I haven't posted this in a long time, so this might help. Based on the extra information you've provided, I'd be inclined to guess.
1) TPS ground (as already mentioned).
2) sticking AIS
3) An SBEC issue
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Below are a several things that can cause high idle problems. Depending on your year/make, certain conditions may or may not apply to your specific vehicle.
I've organized this by the most common causes working down to the more complicated and/or expensive causes. The "Misc" section also provides several cheap fixes that have worked for many people in the past. Good luck!!!
I'm constantly updating this document, so please let me know what else can be added, or if anything is in error.
Issues with a Dirty TB:
Clean both the TB and the IAC motor. Make sure you use "O2 sensor safe" cleaner so you don't fry your O2 sensor. You will need a T-25 torx socket to remove the IAC. A good article on cleaning your TB can be found at:
http://www.off-road.com/jeep/cherok...ngine/tbody.htm
Issues with the TPS and TCU:
Verify the TPS is functioning properly (variable voltage) and that it is set properly. On 87 to 90 models, the TPS is adjustable. On 91 and up, it's not, but still must be checked.
1) There are different TPS sensors used on autos and manuals. Additionally there are different TPS vendors (Benedit and Renix). Refer to your FSM and/or the Renix or Bendex TPS Manuals for setting / testing functions.
2) Verify good ground at TPS (more prevalent on older models). If needed, a jumper ground wire can be spliced into the existing TPS ground to provide a new ground path. On late 80's automatics, with 4.0 it's terminal "D" on the TPS connector.
3) On pre 89's, check the TPS ground wire connection at the "C101" connector which (I believe) is mounted on the firewall above the brake master cylinder. You may want to use dielectric grease on the contacts in the C101 connector. This connector has been the source of many problems on older XJ's. In some cases high idle problems were resolved by removing this connector all together (E.G. splice wires directly together).
The B+ relay energizes the IAS motor at start-up. So after the engineis idling (at a normal speed) you can unplugged the relay to keep the motor in that position. It's on the right side by the battery. I've been told it's the one with pink wires feeding into it.
4) Verify signals from/to the TPS and TCU. These sensors talk to each other to determine the best shift points on an auto, refer to FSM for testing. Disregard for manual. Check your FSM to verify your model year.
5) Verify input/output voltage at the SBEC/ECU/PCM (whatever your year computer is called). Refer to your FSM for specific pin locations to verify main harness condition etc. Be "very careful" when working with your computer not to damage or short the unit!!!! Otherwise you could quickly have a bigger problem then a high idle.
Issues with the IAC:
1) Check the IAC motor voltage with a volt meter using a back probe. You will also want to verify the voltage at the computer end (E.G. ECU or PCM) for the specific IAC pin. You will need to know the voltage ranges and pin number which can be found in your FSM.
2) Have the IAC motor checked for proper movement. This requires a special tool which (to my knowledge) doesn't have a simple work around. The dealer may have to check this.
Vacuum leaks (use a vacuum gauge, don't simply eyeball it):
1) Check all vacuum lines and connections to verify they are 100% sound. If no vacuum leaks are detected, proceed below.
2) Idle RPM that rises as the engine warms up can also be caused by a vacuum leak that gets worse due to thermal expansion of differing materials at their mating points. Many times it can be traced to a either a bad intake manifold gasket, loose intake manifold bolts or a bad TB gasket. Do you hear any kind of high pitched hiss or whine from the engine bay at some point during warm-up? This can imply a vacuum-leak either at the TB or intake manifold. To check the manifold/intake gasket (I'm pretty sure) you will need to remove intake manifold, part of the power steering, fuel rail, and TB. So this is a fair sized job.
Miscellaneous issues that can effect idle (E.G. high, low, rough):
1) Lubricate the gas peddle cable and linkage. Normally this isn't the problem, but I've heard of a few cases where this was the problem.
2) Check all engine harness connectors. There have been several electrical gremlins traced back to engine bay connectors, especially those on or close to the fire wall. In some cases the best fix was to totally eliminate these connectors all together (E.G the C101 connector on older XJ's). Use the "search" function on the board to search for "C101". This can provide valuable information.
3) On late 80's (4.0 liter), check/clean the relay that's bolted to the side of the engine bay next to the air box. I don't know what relay this is (yet)? But it has fixed a few high idle conditions.
4) On 2.5 liter, check the power steering sensor (a.k.a. high pressure switch). It increases idle during high PSI steering efforts.
5) Check the "vehicle speed sensor" located on the T-case (4x4) on the rear output shaft (on 2WD it's on tranny extension housing). I think it works in combo with the TPS to detect when you're "decelerating". The TPS sends low voltage (throttle plate closed) and the speed sensor is sending speed related pulses. Again, this is a long shot. But I've heard of 1 high idle where a bad VSS caused part of the problem.
6) Engine Coolant Sensor can also affect idle speed. However, I've never heard of anyone (yet) that has traced a high idle condition specifically back to this sensor.
7) AC triggers (optional). There are two triggers used when AC is turned on or operating. These can effect idle because when AC is on the ECU/PCM raises the idle a bit to compensate for the extra load the AC is applying. Is either trigger stuck in the "on" position increasing idle?
8) Check the idle set screw adjustor (late 80's, early 90's models). The screw you should never touch, but sometimes it's used to patch either a high or low idle condition. I don't recommend touching this, because once this screw is changed from its factory setting, resetting it back can be impossible as you have no frame of reference. Using this as a "fix" will change your TPS swing voltage, while not correcting the original problem. Your ECU/PCM will get even more confused. In my opinion the only reason to check this is if someone my have already tampered with this? If all sensors, grounds, etc checkout this may be the problem if someone previously tampered with this setting. It is a hidden (recessed) allen screw normally directly behind the TB where the throttle linkage connects. Look for a threaded bolt that the TB arm rests against. The allen screw head is recessed within the base plate.
Here's a photo of the "Idle set screw". Inside the rectangular metal box is the allen screw.
http://community.webshots.com/photo.../40289136eDWflF
9) Check CPS connection (long shot). A poor CPS connection can cause poor idle (normally low or rough) as well as a no start (crank but no fire to plug) situation.
10) Verify your computer hasn't stored any DTC's within memory. To do this (on mid 90's and above), cycle the ignition key, on/off, on/off, on- within 5 seconds. Your check engine light should start to blink within about 10 seconds. It will either provide DTC's (if any are present) or send a code "55" indicating end of memory. Here's a few links to more information and the general codes.
http://www.webejeepin.com/Check-Engine.htm
http://www.jedi.com/obiwan/jeep/dtc.html
11) Check the vehicles recall and TSB's issued for your year/model truck. Several TSB's have been reported concerning "high idle" problems.
For example, a TPS recall ( number 90V177000) was issued for 1990 4.0L XJ's for intermittent high idle.
Here's a link to recall and TSB information:
http://www.edmunds.com/products/mai...html?ID=lin1038
If all else fails there is a way to override the signal sent to your IAC, allowing you to actually set the idle speed you want. It's actually (generally) used for crawling. It costs about $150. I'm not sure if device is still being offered though.
http://www.offroadengineering.com/HandThrottle.htm
12) Specific to XJ's with the "Police package option". There is an "extended idle" option that sends a ground to pin number 10 of the PCM via a simple dash mounted switch. When activated, the idle should increase to roughly 1,000 RPM. If this circuit grounds bypassing the switch, you will have a higher idle then normal.
Notes and Acronyms:
When tracing any kind of idle problem, it can help to know "when" the condition occurs. Your engine computer (ECU, PCM, etc) has two basic modes of operation.
Open Loop - When the engine is warming up and during WOT it disregards some input sensors (E.G. O2 sensor, etc) and manages output devices (E.G. IAC motor) differently then when warmed up.
Closed Loop - When the engine is at operating temperature and cruising or idling, it is in closed loop. More sensors are used (again E.G. O2 sensor) and output devices (E.G. IAC, etc) are managed.
Acronym translations:
AC - Air Conditioning
CPS - Crankshaft Position Sensor
DTC - Diagnostic Trouble Code
Engine management computers:
SBEC - Engine Controller (80's models)
ECU - Electronic Control Unit (early 90's models)
PCM - Powertrain Control Unit with dual processor (93 and above models)
FSM - Factory Service Manual
IAC - Idle Air Control (a.k.a. Idle Stepper Motor")
TB - Throttle Body
TCU - Transmission Control Unit
TPS - Throttle Position Sensor
TSB - Technical Service Bulletin
VSS - Vehicle Speed Sensor (different then Speed Control used by cruise control)
WOT - Wide Open Throttle
Hope this helps,
Kent
