Gonzo1SG

1988 Jeep Cherokee Chief, 4.0 six with 5-speed manual:
Once warm, my vehicle begins to idle very high, 2000 RPM.
I checked the TPS and all parties agree it is working correctly, so I re-installed it.
Next I replaced the MAP sensor; no improvement.
Then I replaced the IAC valve, plus cleaned the intake manifold; no improvement.
Next I replaced the CTS; no change in performance.
Just to be sure, I changed the air filter and cleaned the air box; no change.
Last I replaced the Oxygen sensor and still no improvement.
Please help. Thanks in advance...

jimbo98xj

you might want to check your connectors and wiring since your truck is an 88 just start pulling on some wires to see if they come apart

cruiser54

Renix Jeep C101 Connector Refreshing
　
The C101 connector on 1987 and 1988 Renix Jeeps was a source of electrical resistance when the vehicles were new. So much so that the factory eliminated this connector in the 1989 and 1990 models. The factory recommended cleaning this connector to insure the proper voltage and ground signals between the ECU and the fuel injection sensors. We can only imagine how this connector has become a larger source of voltage loss and increased resistance over a period of almost 25 years. The C101 connector needs to be cleaned at least once in the lifetime of your vehicle. Chances are it’s never been done before.

Almost every critical signal between the engine sensors, injectors, ECU, and some to the TCU, travel this path through the C101 connector.
The C101 connector is located on the driver’s side firewall above and behind the brake booster. It is held together with a single bolt in it’s center. To get the connectors apart, simply remove the bolt and pull the halves apart. You will find the connector is packed with a black tar like substance which has hardened over time.
Take a pocket screwdriver or the like and scrape out all the tar crap you can. Follow up by spraying out both connector halves with brake cleaner and then swabbing out the remainder of the tar. Repeat this procedure until the tar is totally removed. This may require 3 or more repetitions. Wipe out the connectors after spraying with a soft cloth.
If you have a small pick or dental tool, tweak the female connectors on the one side so they grab the pins on the opposite side a bit tighter before bolting both halves back together.
　
Revised 07-17-2012

Gonzo1SG

OK, found the connector, filled with crystallized black stuff just like you said. Instead of cleaning it out however, I bypassed the connector entirely, wire-for-wire, using Scotch-Locks, except for 1 large orange wire pair, which I twisted together (temporarily) with a household electrical twist connector, size: yellow. Unfortunately, I still have the same 'runs-rich & high-idle' problem. The only difference is that when I start the motor, it initially idles much lower than before (about 750 RPM) and takes just a little longer to get up to that crazy 2000+ RPM. Don't know where to go from here. I suppose I could replace the TPS that looked as if it checked good, and perhaps the MAT sensor. Thanks for the connector suggestion, my 2 wise jeep friends nodded approvingly when I told them about that one. Any additional help is greatly appreciated...

cruiser54

I would solder all those wires before proceeding. Scotch-Loks suck.

cruiser54

Here's how to properly test a tPS. Be sure to do the sensor ground test included with it.

Wouldn't hurt to click on the link in my signature and do the ground refreshing while you're at it.

RENIX TPS ADJUSTMENT
Before attempting to adjust your TPS be sure the throttle body has been recently cleaned. It's especially important that the edges of the throttle butterfly are free of any carbon build-up.
IMPORTANT NOTE: With the Key OFF, and using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS. The letters are embossed on the connector itself. Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it parallels the valve cover and also over near the MAP sensor on the firewall. If you see more than 1 ohm of resistance, or fluctuation in your ohms reading, some modifications to the sensor ground harness will be necessary. The harness repair must be performed before proceeding. I can provide an instruction sheet for that if needed.
MANUAL TRANSMISSION:
RENIX manual transmission equipped XJs have only a flat three-wire TPS mounted on the throttle body and it provides data input to the ECU. It has three wires in the connector and they're clearly embossed with the letters A, B, and C. Wire "A" is positive. Wire "B" is ground. Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors. Note the voltage reading--this is your REFERENCE voltage. Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For example: 4.82 volts X .17=.82 volts. Loosen both T-20 Torx screws attaching the TPS to the throttle body and rotate the TPS until you
have achieved your desired output voltage. Tighten the screws carefully while watching to see that your output voltage remains where it is supposed to be. If you can't achieve the correct output voltage, replace the TPS and start over.
Sometimes, after adjusting your TPS the way outlined above, you may experience a high idle upon starting. If that happens, shut the engine off and reconnect your probes to B and C. Start the engine and while watching your meter, turn the TPS clockwise until the idle drops to normal and then rotate it back counterclockwise to your desired output voltage.
AUTOMATIC TRANSMISSION: RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a flat three-wire connector, same as the manual transmission vehicles have, and it is tested the same as the manual transmission equipped vehicles, as outlined above—FOR ALL ENGINE MANAGEMENT RELATED ISSUES.
However, the automatic TPS also has a square four-wire connector, clearly embossed with the letters A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module. THIS SQUARE FOUR WIRE CONNECTOR IS USED FOR TRANSMISSION/SHIFTING RELATED ISSUES ONLY. Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your REFERENCE voltage. Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage. For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage. If you can't, replace the TPS and start over. So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and the other side feeds the TCU.
FOR AUTOMATIC TRANSMISSION RELATED ISSUES: Check the square four-wire connector side of the TPS.
If you have ENGINE ISSUES check the flat three-wire connector side of the TPS.
For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.
Revised 09-22-2012

freegdr

You running a thermostat

Gonzo1SG

Update: removed the temporary scotch-locks and soldered all wires, permanently bypassing that troublesome C101 (?) connector.
No change. Thoroughly checked out wiring harness, measuring resistance from sensors to ECU connectors. All wiring checked good. Replaced old ECU with a rebuilt unit from a national auto parts chain. No improvement. I have only time, tools, and my own two hands, taking this to a pro garage is not an option at this time. I am considering working a trade for a carburetor version of the same engine or, last resort, swapping the vehicle for a domestic pickup.

jakbob

i would check your manifold bolts. when it heats up, the metal contorts, allowing unmetered air to come in. use a shallow 14mm 6 point socket with a 6 inch extention and a socket wrench to get into the tight spots.

cruiser54

That's never a bad idea.

Also good to spray around the intake tract with throttle body cleaner at idle to find vacuum leaks. Only takes a few minutes. You'll know when you hit a leak.

Gonzo1SG

OK ... checked the 3 bolts on the throttle body. Found 1 that was not seated tightly, looked like the threads in the throttle body were stripped near the top of that hole. Bought a slightly longer bolt and easily reached some good threads. Also, the gasket looked like heck so I replaced it. These two things MAY HAVE fixed my problem, however I cannot tell yet because I BROKE the throttle cable stay. The plastic collar just fell apart when I inadvertently whacked it with the butt end of my ratchet. So now when I start the motor, it's as if I'm already pushing down on the gas pedal. Got to fix that before I can know for sure if I have made any progress. So just to recap: 1. Checked TPS, good I think, re-installed 2. replaced MAP sensor 3. replaced CTS sensor 4. replaced O2 sensor 5. checked MAT sensor, good I think, re-installed 6. replaced ECU. So now I must fix or replace that throttle cable and press on.

Gonzo1SG

OK. Throttle cable fixed. Followed suggestion and used throttle-body cleaner to find leaks. I used the whole can and unfortunately saw no evidence of a leak around the throttle body or any of the vacuum lines. Along the lines of tightening down the throttle body bolts, what else (made of metal I imagine) could contort and cause a leak? Because the motor always starts normally and idles nicely - the high RPMs don't occur until the engine is nice and warm, when idle steadily climbs to 2000 RPM and won't decrease unless under load i.e. in-gear rolling down the road. I don't know what else to do at this point.

Malik3404

I have the same problem on my 93 and Im at my wits end.

parts replaced/cleaned: cps, tps, ivac, plugs, wires, cap, rotor, coil, 703 injectors, 02, cat, muffler, radiator, water pump, t stat, belt, k&n filter, cleaned trottle body, battery, battery terminals, alt, AC, big 3.

I have a machanic friend coming to look at my xj later this week. If that fails its off to the dreaded dealer to get **** raped. I dont know what else to do.

Gonzo1SG

Update: On suggestion from a mechanic friend, I replaced my EGR valve. It was really nasty. The valve's nipple was rusted and looked as if it were crimped shut; also, the line to the valve vacuum transducer was completely obstructed. This part obviously was not working. So I replaced the valve, its transducer, & gasket, however no improvement with my problem - engine still idles at 2000 RPM or greater once it warms up. So to recap: PARTS CHECKED GOOD: TPS, MAT Sensor, wiring harness (C101 connector is by-passed). PARTS REPLACED: IAC Valve, MAP Sensor, CTS, O2 Sensor, Throttle Body gasket, Engine Control Unit/Module (Computer), and EGR valve with EGR valve vacuum transducer and gasket. I've done a lot of cleaning, such as the throttle body (inside & out), airbox, etc. Also checked for vacuum leaks, still no luck! Any ideas? Anyone?

redmuck

timing chain? maybe jumped a couple of teeth? my dodge car was the same way and i changed every sensor good or bad and then some and the timing chain was the culprit the whole time.