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Unread 04-14-2009, 06:14 AM   #1
Charlee
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Engine Revs on Start up

What would cause the engine to rev up to 2000 rpm on start up and then it will drop back to 700 rpm and idle . This now happens every time the jeep is started.

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Unread 04-14-2009, 06:32 AM   #2
shortthrow50
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im tryin to figure out why my girlfriends jeep is doing the same thing. she has a 4.7. i cleaned the iac, etc,etc. her idle goes up and down to 1-2k rpms. then she throws it in drive lol. I took the thing to have it scanned and there are no codes. the tps looks good too. Im gonna try a few more things today and ill let you know if i have any luck
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Unread 04-14-2009, 07:08 AM   #3
chris_nm1
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Mine is doing the same thing but I now learning that all my problems are directly related to my BCM going bad. I had the blinking interior lights, Factory alarm going off at random and my overhead console no longer keeps track of mileage. I am in the process of looking for anoth BCM with the exact same P/N as mine.
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Unread 04-20-2009, 07:00 AM   #4
Charlee
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I took the Jeep to the dealer last week. They reset the new idle motor that I put on. That was on Thursday. Said it was good now. It is back there today. Still reving to 2200 on start up. I told them to try another new idle motor . Will see what happens today.
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Unread 04-21-2009, 10:37 AM   #5
Charlee
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There was a computer update that had to be flashed. Seems to be good now.
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Unread 05-19-2009, 12:09 PM   #6
Charlee
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4.7 HO still revs too high on start up. Even put on another new idle motor. That did not help. Seems to work okay for 3 or 4 days after having the PCM flashed but then reverts back to reving high (2200 rpm) on startup.
Has anyone else had this problem and what was the fix.
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Unread 05-20-2009, 11:12 AM   #7
normes
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I still have this problem but when I took the air box off and cleaned the (for lack of a better word) butterfly value the idle was low a start for the first about 10 starts. But know its back up to reving to 2000 at starts. And sometimes when my car is in park it will just rev all on its own up to like 1500 and then come back down and run this cycle. This ussually happens after I drive and put the jeep in park before I shut it off. Idk if these are both part of the same part failing. All I can say is that I would like to get this fixed it cant be good for a cold motor to rev that high.
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Unread 05-20-2009, 01:08 PM   #8
Some_Anchovies
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guys... have you checked for a vacuum leak yet? that will cause this exact problem.

A vacuum leak CAN also cause symptoms such as rpm's that aren't steady (ex. floats between 1000-2000 rpm, up and down.)
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Unread 05-20-2009, 01:25 PM   #9
andar
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My ZJ was doing the same thing a few weeks ago....with engine light on. the OBD said it was a oxygen censor, give that a try maybe
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Unread 05-20-2009, 08:24 PM   #10
Ric02WJ
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Quote:
Originally Posted by Charlee View Post
What would cause the engine to rev up to 2000 rpm on start up and then it will drop back to 700 rpm and idle . This now happens every time the jeep is started.
Quote:
Originally Posted by Some_Anchovies View Post
guys... have you checked for a vacuum leak yet? that will cause this exact problem.

A vacuum leak CAN also cause symptoms such as rpm's that aren't steady (ex. floats between 1000-2000 rpm, up and down.)
I have the exact same problem with my 4.7H.O It rev over 2000rpm on startup and then drop but it's not always steady
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Unread 05-20-2009, 08:30 PM   #11
Some_Anchovies
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o2 sensor can also do things like this, since there is no monitoring of lean/rich mixture.
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Unread 05-21-2009, 12:10 AM   #12
Posers
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Just to add to the mix of possible cures. Back when I was into my Honda's this used to be common after doing engine swaps. Seems that what would happen is if there was any air in the coolant lines it would shift and get stuck in the hoses that would run through the throttle body. So you would get that weird idle. A good flushing of the coolant and making sure to use the bleed valve on the upper rad hose would solve it. It was the tipping of the engine on the hoist that would cause the air bubble to move.
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Unread 05-21-2009, 01:58 AM   #13
pauler
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Mine did that when it was colder, just kinda subsided as of late ...

Not that I mind either way, personally I think it sounds badass when my truck starts up and instantly jumps to 2k RPM, its funny 'cause my truck sounds more powerful starting up than my sisters civic at redline, heh
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Unread 05-21-2009, 03:25 AM   #14
Jet 4.7
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Replace Throttle Position Sensor, my 4.7 had this also, this is very common problem and cheap to fix!
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Unread 05-21-2009, 05:07 AM   #15
Jammer73
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if cleaning the carbon off the IAC motor didn't work, which is typically the cause, then make sure the vacuum line from the side of the throttle body to the EVAP Canister Purge Solenoid(I think) is connected. Youll find the hose is not long enough & will come off when the engine revs hard, as the engine leans over. A common problem, but you will also get an erratic idle as well. Once reconnected it takes up to 10 start cycles to settle back down. Worked for me, as I was about to buy a new IAC, but now don't have to. Just need to cable tie the hose onto the solenoid, or bend the solenoid frame towards the TB. This is the hose:

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