Gorilla369

So I have a 1988 Cherokee Laredo with 4.0 automatic and it has been doing weird stuff lately. Sometimes I start it up and the idle is at 2000 RPM, sometimes I start it up and it's at 1100 RPM. Sometimes when I'm driving it goes all the down to 200 RPM when at a stop and sometimes it sits at 1100 at a stop. It even died on me today because the idle was so low when I was coasting in a parking lot. Things I have replaced so far are; IAC, TPS was adjusted(correctly), pregapped plugs, wires, cap, rotor, coil, throttle cable and fuel filter. It doesn't overheat and had a sea foam treatment. Please help!!!! Thanks.

sik_xj

first thing I would have said even before you replaced all that stuff would be your cps...I would give that shot for sure

Gorilla369

Ya I was reading about that and it makes sense so I have one ordered. Any other thoughts that I should check out just in case?

Radi

Check for vacuum leaks. Intake manifold, vacuum lines, any sensors that have vacuum lines...etc. A vacuum gauge might be helpful. Should hold steady about 18-21 at idle, drop to zero when you open the throttle momentarily, then snap right back when you release it.

Are you getting a miss? Occasional no-starts? Stalling out? If not, I'd hold off on the CPS for now. There is a procedure posted here somewhere to test it, which I'd do first.
You don't want to remove a perfectly functioning OEM unit and install a craptastic aftermarket unit, creating yet another layer of problems. If you ordered OEM the quality is more predictable and should be OK to install.

Gorilla369

I ordered Crown. I'll pick up a gauge. The Haynes manual goes over the testing procedure, I'll check it out and let ya know what I fine. The CPS was pretty cheap so I'll store it on the shelf if mine tests fine.

cdf43

A bad O2 can give you weird idle issues

cruiser54

Renix CPS Testing and Adjusting
　
　
Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.
Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.
The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.
Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It won’t start with the CPS disconnected.
You should get a reading of .5 AC volts.
If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from Napa or the dealer.
Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.
A little trick for increasing the output of your CPS is to drill out it’s mounting holes with the first drill bit that just won’t fit through the original holes. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.
　
Revised 11-29-2011

cruiser54

Radi provides some great suggestions. Eliminate vacuum leaks as a source of your problem. Just go ahead and put a wrench on your intake manifold bolts.

I really don't think it's your CPS but I provided the instructions for you.

Which instructions did you use for adjusting the TPS?

Something else that is quite common is a poor sensor ground circuit. Read the following and check out why.
Cruiser’s Renix Sensor Ground Test
　
This sensor ground circuit affects the CTS, TPS, IAT, MAP, ECU and diagnostic connector grounds. It’s very important and not something to overlook in diagnosing your Renix Jeep as it is common for the harnesses to have poor crimps causing poor grounds. If any or all of the sensors do not have a good ground, the signal the ECU receives from these sensors is inaccurate.
Set your meter to measure Ohms. Be sure the key is in the OFF position. 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 runs parallel to the valve cover and also near the MAP sensor mounted on the firewall. If you have an 87 or 88 with the C101 connector mounted on the firewall above the brake booster, wiggle it, too.
You want to see as close to 0 ohms of resistance as possible. And when wiggling the harnesses/connectors the resistance value should stay low. If there is a variance in the values when wiggling the wires, you have a poor crimp/connection in the wiring harness or a poor ground at the engine dipstick tube stud. On 87 and 88 models, you could have a poor connection at the C101 connector as well.
Revised 06/12/2012

cruiser54

If you've never done this, it should be done before proceeding.

Renix Ground Refreshing
The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, and wasted money replacing unnecessary components.
The components grounding at the dipstick tube stud are:
Distributor Sync Sensor, TCU main ground, TCU "Shift Point Logic", Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal. All extremely important stuff.
The factory was aware of the issues with this ground point and addressed it by suggesting the following:
Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.
Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.
While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.
Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.
First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.
Remove the other end of the cable from the rear of the head using a 3’4" socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.
A suggestion regarding the braided cable:
I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18" long with a 3/8" lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.
A further improvement to the grounding system can be made using a #4 cable, about 10" long with 3/8" terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.
　
　
If you want to upgrade your grounds and battery cables in general, contact Jon at www.kelleyswip.com. He makes an incredible cable upgrade for a very reasonable price.
　
Revised 11-28-2011

cruiser54

And since your 88 has a C101 connector, this is extremely worthwhile to do.

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, and the ECU travel the path through the C101.
The C101 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. Apply a true dielectric grease, not the stuff that came with your brake pads, to the connection and bolt it back together.
　
　
Revised 11-29-2011

Gorilla369

There were some instructions on this forum that I found to adjust the TPS. It was a while ago and I don't remember exactly I did but it had something to do with dividing a voltage by another and staying in a certain range. And I do have the C101 connecter and Cruiser54 posted about this too so I'm going to go check this out right now. Thanks alot. I love this forum!

Gorilla369

Cool, I'm going to check this out cuz I'm actually MECP advanced certified and installed stereos for 8 years so grounding issues and bad connections are right up my alley. Got my bluepoint multimeter and going to put it to work. thanks a ton

Gorilla369

So the O2 sensor is dead (RIP) no voltage, so that's on order. CPS doesn't even read .3 volts so thank god that's already on it's way. I did find a vacuum leak but it's an easy "cap off" fix. My electrical is showing 0.3ohm which isn't bad but could be worse. I'll get these done and let you all know what happens, thanks guys!!!

cruiser54

Get the ground refreshing done first.

Did you wiggle the wires as described in the sensor ground test?

FWIW, be sure you're doing the TPS adjustment (after all the grounds are refreshed and sensor ground circuit test done) using teh flat 3 wire connector.

Gorilla369

Ya I did the tests after I did the grounds. The only thing I haven't done is install the 4awg wire.