History of OEM EFI
and Aftermarket Kits for the Jeep 4.2L Inline Six
1987 Jeep YJ Wrangler with 2.5L TBI four-cylinder engine. This engine
package first appears in the 1984 Jeep XJ Cherokee.
AMC/Jeep 2.5L four-cylinder engines
enjoyed the benefits of OEM throttle body injection (TBI) beginning with the 1984 XJ Cherokee. The trend continued
with the 1987-90 YJ Wrangler 2.5L four. From 1987-up, all 4.0L inline sixes offered multi-point injection, first
the Renix system (1987-90), then the Chrysler MPI system introduced in 1991 4.0L inline sixes and
From 1991 forward, the Jeep
2.5L four and 4.0L inline six each use Chrysler's MPI electronic fuel-and-spark management. ‘LA’ pushrod V-8
5.2L and 5.9L engines, beginning with 1993-up Grand Cherokee models, also use Mopar MPI fuel injection
Note: The1990 YJ Wrangler 4.2L inline six and 1991 Jeep Grand Wagoneer 360 V-8 were the last carbureted Jeep
engines produced by Chrysler for the North American market.
Inspired by the
dramatic gains of EFI, Mopar Performance marketed the first popular retrofit TBI system for the 4.2L
inline six. The results were startling: a stone stock 4.2L with nothing more than an EFI conversion
immediately gained 50 horsepower—and a cleaner tailpipe.
There were efforts
with EFI retrofit kits prior to this first Mopar offering. In the late 1980s, Jack Clifford experimented with a
retrofit AirSensors-based EFI system at Clifford Performance. (See that system in Moses Ludel's
Jeep® Owner's Bible™.) The core was a
Ford central-fuel injection (CFI) unit from a 1980s 302 Ford V-8.
Around that same
time, Lee Hurley (HESCO) utilized Electromotive's laptop programmable EFI on Jeep MPI six-cylinder race
engines. (We discussed Lee's work at Walker Evans' race shop.) Lee custom-programmed Jeep
inline sixes for Baja racing and Archer Brothers' MJ Comanche pickups, which won the SCCA Race
Similarly, Steve Dose experimented with an Electromotive-based system around 1990, using a G.M. TBI unit.
We collaborated on an OFF-ROAD Magazine project, a 4.2L CJ-5 Jeep that I drove from Oakridge, Oregon
to the SCORE Show at Anaheim, California—and back.
Each of these systems
pre-dated the Mopar Performance-Electromotive TBI/EFI venture, which clearly had Lee Hurley's
handprint. This first
Mopar Performance retrofit TBI system relied upon programmable Electromotive fuel-and-spark
Mopar Performance TBI retrofit system used a Holley OEM-type TBI unit. Spark timing was electronically controlled
by the Electromotive microprocessor, which also controlled throttle body fuel flow. EFI quickly revealed its
benefits over the factory BBD carburetor that had been smothered by its emission control
If your Jeep engine must use the Carter BBD system, see the magazine's blueprint for
reliable performance from a stock 4.2L BBD carburetor. We drove
4.2L Jeep CJ and YJ Wrangler 4x4s over the Rubicon Trail ‘in the day’. Restored to factory tune, the Carter BBD can
work as well as other carburetor types—granted, with all the inherent quirks and off-road limitations of
By the mid-‘90s,
Mopar Performance seized on the benefits of OEM type MPI as a retrofit for the 1981-90 Jeep 4.2L inline sixes.
Mopar Performance MPI/EFI Conversion Kit is a 50-state legal, California CARB E.O.
kit uses a slightly modified OEM 4.0L intake manifold and complete multi-point (six injector) injection system, a
PCM-controlled ignition distributor and wires, air intake pieces, a retrofit wiring harness, a high-pressure
two-rail fuel delivery system, necessary sensors, the PCM computer and all required emission control
Mopar EFI Conversion on a fresh Jeep inline six goes into a 1987 Jeep
YJ Wrangler. This kit builds around Jeep OEM components for parts availability, reliability and
Jeep dealership DRB scan tool troubleshooting. Chassis/engine combination and induction
system meet California CARB requirements.
For simplicity and ready parts access,
the core Mopar Performance components were originally ‘off-the-shelf’ 1994-95 4.0L Wrangler or XJ Cherokee
(two-rail fuel supply) pieces.
Later Mopar Performance kits
follow the single-rail 1997-up 4.0L fuel delivery approach and use a modified, late style pressure
regulator. The system has a MIL engine check
provision plus embedded PCM “factory” diagnostic troubleshooting codes (DTCs) for scan tool use.
Mopar Performance MPI retrofit for the 4.2L inline sixes have been ‘how-to’ segments
within Moses Ludel’s Jeep® Owner's
Bible™ (now in 3rd Edition) and the Jeep® CJ
Rebuilder's Manual: 1972-86 (Bentley Publishers,
More recently, others have developed cost-effective TBI solutions for the Jeep 4.2L inline six.
Notably, the Howell Engine Developments system is 50-state legal with a California CARB E.O.
of our installation date (August 2012), Howell Engine Developments and Mopar Performance are the only 4.2L
Jeep inline six retrofit EFI systems available with 50-state legal status. Each of these systems has Jeep
model-specific California CARB E.O. numbers enabling motor vehicle registration and use on public roads
within the State of California.
Howell Engine Developments,
Inc. offers this 50-state legal EFI
conversion for '81-'86 Jeep CJ 4.2L inline six-cylinder engines. Howell kit meets California CARB E.O.
requirements and works independently of the stock Jeep Motorcraft ignition system—which can be retained. (Photo
courtesy of Howell Engine Developments, Inc.)
Both the Howell and MSD systems can work
in conjunction with an OEM Jeep Motorcraft ignition distributor. Additionally, the MSD Atomic EFI can work with an
MSD AMC/Jeep 8516 Pro-Billet distributor. By itself, the MSD 8516 Pro-Billet distributor holds a California CARB
E.O. number. So does the MSD Blaster 2 ignition coil (seen above).
EFI requires either an MSD 6-series box or a 'Ready-to-Run' distributor. A Pro-Billet distributor and MSD 6-series
box make it possible to use the Atomic EFI's electronic spark timing control option.
Our Tailpipe Emissions
Our installation strategy was simple: We would keep the Jeep 4.2L inline
six’s stock intake manifold, exhaust manifold and all compatible emissions devices. Use of stock manifolds enables
use of the OEM exhaust gas recirculation (EGR) system (above). EGR is a key emission device.
Note: The stock
exhaust manifold also provides a location for the MSD Atomic EFI wide-band oxygen sensor. The wide-band oxygen
sensor fits neatly in place of the OEM Jeep oxygen sensor.
system is often misunderstood. This device dilutes the incoming air/fuel charge under specific engine operating
conditions. The aim of EGR is to drop combustion (upper cylinder) temperatures below 2500-degrees
EGR valve has been removed from the intake manifold casting. (Two ports are visible.) Below the EGR valve is
the exhaust manifold's oxygen sensor port with a new wide-band MSD oxygen sensor
An EGR valve
typically operates or receives an opening signal from ported vacuum. The valve opens at throttle tip-in, remains
open through partial throttle, and closes as the throttle opens wider. Like ignition distributor ported vacuum, EGR
should not open when the engine is at idle.
EGR targets NOx tailpipe
emissions, which increase above 2500-degrees F. Without EGR, combustion chamber temperatures on an older carbureted
engine with a conventional distributor can soar to 4,800-degrees F. Some types of aluminum will melt
at one-fourth this temperature!
To meet emission
requirements, get a “free” drop in upper cylinder temperatures, plus reduce the risk of ping or
detonation, leave the EGR system in place and operative. On this Jeep 4.2L EFI project, we
Note: On OEM applications, we leave EGR systems in place and operative. Incremental performance gains
from eliminating the EGR are more than offset by the benefits of lower combustion temperatures and resistance to
ping under load. For off-pavement rock crawling or trailer pulling, the cleaner air EGR system has
prototype, a 1989 Jeep YJ Wrangler, requires a catalytic converter. We retained the entire stock exhaust system,
including the OEM cat.
cat uses a ‘Pulse Air’ passive air supplement. Mopar and Howell find Pulse Air and air injection undesirable and
unnecessary for meeting emission standards with EFI. Each of these EFI retrofit kits earned CARB E.O. numbers
without the use of Pulse Air or an air injection pump system.
the stock catalytic converter in place and fully functional. Following the Howell and Mopar examples, the
Pulse Air inlet tube to the cat got sealed off. So did this upstream exhaust pipe's pulse
A quick way to seal off the catalytic
converter’s air inlet is to either cap the tube or pinch the tube’s end and run a simple weld bead on the pinch
seam (below). A muffler shop can perform this task in
catalytic converter retains full function minus the air supplement. This is emission legal for both Howell and
Mopar Performance retrofit EFI systems under their California CARB E.O. requirements.
earn an E.O. number from CARB, tailpipe emissions cannot exceed standards set for the original engine in good
operating condition, with all of its original emission components in place. A properly programmed retrofit EFI
system can lower the tailpipe emissions substantially.
for a stock OEM carbureted engine, if the emission control system originally had air injection, pulse air, any
other kind of catalytic equipment, an EGR valve, oxygen sensor, any feedback sensors, a thermal air cleaner
assembly or any vacuum devices, each device must be complete, intact and operational in order to pass an
emission compliance test. All intended OEM emission components must be in place and
Vehicles with a non-approved aftermarket air cleaner, non-OEM dual exhaust or header, a non-approved ignition
distributor, intake manifold, or even an incorrect replacement gas cap, will fail a California Smog Check
visual inspection. Each emission-related component must look right and function
with a California CARB E.O. number can be installed, but only in accord with the manufacturer's
instructions. If any required part is missing from the OEM emission system or modified, the vehicle will fail
the visual inspection during a Smog Check for vehicle registration.
California smog check, if a non-CARB approved fuel, vapor, spark, emission, exhaust or induction system component
is on the vehicle or engine, the vehicle will fail the visual inspection. This is true regardless of the
tailpipe emissions readings.