How-to: Troubleshooting Jeep Fuel Injected MPI Engines
Overview: Jeep ‘PCM’ and
Multi-Point Fuel Injection Systems
AMC electronic fuel
injection for Jeep engines dates to 1987. From 1987-90, the 4.0L XJ Cherokee benefited from multi-point fuel
injection. This reliable Renix system could easily have been adapted to the 4.2L six, a move that would likely have
earned praise from YJ Wrangler owners. However, Jeep Corporation would finish out the 4.2L and 5.9L AMC V-8 eras
In 1991, the strategic
shift to Chrysler’s proven multi-point (MPI), sequential fuel injection launched the XJ Cherokee and the YJ
Wrangler to new levels of performance. The engine fuel and spark management computer (ECU) became the Powertrain
Control Module (PCM) with broader responsibilities and more sensor input. Automatic transmission and chassis
functions, including optional ABS, gained electronic controls and feedback sensors that make the 4.0L era YJ
Wrangler (1991-up) and TJ models more versatile and responsive to modern emissions and performance
Introduced on the 1991
2.5L four and the 4.0L six-cylinder Cherokee and Wrangler engines, Chrysler's MPI boosted horsepower while
delivering maximum fuel efficiency and a clean tailpipe. Noticeably, the fours delivered more power and an improved
torque curve. Despite the loss of the long-stroke 258/4.2L engine, the new 4.0L MPI six in YJs refined and
dramatically improved the multi-purpose highway driving and high altitude, backcountry trail-running experience for
Jeep 4WD owners.
These MPI engine
designs, well able to meet emission requirements and fuel economy needs, carry forth into the TJ Wrangler era. YJ
1991-95 engines and the TJ Wrangler powerplants are similar. Small differences on the TJ models include the switch
to a filter/pressure regulator at the fuel tank with a single fuel supply line to the engine fuel rail. Also, the
1997-up MAP sensor mounts at the throttle body area of the intake manifold, eliminating the familiar MAP location
on the firewall.
The XJ and TJ’s triple
32-Way and later TJ triple 35-Way PCM units replace the 60-Way PCM and mount conveniently at the firewall above the
battery. All XJ, ZJ, WJ and TJ Jeep models comply with OBD-II standards for emissions, repairs, diagnostics, PCM
updates and maintenance.
2000-up 4.0L MPI
engines are distributorless, while the 2.5L MPI engine maintains its distributor through 2002 models, the last
offering of the 2.5L AMC-design four. The 2.5L four dutifully served Jeep light utility 4x4s and SUV XJ Cherokees
for two decades. MPI 4.0L inline sixes remain the highly popular engine option for TJ Wranglers, included as
standard equipment on Unlimited and Rubicon models. Beginning in 2003, the 2.4L DOHC inline four replaces the 2.5L
four as the highly reliable and efficient MPI/SFI base engine.
For reference sake and
distinctions, I break the Jeep MPI fuel-and-spark management systems into distributor type and distributorless
type. 2.5L YJ and TJ models through 2002 rely upon the distributor type ignition system. 4.0L engines through 1999
have ignition distributors. From 2000-up, the 4.0L engine is distributorless and uses a camshaft position sensor
(CMP) with oil pump drive in place of the distributor. (1991-99 Jeep 4.0L MPI engines have the camshaft position
sensor built into the distributor assembly.) Ignition coils at the spark plugs eliminate the need for spark plug
wires on distributorless engines.
The camshaft position sensor (CMP) replaces
the ignition distributor on 2000-up 4.0L engines. The oil pump drive and camshaft position sensor are a unit. The
CMP on 4.0L MPI distributorless engines works with the crankshaft position sensor (CPS) to help the PCM separate
and adjust spark timing and fuel injector events. The camshaft rotates at half the speed of the crankshaft, in sync
with the ignition firing cycles. A worn timing chain can affect the CMP signal and conflict with the CPS
Coil gangs mounted in paired cylinder
fashion attach directly to the spark plugs. Although not as convenient for spark plug service, the distributorless
ignition fires far more accurately and fully in sync with millisecond signals from the PCM (powertrain control
module). This advanced fuel-and-spark management brings the 4.0L and 2.4L engines to peak performance and the
cleanest possible tailpipe readings.
Diagnostics for the EFI/MPI systems follow
industry protocols during the YJ and TJ eras. YJs, including the carbureted models, were of the DRB-II® tool era.
OBD-II era TJs benefit from the DRB-III® tool. These tools point out specific problems based upon stored codes and
actual interrogation of the engine and powertrain functions. Diagnostic connector plugs shown here are access
points for troubleshooting early YJs with 6- and 15-Way diagnostics.
The 16-Way (pin) Data Link Connector on TJs
rests just below the dashboard to the left of the steering column (left-hand drive models). This is common OB-II
era diagnostics retrieval. Shown here is the DRB-III® tool in action (at left) and DaimlerChrysler’s StarSCAN®
scanner tool designed to meet ‘Next Generation’ model needs and 2008 Federal/C.A.R.B. diagnostics and vehicle
chassis wiring requirements.
Chrysler vehicles have Controller Area Network (CAN) bus wiring and interactive circuits. StarSCAN® meets the
sophisticated diagnostics capability of CAN-bus systems and advanced chassis and powertrain interfaces. (See the
MPI diagnostics section of this article series for further details on DRB-III® and StarSCAN® diagnostics
Before moving into
actual diagnostics and troubleshooting of Jeep MPI/SFI engines, an orientation to the fuel and spark management
system is valuable. There is no better example of the system’s dynamics than to review the installation of a
Mopar Performance MPI/EFI Conversion package. (See article in this series.)
This kit, derived from
the similar 2.5L and 4.0L Chrysler MPI platforms introduced in 1991, consists largely of off-the-shelf components
for the 1994-95 YJ Wrangler 4.0L engine with later TJ features included. This is essentially the equipment that
launched the TJ Wrangler in 1997. With the exception of the distributorless ignition, introduced on 2000 4.0L
engines, the Mopar Performance EFI Conversion Kit uses components commonly found on 1991-up MPI 2.5L and 4.0L
MPI Devices and
On Chrysler and DaimlerChrysler MPI/SFI systems, components are not difficult to identify. Armed
with a stored DTC or your scan tool diagnostics, you can troubleshoot the suspected circuits or components. The aim
of this section is to familiarize owners and troubleshooters with the devices and areas involved with common MPI
fault codes. Testing and repairing these faults can quickly put a Wrangler back in service.
While the PCM operates in open loop and closed loop
modes, the most significant advantage in closed loop mode is that the oxygen sensors provide input. The heated,
two-wire O2 (oxygen) sensor provides the data for monitoring and adjusting air-fuel ratios. Optimal A/F ratio for
gasoline clean burn (stoichiometric) is 14.7:1.
Oxygen sensor failure or
faulting will send a DTC and force the PCM to run in open loop with a preset A/F or “limp home” mode. Proper oxygen
sensor performance is vital.
Engine performance is considered in
various “modes.” These modes are 1) open loop start-up, 2) open loop warm-up, 3) idle after warm-up in closed loop,
4) cruise mode in closed loop, 5) acceleration in open loop, 6) deceleration in open loop, 7) wide open throttle in
open loop. There is also key-ON engine not running, which is open loop and, of course, the key-OFF mode. Each of
these modes involves specific MPI, ignition and related devices. We can troubleshoot within the framework of the
Engine Not Running: Key-ON and Key-OFF
When the key turns to the ON position, several devices come into play
before the engine cranks over. The PCM activates the Idle Air Control, setting it for start-up. A MAP sensor signal
tells the PCM what the air/fuel needs will be. Additional advice comes from the coolant temp and manifold air temp
sensors. An auto shutdown (ASD) relay turns on for a three-second interval while the fuel pump powers up. The ASD
also starts the oxygen sensor heat up process.
The fuel pump will only run for
three seconds with the key ON unless the engine is running or the starter motor cranks. If you question why the
fuel pump stops running with the key ON, consider how long the key has been ON without cranking the engine. More
than three seconds, and the pump stops. Key ON, engine not running is a busy mode!
Key OFF shuts down the PCM. This cuts off the injectors, the ignition coil(s), and any active
relays for fuel-and-spark management, including the fuel pump and ASD devices. This shuts down the heat leads to
the oxygen sensors. The PCM masterminds the powertrain functions and each mode of operation. Sensors provide
necessary data for each mode of engine operation.
Start-up Mode and Trouble
Start-up involves a number of signals to the PCM. Of immediate concern is the battery state of
charge (voltage), the temperature of the engine’s coolant, getting crankshaft and camshaft position readings, and
processing the TPS, MAP and MAT data. The PCM watches for crankshaft movement, activating the fuel pump relay for
only three seconds if there is no cranking.
Battery voltage is an important signal. The system looks
for a minimum of 12.4 volts. (This is just over a 50% state of charge.) Be aware that running a battery low with an
MPI/PCM system will not provide a strong enough signal to the PCM. Here, this winch-equipped Rubicon TJ has a dual
battery upgrade and a battery isolator to assure that one battery is always at a sufficient charge level for engine
Note—The 13.41 volts
is the fully sealed auxiliary battery at full charge with the engine shut off. These are special batteries, sealed
to prevent leakage in the event of a rollover. The manufacturer advocates use of the battery in this position. It
does not leak or emit explosive fumes.
The coolant temperature sensor (CTS) is a common EFI
device. Coolant is the quickest way to determine engine running temperature. The sensor mounts at the thermostat
housing, the warmest source point to read temperature. This is where coolant exits after circulating through the
engine. The PCM system will not work efficiently below 195-degrees F on a warmed up engine.
On 1991-99 MPI engines with a distributor (2.5L MPI
fours through 2002), the camshaft position sensor (CPS) mounts within the distributor housing. On 2000-up 4.0L
distributorless engines (as shown), the CPS is also the oil pump drive. The CPS device must be timed much like
setting a distributor into place. The signal indicates precise camshaft movement. All MPI engines require a CPS
Caution—Before servicing the distributor or CPS unit, always line up the
crankshaft TDC mark for #1 piston on its compression stroke. Remove the distributor cap or CPS sensor’s cover to
note the shaft or rotor position. Scribe marks to show where the shaft or rotor points with the crankshaft at TDC
on the compression stroke and the housing aligned with the engine block’s locating bolt hole and clamp. Align these
marks during assembly. The distributor and CPS must be “timed” correctly.
The crankshaft position sensor (CKP) is at the back of
the engine above the flywheel or drive plate. This has been the location for the sensor as far back as the TBI 2.5L
engines (shown here). A flywheel or drive plate trigger tooth pattern provides the signal for the sensor. The
crankshaft position sensor (CKP in newer Jeep engines) is a vital PCM signal, sending a precise top-dead-center
piston position reference.
The throttle position sensor (TPS) is essentially a
potentiometer that indicates the voltage setting for various throttle positions. Voltage increases over the opening
range of the throttle valve. Since input to the TPS is 5.0 volts, the output reading should not read at or above
4.5 volts with the throttle wide open. Idle voltage at the throttle stop should be over 0.350 volts and under 0.900
volts. The need for an accurate digital voltmeter that reads 1/1000ths of a volt is obvious. (See the 2.5L
TBI article for more details.)
MAP sensor output voltage is 4 to 5 volts, which drops
to 1.5 to 2.1 volts when the engine warms and the transmission is in neutral at an unloaded idle condition. This is
a crucial signal for the PCM. (See the TBI article for more details.)
MAT sensor is at the intake manifold. Sending an actual
manifold temperature signal to the PCM, this device is part of several operation modes. Mounted to the right of the
throttle body on this TJ OBD-II engine, the MAT is easy to spot and change. If a DTC calls for replacing a MAT,
torque the new MAT sensor to 20 ft-lbs. Do not over-tighten.
The fuel pump relay and ASD relay are each in the Power
Distribution Center. This relay/fuse box unit is easy to access. Near the battery and labeled in detail, the PDC
offers ready service of fuses and relays. Here, the map of relay locations quickly turns up the ASD and fuel pump
The relays lay in the positions indicated on the
fuse/relay map. Find the relay to be tested. You can remove the relay and check across its poles with an ohmmeter
and jumper leads. The voltmeter function can test power to the relay sockets. A Power Distribution Center makes
access easier. Never attempt to “jump” or bypass a relay or fuse. PCM damage could
MPI Operation: A Warming
During the warm-up cycle, the PCM
stays in open loop mode. The PCM looks for each of the signals described in the cold-start and starting mode. There
are two additional devices that enter the fuel-and-spark management equation: 1) whether or not the Park/neutral
position is engaged on automatic transmission models and 2) whether an air conditioner equipped model has the A/C
either selected or set into ON mode. These signals tell the PCM that the warming engine is under additional
load—either in gear or with the A/C on. The PCM adjusts the idle through the Idle Air Control
Note—Four-cylinder models use a power steering pressure switch that signals
an increase in power steering load at an idle after engine warm-up. This switch will increase the idle speed
slightly to prevent the engine from stumbling under steering load. (Six-cylinder engines do not require this power
steering switch or an idle speed change.) The 2.5L power steering switch will operate whenever the engine is
running and warmed up. This switch should be closed with the engine shut off. Test with an ohmmeter across the
switch’s terminals. The switch is near the fan. Be cautious. Even though the engine is shut off for this test,
disconnect the negative battery cable for safety sake.
The power steering pressure switch on four-cylinder
engines is on the power steering hose. This switch indicates a load on the engine when steering pressure increases
at an idle. Placing an automatic transmission into gear will increase load at idle, and so will turning on the air
conditioner (shown here). During warm-up and warm running conditions, the PCM compensates for these loads at an
idle, increasing speed as needed.
Idle Mode: Switching to Closed
As the engine reaches warmed stage, the PCM adds the oxygen sensors to its data resources.
The injectors continue to fire by way of the PCM, which provides the ground for each injector to fire in its
sequence. Injectors have positive (+) current at all times through the ASD relay when the engine is on. The PCM
fires each injector by supplying the on-and-off ground circuit after reading and processing the relevant sensor
Following warm-up, the power
steering pressure switch on 2.5L engines also comes into play. This switch will increase the idle speed slightly to
prevent the engine from stumbling under heavier steering loads at idle. The switch will activate far more readily
with oversized tires and while the Wrangler is rock crawling. TBI and MPI 2.5L engines require the power steering
pressure switch signal to maintain a steady idle.
Now in closed loop, the PCM adjusts injector pulse width (flow), spark timing and the engine
idle. The O2S (oxygen) sensors help maintain optimal air-fuel ratios under cruise and idle driving conditions. In
meeting stoichiometric or 14.7:1 air/fuel ratio for gasoline, the fuel burns as completely as possible, especially
with the dual firing, coil-on-plug (distributorless) systems. The PCM adjusts idle speed at the IAC. Air
conditioning clutch cycling is through the PCM by way of the clutch relay.
The idle air control (IAC) maintains the idle speed. The
PCM adjusts the IAC to hold a steady idle. Four-cylinder engines benefit from a power steering pressure switch as
well. The IAC “motor” mounts alongside the throttle body. This is an electrical device controlled by the PCM.
Mounting screws torque to 60 in-lbs. The IAC has been a reliable, industry-wide mainstay device for MPI engines
since the 1980s.
Warmed and still in closed-loop mode, the PCM operates the cruise mode functions. I equate cruise
to lighter loads, higher manifold vacuum and reasonable highway speeds. The upshift light on manual transmission
models helps train drivers in how to maintain a cruise load level, avoiding unnecessary lugging or over-revving.
Cruise loads are optimal for fuel efficiency and engine longevity.
At cruise mode, all of the warm idle functions are operative, and the PCM functions in the same
way as other closed loop, warm engine modes. Interestingly, the PCM controls ignition timing by switching the
ground path on and off to the coils. At this higher manifold vacuum driving mode, the engine delivers peak
efficiency and fuel economy. Here, spark timing can advance, and fuel mixtures stay at 14.7:1. By contrast, heavier
throttle uses more fuel, and hard acceleration requires fuel enrichment and a switch to less efficient open loop
Whether distributor type or distributorless, the 4.0L
MPI ignition depends upon the PCM for spark advance and retard. This function takes place by the PCM switching the
ground path on-and-off to the ignition coil(s). On engines with coil-on-plug coil packs (shown), the coils fire
cylinders at TDC of the firing and exhaust strokes. To do so, each coil fires cylinders paired as 1-6, 2-5 and 3-4
in the normal 1-5-3-6-2-4 firing sequence.
Note—2.4L DOHC four-cylinder engines have spark plug cables. These engines
also fire atop each compression and exhaust stroke, pairing 1-4 and 3-2 cylinders in the 1-3-4-2 firing order. The
“wasted” fire atop the exhaust strokes helps clean the spark plugs and combustion chambers while lowering
emissions. There is no fresh fuel available, so the spark plug simply cleans itself and continues the exhaust
Open Loop Acceleration and
Ever wonder why MPI engines with modern emissions packages still suffer poor fuel efficiency when
driven hard? Sudden increases in the throttle opening, like a passing situation on a two-way traffic, two-lane road
at the rush hour, tell the PCM to abandon closed loop. In open loop mode, the fuel injector pulse width widens to
accommodate the need for acceleration. The PCM disregards the oxygen sensor signals and shifts to the acceleration
air/fuel ratio programming values.
Air/fuel ratios normally adjusted by oxygen sensor
signals will not work under hard acceleration (shown). In order to move a vehicle’s mass at an accelerated rate,
the engine must run richer. While this impacts emissions, the downstream catalytic converter(s) and spark timing
adjustments can balance out the occasional enrichment mode. The PCM delivers a pre-set, richer injector pulse width
that remains consistent with open loop tailpipe emissions requirements.
Hard deceleration is also an open loop mode. The cruise
mode sensors plus the vehicle speed sensor play a role here. Oxygen sensors have no role, as the system must not
attempt to maintain stoichiometric (14.7:1 A/F) with a fully closed throttle! If the throttle position is fully
closed as shown here, the PCM will cut off all of the injector flow. If deceleration is gradual, the PCM will
adjust injector pulse width (flow), spark timing and the IAC to stabilize the slowdown to idle. All of this is
dependent upon the driver’s throttle demands.
Wide Open Throttle, Open Loop and the
On BBD feedback carburetion systems and early TBI/EFI systems, the wide open throttle (WOT)
switch is either on or off. On the MPI engines, the signals for wide open throttle come from the TPS and other
sensor devices that serve during cruise and acceleration modes. PCM open loop adjustments of the injector pulse
width/flow will meet the wide open throttle demand. The PCM continues to control spark timing by switching the
on-and-off grounding to the coil(s).
The PCM routes the alternator field winding, fuel injectors, coil(s), some relays and some
sensors through the “power ground” circuits. Whether responding to wide open throttle or other injector and
ignition timing demands, the PCM uses these power ground circuits to complete the ground loop at constantly “hot”
(+) devices. Ignition “dwell” and alternator charge rate are PCM functions.
The PCM has a large role, with nearly 30 input signals
from a variety of sensors, powertrain switches and sometimes the 16-Way Data Link Connector. There are nearly as
many PCM outputs, several of bus type to cover a variety of functions on a single circuit. The PCM operates power
ground circuits, monitors the cruise control and interfaces with brake, clutch and transmission signals. Better
yet, with OBD-II, the busy PCM is also programmable and able to download updates as well as upload data and
information! Shown is a late 4.0L TJ Wrangler's PCM with three 35-Way connectors.