88 XJ 4.0. new tps. new cts. new iac. still idles at 1000rpm in gear. seems pretty smooth now though but all my friends jeeps idle at like 500 or 450 in gear. what the heck is wrong with mine. new wires, plugs, cap/rotor. once when i tried the iac i pulled it, cleaned it and put it back in and it idled perfect. next day it was right back to normal. tried oiling it the other day with no change. then switched with my friends jeep that runs fine and still didnt fix it. could it be vacuum? what do i search for? help!!!
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ive tried everything! still idles high!
- Thread starter team510
- Start date
XTurboJ
NAXJA Forum User
- Location
- Orem, Utah
My 90 was doing much the same, but it would Idle around 1500 in neutral. Take it to a shop and have them check for an exhaust leak. I replace the MAP, IAC and the TPS much like you, but in the end all it need was a crack in the exhaust manifold to be welded up. And that dropped my idle right to 650ish in neutral. That was my problem, maybe its yours too.
old_man
NAXJA Forum User
- Location
- Loveland, Colorado
High idle is normally caused by a vacuum leak.
8Mud
NAXJA Forum User
- Location
- Central Germany
Make yourself a selection of short tubing pieces with a screw covered in silicon in one end. Match the tubing size to all of the vacuum ports on the manifold. Replace the vacuum line with a plug/stopper one at a time. I do it with the motor running, so when I pull the vacuum line, I can hear the motor idle up a little then go back to where it was as I put the plug on. When you get one, when removed and the idle stays constant, and actually goes down when you put the plug on, you have a winner. The manifold vacuum lines have little/nothing to do with the motor management, only the MAP vacuum line is absolutely necessary. Throw the pieces of tubing into the bottom of your toolbox, you'll likely need them again sometime.
Intake manifold bolts do come loose. TB gaskets do shrink and the TB needs to be re-tightened.
Like mentioned if you have exactly the right kind of crack in the exhaust manifold (usually above the O2 sensor) it can venturi and suck some extra air into the exhaust, fooling the O2 sensor.
Unplug the TPS plug at idle and see if the idle goes down. Bad ground, poor connection, or even water in the TPS can cause the idle to strange things.
Other less likely causes are weak or broken spring on the TB throttle plate. Somebody tried to adjust the throttle plate stop and or bent the stop (happened to me).
The idle air adjuster on the drivers side of the TB had the aluminum cap fall off and has backed itself out some.
There is a little chunk of solid material stuck to the IAC seat in the TB. You have to remove the TB or get a mirror and flashlight to see the seat.
Leave the idle air adjuster alone. As far as I can tell all it does is a fine adjustment for the IAC position and/or gives you enough air to start with the IAC closed (maybe).
Last but not least, the O2 sensor. I've never had this happen, but the heater circuit has to have some affect. This would likely only show up before or after the motor is above 140-160 degrees or so (warm almost operating temp.
I'm probably forgetting something, like a faulty MAT sensor. If it is faulty and/or covered in junk it can affect your idle. The colder the sensor, the higher the resistance (I believe). Raises th eidle a bit and richens the mix.
Motor temp. sensor (lower left of block) can keep the engine management system in open loop, causing a high idle. Often makes it run rich also. A simple test is on a cold motor (in the shade) the MAT sensor and the engine temp. sensor should have nearly the same resistance. Clean the junk off of the MAT sensor end anyway, it is likely coated with junk, carefully, it is fragile.
Intake manifold bolts do come loose. TB gaskets do shrink and the TB needs to be re-tightened.
Like mentioned if you have exactly the right kind of crack in the exhaust manifold (usually above the O2 sensor) it can venturi and suck some extra air into the exhaust, fooling the O2 sensor.
Unplug the TPS plug at idle and see if the idle goes down. Bad ground, poor connection, or even water in the TPS can cause the idle to strange things.
Other less likely causes are weak or broken spring on the TB throttle plate. Somebody tried to adjust the throttle plate stop and or bent the stop (happened to me).
The idle air adjuster on the drivers side of the TB had the aluminum cap fall off and has backed itself out some.
There is a little chunk of solid material stuck to the IAC seat in the TB. You have to remove the TB or get a mirror and flashlight to see the seat.
Leave the idle air adjuster alone. As far as I can tell all it does is a fine adjustment for the IAC position and/or gives you enough air to start with the IAC closed (maybe).
Last but not least, the O2 sensor. I've never had this happen, but the heater circuit has to have some affect. This would likely only show up before or after the motor is above 140-160 degrees or so (warm almost operating temp.
I'm probably forgetting something, like a faulty MAT sensor. If it is faulty and/or covered in junk it can affect your idle. The colder the sensor, the higher the resistance (I believe). Raises th eidle a bit and richens the mix.
Motor temp. sensor (lower left of block) can keep the engine management system in open loop, causing a high idle. Often makes it run rich also. A simple test is on a cold motor (in the shade) the MAT sensor and the engine temp. sensor should have nearly the same resistance. Clean the junk off of the MAT sensor end anyway, it is likely coated with junk, carefully, it is fragile.
XTurboJ said:
i know i do have a crack in my header. i had my buddy weld it up once and of course it cracked again. the thing is, his header as well as my other friends headers are both cracked as well. we got tired of fixing them, and their jeeps still run perfect. i guess ill have to run down that list 8mud typed up for me. im about ready to swap my longarms, arb's, and everything else to a high output! lol i could probably swap everything over in the time if would take to find my high idle problem lol.
mine is sparadically idling around 2200 and it's ticking me off. Thought it was a stiff cable. Nope. Only way to get it to stop is to shut it down and start it up again (sometimes multiple times till it goes away). I'm at a loss as to what's wrong with it and it's really pissin' me off!
Onkover
NAXJA Forum User
- Location
- Red Bluff CA.
Yep. Sounds like a vacume leak to me also
- Location
- MilkyWay Galaxy
bcsavage said:
I would do two things first. One is fix any intake vacuum leaks, and I mean air tight, this means even very small leaks anywhere on the valve cover as well as the intake manifold, leaks in the brake vacuum booster circuit, a/c - vent damper controls etc.
Second, remove and scotch brite every single ground wire connector on the battery, engine (2) and firewall (2). Then using a multi meter, test and calibrate the TPS to read between 0.80 and 0.82 volts (pin B & C while still connected) at idle with the power on , but the engine off. Then cycle, start, run for a few minutes, stop for 60 seconds, then restart...the jeep 3 times. By the third time chances are your problem will be solved!
Ask me how I know? :banghead:
http://www.naxja.org/forum/showthread.php?t=905849&highlight=High+idle
http://www.naxja.org/forum/showthread.php?t=899397&highlight=High+idle
If these two fail to solve the problem (90% chance it will fix the problem in my opinion) then continue on with the very nice, very complete list below by 8Mud! Nice write up 8Mud!
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- Location
- MilkyWay Galaxy
That (ground issue) is what I thought while I wasted $$$$$$$'s :rattle: and Time, :hang:Big TIME,bcsavage said:
fixing things that probably were not broke.:banghead::banghead::banghead:
Temperature changes, humidity (intermitent condensation and evaporation) and corrosion on a system that depends on an itty bitty 0.82 volt signal for determining idle speed signals where 0.01 volt change is something like a 50 RPM (I actually posted the measured value a few months ago) increase in the idle speed, and it turns out that just a few ohms (in my case it was an average 13 ohms from the engine to the battery, which is now a solid less than 1 ohm) of added resistance can make a huge rpm idle speed increase due to the added resistance. Then add on top of that a variable ground loss, where the condensation and corrosion changes the ground resisitance as the engine block warms up and ground clamp/connection moves just a tiny bit, enough to change that ground resistance a few ohms, and suddenly the idle speed signal jumps up by 500 rpm to 1500 rpm!
I finnaly figured it out when I discovered that my TPS calibration was changing with out moving the TPS. Mine has been fixed for over 4 months now where as before I was contstantly fighting with it for 2 years
blaming it on this and then that part, or sesnor connections.
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