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C101 Elimination

Revised 3/6/2014

As noted in Tip #2, Renix C101 Connector Refreshing, in 1989 the factory wiring harness no longer included the problematic connector. For a long-term fix that will avoid many problems in the future, it is a good idea to just get rid of the thing entirely.

A helpful video can be found in the link below:

Unbolt the 2 halves of the C101 using a ¼” socket. On each half there is a plastic cover where the wires enter. They are removable but you’ll probably end up busting them off. Lightly bolt the C101 back together, away from the firewall. Peel back the split-loom covering from the body side of the C101 connector down to where the harness splits and goes toward the firewall, pretty much below the MAP sensor. On the engine side, remove the split loom about the same distance.

C101 Elimination 1

Now you can see from one side of the C101 to the other. Beginning at the top row, closest to the motor, be absolutely sure you cut the matching wires on each side of the connector off to about 1/2 inch. This will allow you to verify the original position of each wire color in case of mistakes or confusion. CUT AND SOLDER ONE PAIR AT A TIME. The wires may be wound a bit in their looms. See the first photo. Get them unwound neatly and do the following, ONE AT A TIME.

Slide your shrink tube over one wire. In a well ventilated area, solder the wires together and then slide the shrink tubing over the solder joint. Heat the shrink tubing so it seals your completed solder joint. Keep going until you’ve done all 22 or so connections.

Both sides of the C101 connection have a wire that is brown with white tracer. Follow each of these wires back until you come to a point where three wires are crimped together.

C101 Elimination 2

What you want to do here is cut the crappy factory crimp out of each set of three and bring both sets of three wires together and solder them together, using shrink tubing as well. All 6 wires. These particular wires will not end up in your normal C101 elimination loom. The sensor ground upgrade is found in Tip 6.

After all the soldering and shrink tubing is done, bundle the wires together in a new piece of 3/4″ split loom. Tape it up and secure it to the C101 connector’s original bolt hole or somewhere else along the firewall so it will be protected.

C101 Elimination 3

Revised 01-31-2016

Oil Filler Cap Mod


We all get tired of the oil film/leak on top of our 4.0 Jeep aluminum valve covers. Many times the biggest culprit is the loose fitting oil filler cap. Here’s a real easy fix:

  1. Remove the plastic oil filler cap and wrestle the original rubber gasket from it using a small pick or screwdriver.
  2. Make a duplicate of the gasket out of an old bicycle inner tube using scissors and a knife or razor blade.
  3. Install the duplicate on the cap first, and then the original over top of it.

It will be a tight fit on the valve cover the first time, and may require some persuasion with a pair of pliers, but will get easier with time.


Revised 1-31-2016

Cruiser’s Valve Cover Mod

Most early and even later Renix 4.0s could be bothered by excessive oil in the air cleaner box.

A fix from my old days as Service Manager at a Jeep dealership follows. Information was provided to me from a buddy at JeepTech during those days. I don’t believe it ever came out in a Technical Service Bulletin.

Remove the valve cover and turn it over. Next, remove the fluted tubes that are now facing you by removing three screws on each.

Valve cover flute

Chop 1 inch off each of them. Then, drill a half inch hole as shown in the photo. The flat side of the flute that’s closest to the edge of the valve cover. valve cover flutes (2)


Clean and de-burr/sand all rough edges nicely, clean with solvent, and reinstall. Don’t be concerned about the chintzy gaskets that will probably crumble into oblivion when you remove the fluted tubes. They can go back together without gaskets.

Here’s a handy video from my friend Brendan showing more detail.

Revised 09-12-16

4.0L Engine Date Codes

Engine code location

The date code is on the passenger side of the engine, on the block, just forward and up from the distributor on a machined flat surface.

The digits of the code identify:

  • 1st Digit-The year (8 = 1998).
  • 2nd & 3rd Digits-The month (01 – 12).
  • 4th & 5th Digits-The engine type/fuel system/compression ratio (MX = A 4.0 Liter (242 CID) 8.7:1 compression ratio engine with a multi-point fuel injection system).
  • 6th & 7th Digits-The day of engine build (01 – 31).

FOR EXAMPLE: Code * 801MX12 * identifies a 4.0 Liter (242 CID) engine with a multi-point fuel injection system and 8.7:1 compression ratio that was built on January 12, 1998.

Revised 1-31-2016

Renix CPS Advance Mod


The sensor portion of the CPS needs to slide up the perforated bar towards the top of the engine.

Mark the perforated bar’s top end with paint or marker.

Place the CPS in a vice,  just not quite clamping the bar, the sensor resting on the vice jaws, with the Top part protruding.

Place a bolt that fits tightly into the hole at the Top part of the bar.

With a hammer, tap the Top part of the bar and it will slide through the sensor portion of the CPS. Stop when the sensor portion is about 3/8″ from the edge of the bolt hole.

CPS advance mod

Revised 1-31-2016

Renix Vacuum Harnesses


Vac harness 7151367

The vacuum harness that attaches to the front of the valve cover and includes the grommet/fitting, and is called the front harness, is Napa part number BK 715-1367 or is a Dorman 46003.

Vac harness 7151366

The vacuum harness that is closest to the air cleaner, EGR etc, and is called the rear harness, is Napa part number BK 715-1366 or is a Dorman 46004.

Vac harness 7151365

The tube from the rear of the valve cover to the intake manifold is part number BK 715-1365 or Dorman 46005 and comes with the valve cover grommet.

The Throttle Body to MAP hose is no longer available. Click HERE to access a fix for that.

Vac Diagram 4.0 Renix

Revised 2-6-2016

EGR Valve Test


Valve Opening Test

  • With engine at normal operating temperature and at idle, rapidly open and close throttle. Open throttle sufficiently to obtain at least 1500 RPM. Movement should be noticed in EGR diaphragm.
  • If diaphragm does not move, probable causes are: faulty vacuum signal to EGR, defective EGR diaphragm or defective backpressure sensor diaphragm (if equipped), or leaks in vacuum lines or connections.

Valve Closing Test

  • With engine at normal operating temperature and at idle, manually depress EGR valve diaphragm. RPM should immediately drop, indicating that EGR valve is not leaking and had been properly cutting off exhaust gas flow at idle.
  • If there is no change in RPM and engine is idling properly, exhaust gases are not reaching combustion chamber. Check for plugged passage between EGR valve and intake manifold.
  • If engine idles poorly and RPM is not greatly affected by manually moving diaphragm up, EGR valve is not closing off exhaust gas flow. Check for carbon between pintle, leaking EGR valve gasket or bad EGR valve.

Revised 1-31-2016


Shifting the Transfer Case


Here’s how the factory suggests you shift the transfer case and I’ve been doing this since these things were new and I worked at the dealership. Quoted from the owner’s manual. The suggestions in BOLD are mine.

“To engage, shift the transfer case lever from 2H to 4H while the vehicle is moving at any legal speed”. I let off the gas, throw the lever, tap the gas, and let off again.

4L position: ” To engage, slow the vehicle to 2-3 MPH , shift the transmission to Neutral, then shift the transfer lever to the right and pull firmly rearward to 4L”.

To shift out of 4L, stop the vehicle, shift the transmission to Neutral, shift the transfer case lever to 4H, then the transmission to Drive (if you have an automatic), or First gear (if you have a manual), and continue on.

Revised 1-31-2016

Headlight Harness Upgrade

Putco relaysHeadlamp harness

It’s easy to install a supplemental headlight harness.

From the factory, the voltage to the headlight bulbs travels from the battery, through connectors, inside the cabin, to the headlamp switch, and then back out to the lamps via undersized wire and more connectors. It’s not uncommon to find only 10.5 volts at the lamps.

The supplemental harness is installed so that it provides battery voltage to the lamps and is just triggered by the factory wiring. The result is about 35% brighter headlamps and headlight switches that don’t melt and burn out.


Ebay has the harnesses. 2 headlamp H4 with ceramic connectors is what you want. You’ll likely be warned that the harness won’t fit your Jeep. It will.

Absolutely plug and play:

  • Remove grille and headlamp bulbs. I fed my harnesses from the passenger side starting between the battery and the back of the headlamp housing, over to the driver side.
  • Plug the driver side bulb into the new harness.
  • Attach the new harness’s ground wire under one of the small bolts on the radiator support after scraping the paint off under it.
  • Attach the harness to the existing harness behind the grille working toward the passenger side.
  • Plug the new harness plug into passenger headlamp.
  • Plug original headlamp plug into receptacle on new harness.
  • Attach the ground for the passenger side just like you did the driver side under a radiator support bolt.
  • Attach relays with provided bracket on the passenger side inner fender.
  • Connect power wires to battery.
  • Revised 1-31-2016

Improving the Instrument Panel Ground


The ground point for the complete instrument cluster on your XJ or MJ is located up under the driver’s side dash. If you lay on your back and look up under there with a flashlight, without wearing a hat, you will see a black wire attached to a shiny piece of metal almost directly above the hood release knob. The screw will have either a ¼” or 5/16″ head on it.

This ground point is responsible for handling the ground circuit for the following items: Dome lamps, seat belt and key warnings, transmission power/comfort switch, wiper switch, headlamp switch and delay module, fog lamp switch, cargo lamp switch, all instrument panel grounds and illumination, power windows and door locks, cruise control dump valve, and a few more things.

The problem is that where the ground point is located does not share good contact with the chassis where the ground should be. The solution is simple:

  • Make up a jumper wire with #10 gauge wire about 10″ long. On one end, crimp on a ¼” round wire terminal. On the other end, crimp on a 3/8″ round wire terminal.
  • Remove the screw from the existing ground wire and attach the small terminal of your jumper so that the original wire and your new jumper share the same attaching point, one over the other.
  • Look above the driver’s side plastic kick panel just forward of the top of the hood release knob. You will see an 8mm stud there. Attach the large terminal end there with a washer and nut over it tightened securely. Use a coating of OxGard at all ground contact surfaces when attaching the screw and nut.

IP ground location

**Special note for Comanche owners: Make your jumper wire 12″ long and attach it on the driver’s side kick panel close to the fusebox on the 8mm stud.**