position !! I also removed the positive terminal to reset the computer and it still races so high I immediately shut it down . the throttle linkage is NOT sticking it works fine ! Anyone out there had this one happen????? HELP!!!

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  #2  
Old 06-28-2015, 08:38 AM
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Re: help! over-revving

Welcome to the garage.
Very strange that it can rev up with the throttle closed, it should suffer from lack of air. But for some reason the PCM is injecting too much fuel. The number one in fuel injection input is the MAP sensor. Do you have a scanner which shows live data? If so, you can check the MAP. With engine off (but ignition in run), it should be close to local atmospheric pressure.
Also, you did disconnect the TPS, but did you check with live data what the TPS value was at idle? Nomal is around 12%.
If that is ok, i would expect the PCM.
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Old 06-28-2015, 09:35 PM
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Re: help! over-revving

Frango100,I just bought a scanner not sure of it's capabilities but I will know soon ! This one has really had me stumped! If it turns out to be the PCM where is it located and would it be possible to buy one used ? And how much do they cost ? Thanks in advance, you have confirmed a possiblity I was thinking of!!
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Old 06-28-2015, 10:31 PM
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Re: help! over-revving

The PCM is mounted on the bulkhead in the left corner of the engine compartment, when standing in front of the jeep. The overflow reservoir sits just in front of it. But lets first see what the scanner will show. At this point i would more suspect a sensor input problem then the PCM.
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Old 06-30-2015, 10:48 AM
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Re: help! over-revving

Frango100,I am working full time and it looks like I won't get a chance to check this out until Friday,Thank you for a direction to go ! I will keep you posted ! Thanks again sincerely Duane13
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