Summary: There is a known problem with the OBD1 ECU which causes stock
twin and small single cars to run lean between ~4-6k rpms at high
boost. The example below is from an FJO wideband datalog captured from
my '95 TT while doing some street tuning at ~18psi. Further raising the
boost causes the problem to worsen. Attempting to add fuel using an
SAFC-II or similar piggyback does not resolve the problem.
After doing a bunch of research I found that installing an OBD2 ECU
completely resolves the problem. Note: Lagtime also discovered that
this problem could be resolved by using a Greddy E-Manage with optional
injector harness to raise the injector duty cycle.
Another benefit of running the OBD2 ECU is you can connect an OBD2
scantool to read and clear MIL's. It will also allow you to monitor and
datalog engine parameters such as timing, short and long term fuel
trim, coolant temp, TPS, intake air temp, engine load, vehicle speed,
air flow rate, and O2 sensor voltage.
Below is another FJO wideband capture after installing an OBD2 ECU
without any SAFC fuel correction. No other changes were made. As you
can see the problem is no longer present. OBD2's run pretty rich stock.
Below is after tuning the OBD2 ECU with the SAFC was completed.
How to upgrade an OBD1 car to OBD2:
Installation of the ECU is plug and play. Disconnect the battery.
Simply disconnect your OBD1 ECU from the harness and connect an OBD2
ECU. The car should run at this point. The only thing I know of that
doesn't work is the TRAC system. In order to get everything working
correctly the issues below need to be addressed.
1) Like any OBD2 car you will need to run an O2 simulator if you have a
catless downpipe installed. Failure to do so will result in a MIL from
the secondary O2 sensor.
2) On OBD2 cars there an additional ground wire that's not present on
OBD1 cars. Not having this ground wire causes the primary O2 sensor
heater to not function. This will prevent the car from entering closed
loop operation when there is insufficient heat from the exhaust gas to
keep the sensor at operating temp. This happens when the car is cold or
idles long enough (4-5 minutes) for the sensor to cool off. It can
cause the car to run very rich at idle. Adding the ground wire to the
ECU harness fixes the problem.
How to get the primary O2 sensor heater functioning: Since there's an
additional pin needed for the new ground wire you will have to find one
to use. This will require you to either cut a wire off of the ECU
harness or find a donor pin from another harness. I used the pin from
TE2 (E10-19) which is not used on OBD2. On OBD1 it's used for the
special diagnostic test mode.
On the ECU wiring harness remove the yellow protectors, they pop off
with a small screw driver. Remove pin TE2 (E10-19). It's a pink wire
with a green stripe. To accomplish this take a small safety pin and
carefully push up the plastic tab that holds the pin in the harness and
lightly pull on the wire on the other side of the connector. It should
pop right out with a slight tug. Cut the wire a couple of inches away
from the ECU connector.
Insert this pin into the new ground wire location E03 (E9-78). Lengthen
the wire so it's long enough to reach to a suitable ground point. I
used the mounting point for the trac ecu. Attach the wire to the ground
point.
3) OBD2 uses a different communications protocol and connector. The
existing OBD1 diagnostic connector will not work. In order to read and
clear MIL's you will need to install an OBD2 diagnostic port.
How to install an OBD2 diagnostic port: Disconnect the battery. Remove
the wiring harness from the ecu. Remove the yellow protectors, they pop
off with a small screw driver. Next you have to move a pin on the ECU
harness to another location. On obd1 cars pin E9-29 (vf1) is used for
serial communications. This pin is not used on obd2 cars. On obd2 cars
pin E10-8 (sdl) is used for serial communications. This pin is not used
on obd1 cars. So you have to move pin E9-29 to E10-8.
On the ecu wiring harness where it attaches to the ecu move pin E9-29
(vf1) to pin E10-8 (sdl). It's a light green wire. To accomplish this
take a small safety pin and carefully push up the plastic tab that
holds the pin in the harness and lightly pull on the wire on the other
side of the connector. It should pop right out with a slight tug.
Next identify E10-8 and remove the yellow plug that's installed in
unused locations. Now carefully insert the pin into the harness. It
should pop right in.
Next you need an obd2 j1962 female connector. They are hard to find. You could go cut one off a car from a junkyard.
They also sell OBD2 connectors and pins so you can make your own.
Here are the connections that need to be made (4 wires).
Pin 16 on the obd2 connector goes to +12v power. Run this to the a
power source that is 12v with the ignition on. I used the cigarette
lighter circuit. For safety you will want to put a fuse between the
power source and obd2 connector.
Pin 4 on the obd2 connector goes to chassis ground. Run this to a
suitable ground location. I used an existing ground point located just
above the drivers side kick panel.
Pin 2 on the obd2 connector goes to pin E10-8 (SDL) on the ecu, this is
the light green wire you moved on the ecu harness. The easiest way to
do this is to tap in to pin 7 (light green wire) on the OBD1 diagnostic
connector D5.
Pin 5 on the obd2 connector goes to the ECU ground which is E9-69 (E1)
brown wire. The easiest way to do this is to tap in to pin 3 (brown
wire) on the OBD1 diagnostic connector D5.
That's it, now you should have a working obd2 dlc3 port to plug in in whatever obd2 scan tool you want.
- Thanks to Kirk for this awesome write-up
