MJ stumble/not starting

[hagenjc9]

Allright, ive got an odd problem that I can't figure out...My MJ will start right up and has been running pretty well. Recently it has started to have a stumble when you give it gas though. It stumbles any time that you accelerate quickly. If you press the gas pedal very softly it will actually slow the engine down and shut off. The odd part is that it won't fire back up. It will get close the first time you try to start it, and regress to the point where it is just the starter cranking with no chance of spark. Now if I let the truck sit for say 5 hours I can go outside and it will fire right up like nothing happened, but then the process starts all over again....ANY suggestions on what i can/should do to figure this all out......thanks

[cruiser54]

Get out your voltmeter and do this:

 

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

[hagenjc9]

Thanks man!! I'm gonna go test it ASAP and ill let yall know how it turns out. :thumbsup:

[hagenjc9]

Just tested it and looks like its not the CPS....i got readings anywhere form .6 to .8 :dunno:

Any other thoughts?

[cruiser54]

Have you ever refreshed all your grounds? What year MJ?

[hagenjc9]

I have not yet, but defiantly need to look into it.... and I have a 90'

[carnuck]

Spark module under the coil got corroded connections?

[Dadinator]

the way you're describing it, I'd check the TPS as well.

 

Sounds like it doesn't know where the throttle is.

[cruiser54]

Since you're interested, here ya go..

This should be done on every Renix Jeep out there, at least once in it's lifetime.

 

 

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

 

 

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: 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.

If you see more than 1 ohm of resistance 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 a three-wire TPS mounted on the throttle body.

This manual transmission vehicle TPS provides data input to the ECU. The manual transmission

TPS 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. Adjust the TPS until you have achieved this percentage. If

you can't achieve the correct output voltage replace the TPS and start over.

 

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--FOR 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.

 

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. If you have TRANSMISSION issues check the four-wire

connector side of the TPS. If you have ENGINE issues check the 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 11-28-2001

[hagenjc9]

Sweeeet :thumbsup: so much info from you guys it awesome. Ill get out there soon and run down the checklist on the MJ. Thanks alot for all the info its extremely helpfull!

[hagenjc9]

Well I cleaned up the grounds and no luck there...did not get to check the TPS yet, but I think I did find a big clue. I found a connector that wasnt plugged into anything, and it was all melted and corroded. I looked up the wire colors and it seems like it would be the Engine Speed Sensor. Does this sound about right to yall? The problem is after I replace the connector, I searched around the engine, and can't find where it is supposed to be clipped it to. Anybody know where that would be??

[hagenjc9]

BTW this is what that loose connector looks like

[Boots]

I believe that plug is for the low wiper fluid sensor if yours were equipped with one. Mine has the same plug and it isn't plugged into any thing either.