How-to: Jeep Engine
Tuning and Troubleshooting
Rapid advances in engine
fuel-and-spark management span the modern Jeep era. In 1987, the YJ Wrangler 2.5L four became the first
Jeep 4x4 utility model with fuel injection. The 2.5L throttle body injected (TBI) engine was a
Cherokee offering as well.
Simultaneously, the 1987-90 YJ Wranglers with 4.2L sixes were among the last North American-built Jeep
engines to use a carburetor. In 1991, the full-size Grand Wagoneer finished its nearly thirty-year history as
the last U.S.-market Jeep to offer carburetion and an AMC-designed V-8 engine.
YJ’s 4.2L six was essentially the engine used in AMC-era CJ models, this engine also shared many design
features with the XJ Cherokee’s 4.0L inline six introduced in 1987. From 1987-90, the 4.0L six featured a
multi-point “Renix” MPI fuel injection system that worked well in this application. As Jeep Corporation
launched the 1991 models, Chrysler’s influence would be seen with the introduction of its own 2.5L and 4.0L
MPI fuel injection systems for the YJ and XJ models.
This entire section of the
magazine is devoted to 'how-to' troubleshooting and proper tuning of Jeep engines. See the following
The 1987-90 YJ 4.2L inline six
cylinder engine carries forth the CJ-era carburetion and induction system. This is a feedback BBD Carter
2-barrel carburetor that works with the MCU (micro-computer unit) and a series of engine sensors. A closed
loop design, this is the last generation carburetor. The ignition uses a distributor. Focused on lower
tailpipe emissions, these fuel and spark systems were somewhat complex.
The YJ Wrangler launched the
first utility Jeep 4x4 with fuel injection. 1987-90 2.5L throttle body injection (TBI) engines provided much
needed on- and off-pavement performance gains. EFI quickly proved its versatility on slopes and at altitude.
Over-enrichment at higher altitudes is non-existent with EFI, as feedback from the oxygen sensor and manifold
absolute pressure (MAP) sensor help adjust injector flow. 1991 2.5L engines featured multi-point fuel
injection (MPI) for improved fuel distribution and performance.
the era of carburetion and also the 4.2L six. In its place were formidable 2.5L MPI four cylinder engines and
the powerful MPI 4.0L inline six, a powerplant option that served the YJ Wrangler, XJ Cherokee and the
all-new ZJ Grand Cherokee introduced as a 1993 model. These proven 2.5L and 4.0L MPI engines also launched
the TJ Wrangler in 1997. 2.5L and 4.0L engines served the XJ Cherokee through its last model year (2001).
4.0L sixes remained the base engine for the 1999-2004 WJ Grand Cherokee and served as the more powerful
engine option for all TJ Wranglers.
strength and ruggedness of these engines are indisputable. By design, the only deficit of the carbureted
4.2L six was its continued use of carburetion. In an era of emission system constraints, the 4.2L paid
significantly for its clean tailpipe.
electronic fuel injection could easily have kept this long-stroke, stump pulling six in the Jeep lineup.
Proof of the 4.2L six’s ability to run stronger and cleaner is the retrofit of a Mopar Performance 50-State
legal MPI induction system on a 4.2L engine in good condition. Tailpipe emissions drop to levels rivaling the
4.0L engine, and horsepower increases—often 50 or more—with no other engine
While throttle body EFI was a
major breakthrough for the 2.5L engine, the switch to multi-point, port fuel injection provides a significant
gain for any inline engine. Inherently, inline engines, especially six-cylinder types, do not respond well to
a single carburetor and long intake plenum runners. The Jeep 232/258 (4.2L) six suffers significantly from
this phenomenon. A carburetor mounted mid-manifold cannot distribute fuel uniformly to the outer
body injection, which does deliver improved fuel efficiency and lower tailpipe emissions, does not solve the
plenum runner length issue. In fact, a TBI unit and use of a single oxygen sensor at the exhaust manifold
collector can still cause lean burning on outer cylinders.
Note—I highly respected the late Jack Clifford (Clifford Performance) and his contribution to
inline six-cylinder engine performance. In the dozen years that I served as technical editor of OFF-ROAD
Magazine, Jack and I shared many stories about inline six racing successes. Jack developed some of the best
induction systems ever for carburetion. His intake manifolds, headers and matching carburetion, camshaft and
ignition packages worked very well. Jack was also a pioneer in the use of retrofit EFI systems. He highly
valued the fuel flow control and distribution advantages of fuel injection. Jack and I fully agreed that
multi-point electronic fuel injection would revolutionize inline six-cylinder performance—and the XJ Cherokee
and Wranglers proved it!
The YJ Wrangler sixes needed multi-point, port fuel injection, and Chrysler responded with the
1991-up MPI 4.0L engines. Port injection provides optimal, uniform air/fuel ratios per cylinder and maximum
performance with cleaner emissions.
section of the magazine, I begin with carburetion system needs that apply only to the 4.2L inline sixes. You
will also find steps and full details on installing the Mopar 50-State legal 4.2L MPI conversion. That
induction system is patterned after 4.0L original equipment components and serves as an excellent prototype
for illustrating the layout and function of Jeep 4.0L fuel-and-spark management systems. Jeep 4.0L
models with distributor-type ignitions share similar service steps. These also apply to the 2.5L MPI
Note—Newer "distributorless" Jeep engines do not require the routine distributor
cap, rotor and spark plug wire replacement. Otherwise, the distributorless MPI systems have needs and
components much like earlier Chrysler MPI systems.
1991-up Jeep Wrangler 4.0L and
2.5L models share the added benefits of multi-point fuel injection. MPI gives long inline engines the uniform
fuel distribution that will maximize performance yet maintain the lowest possible tailpipe emissions. Both
the 2.5L and 4.0L engines benefit from Chrysler’s MPI system, illustrated here on a TJ 4.0L engine. MPI
brought out the true performance potential of the 4.0L engine. The Mopar Performance retrofit MPI package is
a major gain for early YJ Wrangler 4.2L sixes.
share fundamental troubleshooting guidelines for the 2.5L TBI four-cylinder engine, an engine available in
the early XJ Cherokee and YJ Wrangler. The TBI owner will find basic tuning and field troubleshooting are
well within reach. These are reliable systems with basic components.
YJ 2.5L four depicted in these articles had 203,000 miles on its odometer with nearly all of its
original components still in place and reliable. Many parts and tuning components for early XJ and YJ
models are still available from Mopar as well as aftermarket sources.
also included details on the distributorless fuel and spark management systems that power up the
later 2.4L and 4.0L Jeep engines. These components serve throughout the Jeep vehicle lines and
represent state-of-the-art systems. There are addition sensors on the distributorless engines that enable the
on-board OBD-II diagnostics system to pinpoint trouble spots and even signal significant internal engine
Note: Beginning with the TJ Wrangler era, on-board diagnostics meet the Federal
mandates of OBD-II, which provides consumers and service professionals with previously unavailable
and special scan tools help us perform advanced diagnostics and troubleshooting. The on-board programming
also provides the Jeep dealership with the ability to “flash” the on-board computer with approved and
emission legal updates of software for tuning and improved functions.
today’s Jeep dealership, a technical service bulletin (TSB) is often the call for a simple re-flash or
reloading of updated software. Today’s engine “tuning,” for the most part, is an update of on-board
For those new to modern
fuel-and-spark management, these articles provide a learning tool for understanding the systems. Most fixes,
especially in the field, are not complicated. It is infrequent that the powertrain control module (PCM or
computer) fails. Electronic problems are more often related to sensors that fail to provide the right
signals—or even something as simple as a loose connection, vacuum hose or faulting, low voltage signal
caused by dirty battery connections!
with the fundamental carburetion and conventional ignition of the 4.2L engine. Here, fuel-and-spark
management represents the more primitive functions of a feedback carburetor and an MCU (micro-computer unit).
Yes, even the carbureted YJ has a computer that to some degree controls fuel flow while also playing a
limited role around ignition spark timing.
understanding the needs of a basic fuel system and ignition, owners of modern Jeep vehicles will find the TBI
and MPI systems easier to troubleshoot and diagnose. The TJ 4.2L six provides a good training ground for
learning the basic fuel and spark needs of an engine.