Engine Misfire Fix - 2001 Cherokee Sport - JeepForum.com
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post #1 of 1 Old 05-03-2016, 11:30 PM Thread Starter
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Engine Misfire Fix - 2001 Cherokee Sport

Summary: Engine misfire OBD codes stopped by adding new relay to run electric radiator fan when engine is running.

  • Engine symbol illuminated on dash
  • OBD misfire codes (P030x)
  • Engine compartment too hot to raise hood after driving on warm day
Deduction Fuel is percolating in fuel rail and or fuel injectors due to proximity of exhaust manifold to intake manifold and to fuel rail, the Jeep’s high operating temperature (210°F), and the electric radiator fan only running in limited conditions.
Goal: Run the electric cooling fan whenever engine is running to cool engine compartment and eliminate fuel percolation.
1. Bought A2C 775 Universal Automotive Relay on Amazon for about $8
2. Also used about 2 feet of 12 gauge wire
New Relay Control circuit:
1. Cut out about 4 inches of the wire cover at right firewall to left of hood hinge to expose wires (see photo)
2. To switch the new relay on and off, located a white wire in this harness by referring to wiring diagram and testing with multimeter
a. Note: There are several white wires in this harness; any of them will probably work
3. Cut away about ˝ inch of white wire’s cover, soldered white wire from new relay to it, and wrapped with electrical tape.
4. Connected the new relay’s black wire to ground.
a. First formed loop in wire end and soldered around loop to keep wire together
b. Then unscrewed existing screw on firewall and secured wire loop with screw. See photo.
New Relay Fan Motor Circuit
1. Determined that the LB (light blue) wire in this harness is used to run the radiator fan motor.
a. Used it to avoid running a separate wire.
b. Located this wire, cut away about ˝ inch of its cover, soldered the new relay’s yellow wire to it, and wrapped with electrical tape – see photo.
2. To provide properly fused power to the LB wire, decided to use the LG/RD (light green with red stripe) wire under PDC (Power Distribution Center) – see photo
a. This is the same wire that provides power to the fan when commanded by the ECU (Engine Control Unit). Since it has a 40amp fuse, using it eliminates the need for a separate fuse.
b. Accessed the LG/RD wire at PDC
1. First removed battery terminal plastic cover at front of PDC
2. Then removed nut securing two wires under cover
3. Next released two metal tabs and lifted up PDC
4. Next removed bottom from PDC after releasing 7 plastic clips
5. Located LG/RD wire running from fan’s 40 amp fuse to existing fan relay (see photo)
  • Referred to schematic on inside of PDC to locate fuse and relay
6. Cut away about ˝” of LG/RD wire cover, soldered new red 12 gauge wire to it, and wrapped with electrical tape.
3. Connected other end of new red wire to the new relay
a. Removed the DB (dark blue) wire from new relay
b. Cut off the wire connector from DB wire and opened crimp
c. Soldered red wire to this wire connector
d. Reinserted wire connector back into relay
Relay Mount and Finish Work
1. Connected new relay to firewall using hood hinge screw (see photo)
  • First enlarged hole in new relay’s metal bracket to size of hood hinge screw
2. Put the piece of wire cover removed earlier back in place and wrapped over it with electrical tape.
How it works
  • When the engine is running, power flows from the white wire in the harness to the relay’s white wire, to relay terminal 85, through the relay’s electromagnet, to relay terminal 86, then through the relay’s black wire to ground completing the control circuit.
  • The relay’s electromagnet then causes the relay’s internal switch to connect terminals 30 and 87 and power flows from the LG/RD wire at the PDC, through the new red wire, through the new relay, to the LB wire, to the fan motor, and to ground, completing the fan motor circuit.
Note: This modification is completely unknown to the ECU. So, no unwanted OBD codes are set.
  • Did it eliminate the OBD misfire codes? Yes
  • Did it cool down the engine compartment on warm day so hood can be raised without burning fingers? Yes
  • Did this mod trigger any new OBD code? No.
Credit: Thanks to all the folks who contributed to this posting which gave me the idea for this mod: http://www.naxja.org/forum/showthread.php?t=1025399

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getterdun is offline  

cooling fan , engine misfire , misfire , p0300 , p0301

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