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Topic Review (Newest First)
05-03-2017 07:36 PM
TJ2021 Just a late update, I found the problem to be the parts store TPS combine with a misfire on cyl 1. once i took care of the misfire the idle issues ended untill the misfire returned (sticky valve but with 236k im not pulling the top end apart) the idle/rpm jumping started again. Seafoamed the engine misfire subsided and the idle issue did as well!
Thanks again guys!

-TJ2021
10-16-2016 11:43 PM
bigeasy911 This may not be direct help but one thing I have learned working on electronic controlled engines over the years is when it comes to any sensors with only a few exceptions always use factory/OEM parts for these. One of the exceptions is when the actual manf and part number is known for that sensor so you can get it aftermarket. You will save yourself a ton of grief.

When you end up using a box store sensor you introduce a POSSIBLE new issues. But in your mind you have replaced it so that can no longer be the issue thus sending you down the road of finding what else could be the issue.

It sucks as the dealers mark up the parts so high. But it will save so much grief. As an example look at all the various threads on countless jeep forums with an issue that is very likely a Crank Position Sensor fault with flashing CEL and mis fire codes, limp mode. Yet people are changing all sorts of stuff out because they already changed the CPS with a box store one. I have fixed 15 of these in the last 3-4 yrs for friends. Many were fighting the issues for months if not longer and when the CPS was swapped for a mopar all was good. What got me is 3 of them had taken it to the dealership and they wanted to do intake and valve cleaners etc but not one of them swapped out the CPS for one they had in stock. h be purchases from the box stores is the O2 sensors as they are NTK and supposedly the same. But all else I would not chance out side of MOPAR.

My concern here is what possible issues may have been introduced using box store sensors. I had one of those were the person had changed basically every sensor out with cheap box store knockoffs. Ended up chasing out tails till I finally started swapping out all the various sensors one at a time with mopar ones. The TPS ended up being the issue and was the third one. See this they guy went and replaced all the others as well with mopar parts. (He had thrown away all his old ones) costly mistake.


The other issue that can create all sorts of weird gremlins that can have you scratching your head on these PCM controlled engines is ground wires. They can be very very picky about good grounds. One of the first things to check when issues do not add up to one specific thing. Personally I like to change out the OEM grounds with higher gauge ones and to add a couple extra to be sure. Being sure to having good solid contact point


Another issue can be shared grounds. If say a wiper motor shares the same harness ground as say the sensor the dirty feedback from the wiper motor especially as it ages can make the sensor do some weird freaky things. We had this issue with cam position sensors on 7.3 diesels. When the wipers were turned on the engine would start to cut in and out just as a bad CPS. Gave the wiper motor its own ground problem solved.

Frankly the electronics these days are a PITA and is one reason I love mechanical P pump diesels with manual trans. No matter what electronically gets screwed as long as the battery can start it she will run and run fine. Mhy fav combo being IH DT360> Fuller 6 or 8 speed which allows for a larger range of dif gearing and still plenty of power.
09-26-2016 11:03 AM
desert dogg On my 2000 TJ, I was getting the P0123 code for the TPS, and the jeep's RPMs would randomly surge for no reason while driving.

I tried just about everything to fix it including, Mopar TPS/MAP/AIC sensors, thorough check for vacuum leaks, clock spring, battery voltage and connections, etc.

After 3 months of focking with it, I found the problem: The ground pin on the TPS connector was a little corroded and loosely connected. I went ahead and put a new connector on it and haven't had an issue or CEL again.

Almost 20 years of TJ ownership has taught me that the components/PCM/sensors on these things are VERY sensitive to small grounding issues, more than my YJ and JK have ever been. You could easily find yourself spending hundreds of $$$ tracking down issues when the problem is just a little corrosion on the ground lug to the body or a worn woven ground strap from the block to the tub.
09-24-2016 03:45 PM
Marty, SoCal I went back up to the original post and reread this:

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

A very common cause is that the clock spring in the steering column for the airbag , speed control switches and horn is internally shorted! Remove the lower steering column shroud to access the airbag clock spring connector, (6 pins IIRC) disconnect it, the two codes will go away if that's the cause.

What happens is that the ground feed for the speed control wiring shorts out to the power feed for the speed control switches that may or may not be installed on the steering wheel, this throws off the map and TPS relationship and throws the code. The MAP and TPS sensors share the same ground with the speed control switches.

The ground wire in the steering column harness to the speed control switch to the clock spring is colored dark Blue with a dark green stripe. If you don't have speed control, and don't want to buy a new clock spring, clip that wire near the airbag connector and tape it off on both ends. I would recommend buying a new one, because usually, the airbag wiring or horn connection is ready to fail, or has failed, too.

Hope this helps!

P.S., Yep, no MAF, just old, simple Speed Density!

P.P.S. The shorted clock spring will screw with the MAP reading, check the key on, engine off Barometric Pressure reading, it should closely match the ambient barometric pressure for your current altitude and weather conditions.
09-24-2016 03:36 PM
Jerpies I thought 05+ was maf? All my jeeps are old disregard me then.
09-24-2016 03:18 PM
Marty, SoCal A vacuum leak on the 4.0l Mopar speed density EFI system will not change the fuel trims, except if it's on the MAP vacuum line. (Engines with Mass airflow sensors will show fuel trim changes with vacuum leaks) You will just get a high idle if it's large enough for the IAC not to be able to compensate for it. Basically, the system reacts as if you opened the throttle a bit. If you want to see it in action on your Jeep and you have a scan tool, pull off the vacuum booster hose and create a large vacuum leak. You will see the engine RPM increase quite a bit, (maybe to 2000 rpm) then scroll over to the fuel trims and IAC steps. You will see the trims stay normal, the O2 sensor will switch normally, and the IAC steps will drop to zero as the PCM starts to try to compensate for the high idle.

With a vacuum leak that is large enough, it might set a code for "target idle speed not met", if it idles high too long and the IAC steps go to zero trying to slow down the idle. You will see low IAC step counts if it does have a vacuum leak, so you can look for that. Usual steps are around 30, certainly should not be below about 10 unless someone messed with the base idle screw. If the throttle body is really dirty, the steps may be up to 50 or so to compensate for the lower airflow past the blade.

Tightening the top end intake manifold bolts takes all of 30 seconds with a 9/16" box wrench and costs nothing. If you find those tight, OK, you know they rest are probably OK, but if they are loose, check all the others. It's a very common cause of high idle issues. on 4.0l's and rough running on carbed 4.2ls. I've seen it enough that I always check them on customer jeeps while doing oil changes or tune ups. I have two friends that ran with the manifolds loose long enough that the engines sucked dust through the leaks and wore out the two end cylinders rings and bores. in about 20k miles.

The issue with the tiny hole in the map sensor line causing rich running just happened to my buddy on his 4.0l. Sometimes it would seal against the fuel rail where it was worn through, then hit a bump on the road and the hose would bounce a bit and start leaking again. Drove him nuts figuring it out.

It's rare, but I've also seen a loose throttle blade on a throttle body once, that caused an intermittent high idle issue. (I worked as a Jeep/Chrysler dealership master tech for 18 years from 1990-2008)

Hope this helps!
09-24-2016 01:57 PM
Jerpies
Quote:
Originally Posted by ranger4.0 View Post
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?
09-24-2016 11:35 AM
ranger4.0
Quote:
Originally Posted by Jerpies View Post
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
09-24-2016 10:48 AM
Jerpies 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.
09-24-2016 06:27 AM
Marty, SoCal 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!
09-21-2016 05:15 AM
RebelRider
TJ idle need to fix befor winter!

Quote:
Originally Posted by TJ2021 View Post
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.
09-20-2016 05:37 PM
TJ2021
Quote:
Originally Posted by RebelRider View Post
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
09-19-2016 08:20 PM
RebelRider 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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09-19-2016 05:40 PM
TJ2021 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
09-18-2016 08:26 PM
Marty, SoCal 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!
09-18-2016 04:56 PM
TJ2021 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
09-18-2016 04:44 PM
RebelRider How clean in the throttle body? Have you tested for vacuum leaks. Does the A/C blow out of the vents at idle?
09-18-2016 04:38 PM
TJ2021 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
08-06-2016 08:01 PM
redsand723
Quote:
Originally Posted by TJ2021 View Post
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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08-06-2016 07:37 PM
TJ2021
Quote:
Originally Posted by PapaSmurf789 View Post
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
08-06-2016 07:31 PM
TJ2021 Why would a crank position sensor cause an idle issue? Im going to replace it anyway just wondering. Thanks again

-TJ2021
08-06-2016 07:28 PM
PapaSmurf789 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
08-06-2016 05:55 PM
redsand723 It's easy to get to. It's on the driver's of the bellhousing, towards the top

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08-06-2016 05:30 PM
TJ2021 Thanks man, im spending tomorrow checking over everything, ill add that to the list!

TJ2021
08-06-2016 05:11 PM
redsand723 Crank position sensor

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