Jeep How-to: Advanced
Fuel-Injection Troubleshooting and Diagnostics
On-board computers and
self-diagnosis have been the most significant advancements in the automotive service field. In the earliest years
of my professional career, engine oscilloscope diagnostic equipment provided a professional edge. At my shop stands
a mid-1960s circa Sun 720 Electronic Engine Tester built by Sun Service Equipment. The machine was state-of-art
during the muscle car era, that exciting period when pony cars and the likes of Mopar 426 ‘Hemi’ V-8s were making
racing history in NASCAR, NHRA and most backcountry lanes of North America.
In the rugged
surroundings of the Eastern Sierra, with its practical need for 4WD utility vehicles, our local burger joint’s
parking lot and Highway 88 looked no different on Friday night than Woodward Avenue in Detroit, Michigan. From the
fall of 1963 when I began high school until my graduation in 1967, the local new car dealerships provided the most
sought-after performance cars that enthusiast magazines could promote.
Despite ready access
to fishing, hunting and exploring ghost towns, with primitive back roads virtually leading from our doorsteps, the
high performance virus was infectious. Even the local flat-fender hunting rigs saw occasional small-block V-8
transplants, with a close-ratio, muscle car T-10 four-speed transmission sandwiched between the engine and Model 18
Performance engine building and tuning were popular, and I soaked up all of the
information the magazines, professional racers and seasoned mechanics could mete out. In an era of breaker
point ignitions and the rare “transistor” distributor, blueprinting and tuning involved a series of
adjustments and settings, typically valve clearances, ignition point dwell/cam angle, spark timing,
distributor re-curving and building just the right carburetor for a refined level of
Common tools of the tuning trade were the dwell-meter/tachometer, a breaker
point spring scale, a vacuum gauge, a valve timing degree wheel, small gauges for detailed carburetor work
and an induction timing light, preferably with built-in advance. My burgeoning library and stuffed magazine
rack consisted of every factory tuning specification I could lay hands on. These materials grew to
proportions that served well years later when I was a technical editor and performance feature
contributor to both OFF-ROAD and Popular Hot Rodding
magazines from 1985-1997.
Equipment like the Sun 720
Tester and distributor strobe machines lined the walls of quality dealerships and automotive performance shops
during my early years as a professional mechanic. The centerpiece of the Sun 720 Tester was an oscilloscope that
could display ignition patterns under a variety of load conditions. A parade sequence showed the spark plug fire,
coil oscillation, points closure and dwell duration for each of the cylinders on a running engine. This became very
useful once you understood the patterns and engine tuning dynamics!
Breaker point ignitions, of course, were limited to 20-40KV range coil
performance, which made adequate spark a constant challenge. Dual point ignition distributors were a popular
performance item, capable of increasing the dwell/cam angle for better coil saturation—in other words,
allowing the coil to build enough voltage to set off a better spark at each cylinder during high rpm engine
operation. The breaker-less transistor ignitions, predecessors to the common high-energy electronic
distributors of the mid-1970s, were seen as exotic components found only on a select number of factory muscle
The 720 Tester features a 10,000 rpm tachometer with cylinder balance/rpm load test
capability, a combustion analyzer for reading CO and hydrocarbons, a dwell/cam angle meter for breaker point
setting, a condenser tester, a cylinder leakdown tester and a vacuum pump and gauge. The timing light with built-in
spark advance adjustment can analyze an advance mechanism’s performance and the degrees of spark timing
The Sun 720 Electronic Engine Tester was a
desirable piece of equipment in the 1960s and ‘70s. A race car shop might add a distributor strobe machine like my
Allen unit (at right) for ignition spark timing re-curves. An upscale facility would have a Clayton chassis
dynamometer to simulate engine loads, road speeds and hard acceleration. An R&D laboratory might add
atmospheric simulation, heat and combustion-emissions analyzing equipment and a wind tunnel.
So, now we
fast-forward to the 1990s and on-board vehicle diagnostics. Despite the wealth of once-prized information that we
surviving racers and tuners of the 1960s and ‘70s possess, your modern Jeep's on-board PCM and sensors can perform
a better job of tuning, optimizing performance and meeting clean air standards than any of us ever could!
While I still enjoy “bench racing” muscle cars of the 1960s, my years as a technical contributor and columnist
for Popular Hot Rodding Magazine clarified how truly remarkable electronic computer
controls and fuel-and-spark management can be.
This late TJ Wrangler MPI 4.0L engine
self-tunes continuously, adjusting fuel-and-spark functions to meet driver demands, loads and atmospheric changes.
In milliseconds, the PCM processes information from sensors, maintains optimal injector fuel flow and adjusts spark
timing to ever-changing demands. The on-board PCM replaces dynamometer “tuning” of a bygone era with
software-driven spark timing and fuel maps. The Wrangler’s powertrain control module and sensors operate from a
data base unknown to factory R&D labs of the muscle car era.
EFI saved the internal combustion engine from extinction. For four-wheelers,
electronic fuel and spark management made all-angle, high altitude rock crawling far more accessible and far
less frustrating. Capable of optimizing air/fuel mixtures and spark timing in milliseconds, modern electronic
fuel-and-spark management also checks and balances itself—and even notes when important functions are out of
tolerance by storing a diagnostic trouble code (DTC)!
Although one’s first glance at PCM and electronic fuel-and-spark management may
be daunting, we have grown to value computers and their ability to store and analyze information. In effect,
the OBD-I or OBD-II diagnostics and troubleshooting built into the Jeep PCM is hugely more precise and
accurate than the carburetor and rigid ignition distributor technology of the muscle car
Few still argue
about the virtues of simply changing breaker points and setting the base timing by moving the distributor
housing. With a PCM, we benefit from continually changing spark timing, a distributorless ignition that
references a precise TDC reference on the flywheel. Add to that the ongoing feedback from the camshaft
position sensor, a great way to monitor valve timing and the timing chain or belt wear!
The PCM handles this
and far more. Air/fuel ratios, once determined by fixed jets that were installed trackside for a particular
elevation and weather conditions, is now adjusted—in milliseconds—to the ever changing variables of altitude,
engine load, atmosphere, engine temperature, cold-start behavior, throttle opening and demand, transmission
shifting characteristics, axle ratios, tire sizes and way, way more!
Tuning objectives for the
Sun 720 Tester and the master mechanic/tuners of the muscle car era were forever outstripped by the introduction of
lambda-based oxygen sensors and electronic fuel injection systems in the 1970s. The 2.5L TBI Jeep engine found in
1987 YJ Wrangler models had more ability to tune itself than any 1960s Grand Prix circuit or Indianapolis 500 race
car. Chrysler’s 1991-up MPI system and 1996-up OBD-II on-board diagnostics are quantum leaps forward from the TBI
If you could tow the Sun 720 Tester behind a microprocessor
equipped Jeep, the bulky machine would provide a mere fraction of the analytical tools found in the MCU
unit of a feedback carbureted 4.2L CJ or YJ Wrangler! A closed loop fuel system and carburetor with a stepper
motor, coupled with an electronic ignition distributor, were dramatic gains. By contrast, PCM fuel-and-spark management of the modern Jeep era can
not only interrogate the powertrain but also provides a platform for reloading or “re-flashing” software in
need of factory updates and improved tuning.
computers have provisions for reprogramming and upgrades that improve drivability and reliability while delivering
cleaner tailpipe emissions. Aside from routine service parts replacement, “tuning” is now the domain of specialized
fuel, ignition and emissions engineers and software programmers.
MPI/PCM On-Board Diagnostics and
‘DTC’ Trouble Codes
The PCM and on-board
diagnostics have virtually ended the guesswork of engine service. If the engine is in sound mechanical condition,
the electronic control module will monitor and troubleshoot the ignition and fuel flow functions. The PCM
constantly checks device functions and in later Jeep models can even monitor some systems.
A Diagnostic Trouble
Code (DTC) reflects troubles at individual sensors, electrical connections, electrical grounds and faulting
ignition/fuel or emissions parts—even if the problem is infrequent or intermittent. Intermittent problems have
always posed a challenge. The DTC will point to the system or device where a problem exists.
While the modern service tech is still responsible for quality repairs and replacing only
the components required, isolating a faulty system is no longer a problem. On-board diagnostics can provide a DTC
for critical powertrain and chassis functions and devices. On-board diagnostics is only limited by the number of
sensors in the system and the programming of the PCM to isolate sensors and troubled devices.
Note—From 1991-onward, the Chrysler MPI systems and the Diagnostic Readout Box (DRB-II® and DRB-III®)
scan tools have made diagnostics quicker, more accurate and ultimately less costly to the Jeep dealership's
Wranglers or Cherokees
do not fit the concept of an ordinary “town car.” Yes, it is reassuring that your local Jeep dealership has the
right tools to rapidly troubleshoot a failed MPI or spark management system. For the backcountry traveler, however,
a bigger concern might be how to keep the XJ, YJ or TJ moving along a narrow, rocky shale trail at 13,000 feet
elevation. This raises the bar, for sure, and owners must rely on simpler troubleshooting techniques for such
will offer a snapshot of an existing problem or trouble pending. DTC charts for particular Jeep models can help
trail runners quickly isolate a problem that occurs far from DRB scanners and those other valuable diagnostic tools
typically found at a Jeep dealership or a well-equipped shop. While a Five-Star® Jeep
dealership wants to earn your business with prompt, accurate and cost-effective repairs, on the remote trail
that option does not exist.
In articles covering
hands-on troubleshooting, I include backcountry field methods for keeping your Jeep running. The on-board
computer is capable of analyzing and compensating for a variety of troubles. When that fails, there is
a limp-home mode to get your Jeep and its occupants safely back to civilization.
If limping home is not
good enough, however, see the article on ‘Quick Troubleshooting’ in this section of the magazine. The tools I
describe can be carried in your Jeep’s backcountry tool box or readily used at your home shop. These devices
are even popular with professional Jeep technicians who want a fast sense for a system’s ignition and fuel flow
MIL or Engine Check Light
The engine check light or MIL (Malfunction Indicator Lamp) is usually the driver’s first
sense that the engine’s emissions, injection or ignition could be faulty. The light indicates that a DTC has been
established and stored in the computer.
This DTC information
will remain until either 1) the problem is repaired and the code erased or 2) the problem does not persist and
after a given (typically 40) warm-up cycles of the engine, the PCM erases the code. For this reason, a one-time
fault will not be stored for the life of the engine and may not show up months later as a stored
The MIL is a warning.
You may not be aware of any troubles, and perhaps the Jeep is running well. The DTC could be related to a device or
code that does not readily have a symptom—like a battery temperature sensor malfunction that does not impact
performance or engine starting ability.
Sometimes, there is no
sign of trouble because a 1996-up OBD-II system is throwing a trouble code based on a software conflict rather
than a hardware defect. In these cases, your Chrysler/Jeep dealership will re-flash (download) updated
software that remedies the problem.
Note—If your OBD-II Jeep has a persistent MIL signal
at the instrument panel but no symptoms of trouble, see your local Jeep dealership. There may a simple
software re-flash available for your specific model year, chassis VIN and emissions
The most significant distinction in
OBD-II Jeep PCMs is the data and downloading capability of these systems. The Cherokee and Wrangler 60-Way (pin)
PCM has been replaced by a PCM with three 32-Way or 35-Way (later Jeep models) connectors. On 1997-up TJ Wranglers,
the PCM mounts above the battery at the firewall. The fuse-and-relay box (Power Distribution Center or PDC) is
nearby. Troubleshooting is easier and more logical than earlier systems.
The best way to read a DTC is with a scanner tool (preferably the DRB-III® tool if
available). However, you can retrieve a DTC code on many Jeep vehicles with a very simple procedure: cycling the
ignition key switch. Turn the key ON-OFF-ON-OFF-ON rapidly without cranking the engine. The MIL lamp will begin
flashing the DTC.
The flashes will come
in a series, with a pause between flash sets. For example, a DTC code of 21 (indicating that the engine is cold too
long) would be two flashes, a pause, and then one flash. The code can repeat itself. If your Jeep
model has this function, retrieving the code will lead to a DTC chart for pinpointing the specific
problem or trouble area.
To retrieve DTC information with a scan tool on an OBD-II system (the TJs), use
the 16-pin Data Link at the base of the dashboard. The Data Link is easy to reach at the left of the steering
column. On 1987-90 YJs, the data plugs are next to the battery; ’91-up YJs have the data plug near the PCM at
the left side of the firewall.
Jeep dealerships use
the DRB-III® scanner for most YJ and TJ models. Loaded with the correct software, this tool will read data
from pre- and post-OBD-II Chrysler vehicles. A well-equipped dealership can retrieve DTC information from any
year Jeep equipped with a micro-processor (ECU) or PCM.
The DRB-III® scan tool is the diagnostic
tester of choice for Jeep dealerships. This tool can retrieve and erase codes, test devices and circuits and even
actuate devices to watch their function! Hooked to this TJ’s 16-Way Data Link Connector, the DRB-III® scan tool is
a fast and accurate diagnostic instrument. This is a wise, pinpoint troubleshooting measure. A StarScan®
tool is at the right of the DRB-III scan tool.
Note—If you cannot fix a problem readily, it is well worth having your local dealership perform a
DRB-III®, StarSCAN® or WiTech (latest tool) scan of systems and devices.
Considering the complexity of a modern Jeep fuel-and-spark management system, the
ability of the PCM to monitor and troubleshoot is valuable. Parts replacement is narrowed to actual devices that
test faulty. Often, the problem has a simpler cure like improving a ground or plug connection.
The Cherokee, Grand
Cherokee or Wrangler is often a backcountry access vehicle subjected to trail debris, dust hazards, stream
crossings and constant jarring. While the Jeep chassis and engine provide a rugged and highly reliable platform,
the vehicle’s use is often a source of troubles.
Rocky trails and
limb-strewn byways can tear at wiring and even crush modules and connectors. Despite these hazards, Jeep 4WD
utility models remain the rough and ready vehicles that owners expect.