Module 1
The PGMFI System Overview - Part 1
Author: Grant Swaim IMPORTANT - READ !
E-mail: sureseal@nr.infi.net Do not read or study this information unless you agree to the
following conditions:
URL: www.tech2tech.net The information in this training module is the intellectual
property of N. Grant Swaim and is copyrighted by Sure Seal
Phone: (336) 632-9882 Products Inc.
Fax: (336) 632-9688 Subscribers to the Tech-2-Tech website, and persons partici-
pating in Tech-2-Tech’s on-line training program are entitled
to read this material on-line.
Postal Address: Tech-2-Tech Website
PO Box 18443 You may also click on the “save” icon on the Acrobat viewer
Greensboro, NC 27419 and save a copy to your local computer. You may save a
copy of this file on one computer and it must be viewed from
that one computer.
Physical Address: 220-4 Swing Rd
Greensboro, NC 27409 You may also print one copy of this file for your viewing. If
the printed copy becomes illegible, or lost, an additional
Last Update: April 2000 copy may be printed.
Tech-2-Tech offers the following training modules in printed manual, CD-ROM, and on-line formats.
PGMFI Training Modules OBD-II Training Modules
• The PGMFI System Overview—Part 1 • On Board Diagnostics—General Overview
• The PGMFI System Overview—Part 2 • Diagnostic Trouble Codes
• PGMFI Flash Type DTCs • MIL / Freeze Frame
• Inputs / Outputs—Part 1 • Scan Tool
• Inputs / Outputs—Part 2 • Scan Tool—Advanced
• Engine Control Module • Monitor Tests—Overview
• Air Flow / MAP Sensor—Base Inj Pulse Width • Comprehensive Component Monitor
• Fuel Delivery System • Catalyst Monitor
• Closed Loop Strategies—Theory • EGR Monitor
• Closed Loop Strategies—Case Studies • Evaporative Monitor
• Thermistor Inputs • Fuel System Monitor
• Throttle Position Sensor • Misfire Monitor
• EGR Valve Lift Sensor • Oxygen Sensor Monitor
• MAP / BARO Sensor • Oxygen Sensor Heater Monitor
• Ignition Inputs • “P” Codes
• Vehicle Speed Sensor
• Oxygen Sensor Miscellaneous Training Material
• Lean Air Fuel Sensor
• Glossary of Terms
• Miscellaneous Input Signals
• Fuel Injectors—Multi-Port Injection
• Fuel Injectors—Dual Point Injection
• Ignition System—Outputs
• Idle Air Control Valve
2000 © - All Rights Reserved Sure Seal Products Inc - Greensboro, NC
Page 1-2 Visit Our Website: www.tech2tech.net
1 The PGMFI System Overview - Part 1
1.1 The Formative Years
For many years, the primary goal in engine design was for power and driveability.
The earlier engine control systems could reach these goals by using mechanical
and vacuum controls. Carbureted engines with a traditional mechanical / vacuum
distributor were used for over 15 years on most Hondas sold in the USA.
In the late 1970s legislation was passed that mandated a maximum tailpipe emis-
sions standard. Fuel-efficient cars were in demand by consumers and there was
also legislation that mandated minimum fuel economy standards. The tailpipe
emissions standards got progressively lower and the demand for high efficient
vehicles got higher. It was impossible to deliver the low emissions and high fuel
economy without better fuel and ignition control. It could no longer be done using
mechanical and vacuum controls. It was this demand for cleaner and more effi-
cient engines that required the use of electronic engine control systems.
1.2 PGMFI as an Emission Control Device
Tailpipe emissions can be put into two main categories: pre-combustion and post-
combustion. Pre-combustion emission controls are all the things that are done to
reduce the emissions before the exhaust leaves the combustion chamber. Post-
combustion controls are the systems that further reduce the emissions after the
exhaust has left the combustion chamber.
1.2.1 Examples of pre-combustion emission controls
Some examples of pre-combustion emission controls are: engine design, EGR
systems, fuel and ignition timing controls. It may seem a little odd to think of the
fuel control system as an emission control device, buy that is a large part of what
it is! The PGMFI fuel injection system is a major player in pre-combustion emis-
sions. By designing a good fuel control system, many post-combustion systems
can be down sized or, in some cases, eliminated.
Honda's approach from the very beginning was to control tailpipe emissions as
much as possible at the pre-combustion stage. The Honda engineers control as
much as possible from superior engine design, then by superior fuel and ignition
controls, and lastly by post-combustion systems.
It was this approach that led to the Compound Vortex Controlled Combustion
(CVCC) engine design that allowed Hondas to meet emission standards, until the
early 1980s, without a catalytic converter (5 years later than most domestic cars).
In this example, Honda was able to totally eliminate a post-combustion system by
superior engine design.
All Rights Reserved 2000 Sure Seal Products Inc
This manual printed 4/9/00 from the file pgmfiobd_002.
Visit Our Website: www.tech2tech.net Page 1-3
Honda even offered to license the CVCC technology to General Motors so they
too could eliminate the need for installing a catalytic converter, on their cars, in
1975. GM did not want to use the CVCC technology. Their engineers said it
would not work on a larger displacement engine.
Honda engineers took a Chevy Impala to Japan and hand built a set of heads that
used the CVCC technology and proved to GM that it would work. Even after it
was proven that GM cars could meet the emission standards without using a cata-
lytic converter, GM still did not change its mind. Apparently GM had committed
to using converters in 1975, years before. It seems that GM had made heavy in-
vestments in platinum mines and the equipment to manufacture catalytic convert-
ers.
Some of the areas of engine design that effect pre-combustion emissions are:
• Valve timing (think of VTEC as an emission control device too)
• Number of valves and valve placement
• Combustion chamber design
• Intake manifold design
It has been Honda's attention to detail in the pre-combustion area that has placed
it as a leader in producing vehicles with low emissions and good economy. For
many years, car manufacturers struggled to keep up with the increasingly tougher
emission standards. Honda is now producing cars that meet standards many years
into the future. They are the industry leader in the low emission (LEV) and "Zero"
emission technology today.
Post-combustion emission controls fall into three main categories: evaporative,
crankcase, and tailpipe emissions. Tailpipe emissions are the most important and
are comprised of catalytic converters and pulsed air injection (on some models).
These systems further reduce the emissions after the exhaust has left the combus-
tion chamber.
1.3 The Evolution of Fuel Control - From Carbs to PGMFI
1.3.1 Vacuum / Mechanical Systems
From 1973 to the early 1980s, Honda used traditional vacuum and mechanical
systems to control ignition and fuel. They used a traditional carburetor and a me-
chanical / vacuum ignition distributor. The carburetors used on the CVCC engines
are somewhat different than most since it is actually two carbs in one. It had a
section that supplied a rich mixture to the pre-combustion chamber and a section
that provided a lean mixture to the main combustion chamber. This technique al-
lowed the engines to run on a much leaner mixture than was possible with a tradi-
tional combustion chamber.
All Rights Reserved 2000 Sure Seal Products Inc
This manual printed 4/9/00 from the file pgmfiobd_002.
Page 1-4 Visit Our Website: www.tech2tech.net
These models had very basic timing and fuel controls. Most controls were me-
chanical or vacuum in nature. About the only unusual control found on these
carbs was an altitude compensating device that seemed to be reserved for the Hi
Altitude cars. It was a barometric pressure-sensing device that could add extra air
to the air bleeds as needed.
1.3.2 Feedback Carb
A forerunner to the PGMFI system was the feedback carburetor system. It was
used; beginning in 1984, on all models and was used until the carb was replaced
by a PGMFI system. If PGMFI were one of Honda's brightest engineering mo-
ments, then the feedback carburetors would have to be one of Honda's darkest en-
gineering moments (author's opinion only). The Honda feedback carburetor
systems must hold the record for the most vacuum lines and air control valves
ever put under one hood!
The feedback carb system utilized an O2 sensor to monitor the exhaust oxygen
content. It also used an electronic control unit to monitor several inputs, including
the oxygen sensor. When certain conditions were met the control unit would use
the oxygen sensor input to adjust the air / fuel (A/F) ratio. This condition is called
closed loop (CL) operation and works a lot like the CL operation of a PGMFI sys-
tem.
To control the A/F ratio, the control unit used air valves to dump air into the in-
take manifold. The carb feedback systems could only lean the mixture by adding
more air to the intake manifold. Two different air control systems were used by
Honda, the "M" and "X" system. The “X” system was used for large air adjust-
ments and the “M” system was used for finer air adjustments.
Since the carb feedback system did not operate in CL mode at idle speeds, the
Honda carb feedback system should always be checked at 2000 RPM and up. The
feedback carb systems used a ported vacuum switch (instead of a throttle position
sensor) to confirm the engine was not at idle.
If this system was working correctly it was “OK”, but if it gave trouble it could
get nasty! Tracking down vacuum leaks and diagnosing air valve malfunctions
could easily turn into an afternoon project!
Most car manufacturers utilized feedback carburetor systems to help make the
transition from carburetors to fuel injection systems. There were basically three
approaches used to control the A/F ratio on carb feedback systems.
Control the Fuel
The controlling of the fuel was the most widely used system. Most manu-
facturers put a fuel mixture control solenoid inside the carb and it worked
All Rights Reserved 2000 Sure Seal Products Inc
This manual printed 4/9/00 from the file pgmfiobd_002.
Visit Our Website: www.tech2tech.net Page 1-5
well. Basically, this system only added a wire or two to the underhood of
the car.
Control the Air Bleeds
Some manufacturers controlled the A/F ratio by controlling the air bleeds.
This also worked well and only added a few small vacuum lines.
Control the Air
To my knowledge; Honda was the only major manufacturer that chose to
control the A/F mixture by directly adding air to the intake manifold. To
make a difference in the A/F ratio by changing air it took some significant
volumes of air. This system turned the engine compartment into a vacuum
line / air control valve smorgasbord.
1.3.3 PGMFI / Pre OBD-II
In 1985, Honda debuted its PGMFI system on certain models. In 1988 all Civic
models used the PGMFI system. Accords went full injection in 1990 and Preludes
went full injection in 1991. The PGMFI system is engineered by Honda and is
used exclusively on all fuel injected Hondas (except Passports). It is a well-
engineered system and its basic design has stayed relatively unchanged over the
years. The newer PGMFI systems use more inputs, control more systems, and
have more powerful processors than the earlier systems, but still are very similar
to the first PGMFI system used in1985.
An electronic engine management system has many advantages over the earlier
vacuum / mechanical systems and even the feedback carb systems. With the
PGMFI system all the individual control systems could be brought under the con-
trol of one processor. Here are some more reasons why an integrated electronic
control system such as the PGMFI system is so much better and effective than
earlier control systems:
1. Electronic control can constantly adjust outputs based on changing inputs.
2. The system can produce the lowest emissions by keeping all the fuel and
ignition controls close to the optimum setting.
3. If an input to the engine control module ECM is lost or become corrupt,
the ECM can ignore that input and resort to an internal stored value.
4. Electronic controls can provide a certain level of self-diagnostics to the
service tech.
The PGMFI systems were offered in a throttle body style system (called dual
point injection - DPI, by Honda) and a multi-port fuel injection (MPI) system.
The DPI system was only used on certain 1988-91 Civics. The performance and
high fuel efficient models still used the MPI system. The DPI has two injectors,
one below the throttle plate and one above the throttle plate. It also utilizes a flap
All Rights Reserved 2000 Sure Seal Products Inc
This manual printed 4/9/00 from the file pgmfiobd_002.
Page 1-6 Visit Our Website: www.tech2tech.net
in the air horn that
Image 1-1 The PGMFI DPI System
closes under low airflow
to help speed up the air
at the top injector.
Image 1-1 shows a
PGMFI DPI system
used on a 1991 Civic
DX.
Image 1-2 shows a
PGMFI MPI system
used on a 1989 Accord.
The MPI system was, by
far, the most popular
and widely used system.
The bulk of the informa-
tion in this manual is
focused at the MPI system; however, most of the information applies to both sys-
tems.
The major operating conditions that the PGMFI system must recognize and con-
trol are: cold start, warm
Image 1-2 The PGMFI MPI System up, acceleration, decel-
eration, cruise, full load
conditions, and idle. The
PGMFI system accom-
plishes all these tasks by
using information pro-
vided from inputs and
internal tables pro-
grammed into the mem-
ory of the ECM.
For More Information About Go To
The DPI Injection System Chapter 21
The MPI Injection System Chapter 20
All Rights Reserved 2000 Sure Seal Products Inc
This manual printed 4/9/00 from the file pgmfiobd_002.
Visit Our Website: www.tech2tech.net Page 1-7
1.3.4 PGMFI With ECM Data Stream / OBD-II
The PGMFI systems, with the addition of a data link connector (DLC) for serial
data stream retrieval, bring us up to the current systems. The basic PGMFI system
did not change much, but the added ability to retrieve a live data stream takes this
system to a higher level. The ability for a tech to pull diagnostic trouble code
(DTCs) and view live engine parameters from a DLC added tremendous diagnos-
tic capabilities.
A 3-pin DLC was added to 1992 Civics / Preludes and 1994 Accords. This DLC
uses a proprietary protocol and is not OBD-II compliant. A 16-pin OBD-II DLC
was added to 1995 V-6 Accords and all 1996 and up Hondas. The OBD-II
equipped Hondas offer the most diagnostic information through the DLC.
For More Information About Go To
Honda’s Data Stream Chapter 26
1.4 ECU, ECM, TCM, or a PCM?
Is it an ECU, a TCM, an ECM, or a PCM? Whew.... It depends on the year, model
and whether you want to be OBD-II "politically correct". Let's take a look at the
differences:
ECU
Honda, in the past, called all its PGMFI controllers an Electronic Control
Unit (ECU). Since the standardization of terms under the OBD-II regula-
tions J1930, the term ECU is no longer used.
ECM
Engine Control Module (ECM) is the “OBD-II correct" term for a proces-
sor that primarily controls the engine management systems. An ECM typi-
cally does not control any major transmission functions.
TCM
Transmission Control Module (TCM) is the “OBD-II correct" term for a
processor that primarily controls transmission systems.
PCM
Powertrain Control Module (PCM) is the “OBD-II correct" term for a
processor that controls both engine and transmission systems.
Automatic transmission equipped Accords, from 1990 to 1995, used a separate
TCM and combined the ECM and TCM into a PCM on 1996 and newer models.
Civics never used a separate TCM. On 1996 and newer Civics, the ECM did take
on significant transmission functions and therefore is called a PCM. The 1995
Odyssey had a separate TCM and a ECM. They were combined in 1996 to one
All Rights Reserved 2000 Sure Seal Products Inc
This manual printed 4/9/00 from the file pgmfiobd_002.
Page 1-8 Visit Our Website: www.tech2tech.net
unit, a PCM. Preludes have maintained a separate TCM for A/T equipped models
from 1988 to present.
It is important to understand that Honda automatic transmissions either had their
own controller (a TCM) or a specific, dedicated section within the PCM control-
ler. The automatic transmission controller maintains its own DTC set and will il-
luminate either the "S" "S3" or the "D4" dash light. The MIL is only illuminated if
the transmission malfunction would affect tail-pipe emissions.
If you are using a scan tool to retrieve automatic transmission DTCs from a Hon-
da, be sure you are polling the transmission processor. You can specify the fuel
delivery controller or the transmission controller (even if they are combined into a
PCM) from your scan tool.
For simplicity, within this training module, “ECM” will be used to mean the
processor that controls the PGMFI system, even though it may technically be a
PCM.
1.4.1 ECM Location
The Honda ECM has been stuck in a lot of different places. The earlier systems
had them under either the left or right front seat. The early Preludes stuck them
behind the interior panel at the left of the back seat and you read the DTC flashes
by removing the ashtray. All the later systems have moved the ECM and the TCM
(if equipped) to the right front floorboard / firewall area. Some are mounted verti-
cal in the right front kick panel area. The units that mounted flat on the floorboard
were susceptible to water damage from even minor flooding.
1.4.2 ECM Construction
All Honda ECMs are programmed with the correct engine parameters at the fac-
tory and are not field reprogramable. Each ECM is programmed with information
that is specific to the vehicle it will be installed in. Some of the information added
to the ECM memory is engine displacement, compression ratio, and various other
engine and drivetrain parameters. In addition to not being reprogramable in the
field, the Honda ECMs have no removable chips. You cannot use a scan tool to
change any of the engine parameters. Some scan tools will allow you to use a bi-
directional mode to test transmission solenoids by activating them.
A Honda ECM very rarely gives trouble. They are very durable and usually is the
last item to suspect when diagnosing a problem. Most abnormal output readings
can be traced back to a bad input signal. There has only been one ECU recall,
which only affected a small number of Honda Civics.
For More Information About Go To
ECM Construction & Theory Chapter 6
All Rights Reserved 2000 Sure Seal Products Inc
This manual printed 4/9/00 from the file pgmfiobd_002.
Visit Our Website: www.tech2tech.net Page 1-9
1.5 Adaptive Learning
Screen Capture 1-1 Beginning with the OBD-II equipped Hondas (95 V-
6 Accords and all 96 and newer models) the ECM
will "learn" the fuel requirement characteristics of
each car. The concept of adaptive learning is similar
to "block learning" used by many domestic car
manufacturers. Honda uses adaptive learning to
learn a car's fuel trim requirements and apply it to
the base pulse width (PW).
One of the parameters the ECM monitors is long
term fuel trim (LT FT). Screen Capture 1-1 shows
the LT FT at .97. This means that this vehicle runs a
3% narrower PW than the factory default base pulse width.
This parameter indicates if the ECM must run the PW wider or narrower than the
factory default base pulse width to maintain proper fuel control. The LT FT be-
gins at 1. If the ECM has to consistently widen the PW to keep the fuel mixture
correct, the number will increase by that percentage. For example, if the ECM has
to consistently increase the PW by 10% to keep the A/F ratio correct, the Lt FT
will be 1.1
Also note that a generic scan tool and the Honda OEM scan tool report the LT FT
parameter differently. A generic scan tool always reports the deviation from the
default PW as a percentage. The Honda OEM scan tool uses the index of “1” as
the factory default PW. For example a Honda with a 3% wider PW would be re-
ported as +3% by a generic scan tool, but reported as 1.03 by the Honda OEM
scan tool.
It needs to be understood that the LT FT parameter is reflecting the variance in
the PW from the factory default base PW and is not necessarily reflecting a
change in the actual amount of fuel being delivered. Take this scenario as an ex-
ample.
• A Honda has a fuel filter that has become partially clogged with trash.
• This situation ends up dropping the fuel pressure at the injector rail by 5
pounds
• With the lower fuel pressure, less fuel is delivered for the same injector
PW
• The oxygen sensor reports a lean condition and calls for more fuel from
the ECM
• The ECM widens the PW to deliver the correct amount of fuel to satisfy
the oxygen sensor
All Rights Reserved 2000 Sure Seal Products Inc
This manual printed 4/9/00 from the file pgmfiobd_002.
Page 1-10 Visit Our Website: www.tech2tech.net
In this scenario the increase in the PW was to compensate for the drop in fuel
pressure. In this case, the same amount of fuel was delivered to the engine even
though the PW was widened. This shows how the LT FT parameter can often be
used to indicate a problem that is developing within the fuel delivery system.
This variance from normal is learned by the ECM and reflected by the LT FT pa-
rameter. This learned parameter is also used to change the fuel mixture while the
car is operating in open loop (OL). When a Honda is in OL operation, it must rely
on internal tables to determine the base PW. If during CL operation, it is "learned"
that the engine needs a 10% wider PW than the ECM expected, then it is logical
that it should need a 10% wider PW when operating in OL. If power is lost to the
ECM, the learned LT FT parameter will be set back to 1. It may take several trips
for the ECM to relearn any fuel delivery abnormalities.
It is possible to have an OL driveability problem develop from just replacing a
battery and loosing the learned LT FT parameter. It is better to reset a DTC using
a scan tool (instead of pulling the ECM fuse) so that the adaptive learning will not
be lost. It may be a good idea to mention to customers that after a battery change
it is possible for the initial cranking and warm up period to seem different until
the ECM has relearned the characteristics of the car's fuel requirements again.
Slight deviations from the standard of “1” are normal. A major deviation, like ±
5%, may be an indication of a problem developing with the fuel system. The
ECM will set a code and illuminate the MIL if the LT FT parameter exceeds ±
20%.
1.6 Fail Safe
The Honda engineers have done a good job of designing the PGMFI system to be
fail-safe. It is very rare that a car is rendered undriveable by the PGMFI system.
For beginners, all the outputs that the ECM has to activate are usually supplied
with power and the ECM completes the circuit by providing a ground. This means
that if the wire from the output device to the ECM gets shorted to ground, it will
not cause any damage to the ECM, since it was a ground to start with.
If the input signal is lost or becomes corrupt, the ECM will go into a fail-safe
mode. It will ignore the input and use an internal standard that is pre-programmed
into the ECM. It will also illuminate the MIL and store a DTC. The DTC will re-
main in the ECM until it is cleared from the memory, or power is lost to the ECM.
The next time the car is started, the ECM will try to use the input again. If the in-
put is missing or corrupt, the ECM will repeat the process of ignoring the input,
resorting to an internal standard, and illuminating the MIL, again.
Note that OBD-II equipped Hondas have a different MIL illuminating and DTC
storing strategy. These are covered in depth in Chapter 24.
All Rights Reserved 2000 Sure Seal Products Inc
This manual printed 4/9/00 from the file pgmfiobd_002.
Visit Our Website: www.tech2tech.net Page 1-11
The ability of the ECM to ignore bad inputs and run the car on an internal ECM
standard makes the system almost fail proof. You can unplug virtually all inputs
except the ignition and manifold absolute pressure (MAP) Sensor inputs and a
Honda will run remarkably well.
1.7 Back-Up Mode
If a major malfunction is detected in the controller part of the ECM, the ECM can
switch over to a “limp home” section. This is referred to as going into Back-Up
mode. When the vehicle is in back up mode the car should be driven as little as
possible till the vehicle can be checked out. The MIL will usually come on and
flash even when the service check connector is shorted if the car is operating in
the back-up mode.
1.8 Self Diagnostics
Each input has specific operating parameters. When an input operates outside
these values, the ECM will ignore the input. It then relies on an internal standard
and illuminates the MIL. You can retrieve the stored DTCs by either reading the
code from the ECM or by shorting a service check connector and counting the
flashes of the MIL.
While this self-diagnostic feature is usually helpful, it will not catch all problems.
The OBD-I systems only compares most inputs to a high / low range. The OBD-II
systems did add rationality and functionality checks to most input and output sig-
nals. Technicians should not rely on the self-diagnostics of the ECM to identify
all problems. In many cases there is a legitimate problem that can be effectively
diagnosed with proper equipment, yet the car will have no stored DTC.
On Hondas equipped with a DLC, a scan tool can be used to retrieve codes. The
three pin DLCs used on Hondas prior to OBD-II produce a numeric code similar
to the one displayed by the flashing MIL, but it is more detailed. On OBD-II
equipped Hondas a scan tool can be used to pull the standard OBD-II "P" codes
This PGMFI system overview is continued in Chapter 2!
All Rights Reserved 2000 Sure Seal Products Inc
This manual printed 4/9/00 from the file pgmfiobd_002.
