Re: 87 - 258 Altitude jumper ground
Though we spoke on the phone , I felt that my experience re : high altitude wire was worth sharing . The double wire grounded to the engine block near the coil correponds with the 2 ground wires from the MCU/PCM/ ECU . These 2 black wires originate at the MCU wire harness terminals #20 and #60 . They are part of the MCU wires that pass through the firewall above the gas pedal . The altitude wire also is part of this bunch of wires . It has its origin at MCU harness pin # 11. Below 4000 ft.altitude , you do not want this wire grounded. I encased mine within a rubber hose so that it would not ground .
There was an Information Service bulletin number 7E issued Sept 1987 for the 87 YJ Wrangler with 4.2 and the 2.46 TBI for vehicles which were built for one altitude , yet operate at another . The bulletin describes how to get better performance and is to be offered at no charge to the public. Any vehicle undergoing this change should have an updated VECI label. In a nutshell, the procedure either places or removes a metal U- shaped jumper connector in the duo-terminal connector located near the MCU wiring harness. Set the timing to 16 degrees BTDC above 4000 ft or to 9 degrees BTDC below 4000 ft altitude . When traveling below 4000, reset timing to 9 degrees BTDC , above 4000 reset to 16.