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Discussion Starter #1
I have a 05 TJ, 4.0L, its having the oddest idle issue, at first start up it revs to 2k after a few sec it drop to 1500, then once warm it drops and stays at 1100(drops down to about 900 once in gear). so far Ive replaced the IAC, TPS, and the MAP. Ive checked for vacuum leaks and i cant find anything. ive searched a bunch of different sites and still cant find anyone who has any idea what it could be. The other problem is when driving above 50mph the rpms with no input from the throttle jump 500rpm the drop back down then up again, it dosent do this all the time. The only thing i can think of is its a vacuum leak some where in the EVAP system, any help would be awesome, thanks guys

-TJ2021

p.s. it is throwing 2 codes P122 TPS circuit low input and P68 MAP sensor throttle position correlation.
 

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Where did you get the tps? Sounds like the symptoms of a bad one and if you have a code for it. I've found the box store ones to be unreliable and can go bad in just a couple of days. Stick to dealer part
 

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Discussion Starter #6
Why would a crank position sensor cause an idle issue? Im going to replace it anyway just wondering. Thanks again

-TJ2021
 

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Discussion Starter #7
Where did you get the tps? Sounds like the symptoms of a bad one and if you have a code for it. I've found the box store ones to be unreliable and can go bad in just a couple of days. Stick to dealer part
its a box store part, i havent driven it since i installed it but the idle is still high

-TJ2021
 

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Why would a crank position sensor cause an idle issue? Im going to replace it anyway just wondering. Thanks again

-TJ2021
When mine went out it wouldn't idle for shit, backfire every shift,, buck, cut out, die at stop signs, die on bumps, etc. I like to never made it home

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Discussion Starter #9
Update, the jeep began to run fine for about a week then the problems slowly returned, i replaced the TPS again(warranty from advanced auto) let it sit with the battery disconnected for about 5-6 hours. Hooked up the battery started it up still idles high, same as before 2k at start-up then down to 1500. Would replaceing the TPS with an OEM part fix this or could it be something else? No codes thrown yet (last time it only threw the code once)

-TJ2021
 

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Discussion Starter #11
I cleaned the throttle body and checked for vacuum leaks the first time it acted up also replaced the IAC and MAP, the A/C does blow throw the vents at idle. It also runs rich when it idles high. I dont know if it matters but it does misfire once and awhile on cyl 1 (sticky valve, been misfireing on and off for almost 3 years but with 225k im not going to open up the motor. it will misfire for a day then run great for a month)

-TJ2021
 

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Check and tighten the intake manifold bolts, the tend to loosen over time and cause a vacuum leak. I recommend checking and re-torqueing them every oil change.

You can plug each vacuum line on the intake except the map sensor and see if that corrects the idle, sometimes a brake booster leaks internally.

Hope this helps!
 

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Discussion Starter #13 (Edited)
Thanks guys, i drove it today (about 80miles) the idle now hunts when stopped in gear from about 500rpm to 800rpm, it doesn't do it when in park or neutral(1200-1500rpm not in gear). Ive never removed the intake manifold but ill check the bolts

-TJ2021
 

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If you can hook it up to a basic hand held scanner with live data, it will tell you a lot. The symptoms sure seem to point to a faulty coolant temperature sensor. That would explain the running fat.


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Discussion Starter #15 (Edited)
If you can hook it up to a basic hand held scanner with live data, it will tell you a lot. The symptoms sure seem to point to a faulty coolant temperature sensor. That would explain the running fat.


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Would that cause the idle at start up go from 1500 to 2000rpm? I wish i saw your post before i left work I ran the jeep on a live streaming scanner and the TPS would read 12.2% absolute throttle position witch in normal but with no input from the throttle it would read 25.5%. When it would read 25.5% it would bog down to 500rpm and run rich. after about a sec it would go back to 12.2% and the idle would go back to 750rpm. I didnt think to check what the temp sensor was reading. Not to sound like an idiot but what should i look for on the scanner to check the coolant temp sensor, what the scanner is reading compared to what the jeep is reading? (I do auto body for a living not a mechanic)

-TJ2021
 

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Would that cause the idle at start up go from 1500 to 2000rpm? I wish i saw your post before i left work I ran the jeep on a live streaming scanner and the TPS would read 12.2% absolute throttle position witch in normal but with no input from the throttle it would read 25.5%. When it would read 25.5% it would bog down to 500rpm and run rich. after about a sec it would go back to 12.2% and the idle would go back to 750rpm. I didnt think to check what the temp sensor was reading. Not to sound like an idiot but what should i look for on the scanner to check the coolant temp sensor, what the scanner is reading compared to what the jeep is reading? (I do auto body for a living not a mechanic)

-TJ2021


It would be as simple as comparing the live data temperature reading to your dash gauge. It could be giving the computer a false temp reading therefore dumping or leaning fuel. At cold start up it should be reading well into the double digits and climb as the engine warms up. Look for spikes in the coolant readings throughout this entire process. If it is nice and steady and stops at operating temperature, the sensor should be good. I'm not saying this is your issue, but it's definitely something to look at.
 

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I've seen where the MAP sensor plastic vacuum line has a small hole worn in it from rubbing on another component like the fuel rail. Sometimes it seals against that object and sometimes it doesn't. A leaking MAP sensor vacuum line will have a much greater effect than the coolant temp sensor! It will cause very rich running. Remove it and inspect it carefully for small splits or tiny holes. If you have a hand held vacuum pump, you can leak test the line by leaving it connected to the MAP sensor and applying vacuum to the manifold side, it should hold vacuum. If in doubt, replace it.

I would still put a wrench to the manifold bolts, the end ones tend to loosen over time due to the differing expansion rates of the aluminum manifold and iron head.

Hope this helps!
 

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Check your fuel trims first and foremost. Crank sensor won't cause a high idle.

Don't bother tightening any bolts or replacing any parts until you check fuel trim data.
 

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Check your fuel trims first and foremost. Crank sensor won't cause a high idle.

Don't bother tightening any bolts or replacing any parts until you check fuel trim data.
This is not true. Crank sensor in my 2000 xj caused my idle to be all fucked up. Idle high and big down low and stall out. Eventually it just quit and the jeep would no longer start
 

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This is not true. Crank sensor in my 2000 xj caused my idle to be all fucked up. Idle high and big down low and stall out. Eventually it just quit and the jeep would no longer start
Bad crank sensor is most likely to cause a stall, he never said his jeep dies out. Not saying it couldn't cause poor idle, but every crank sensor I've replaced on jeeps was due to stalling condition. What codes are stored now?
 
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