TECHNICAL NOTES ON THE EEC-IV MCU - Auto diagnostics

Eectch98-Part5.fm
16d (Table 1). Also, fill in the lower rpm VE tables (Table 3) with 30, and try to
start the engine without grinding the starter too much. The engine will warm up
enough to wake up the O2 sensor, look at the scanner, and see where the BL is, and
IAC Counts. The IAC Counts are a vacuum leak. So they are critical to a decent
idle.
Now you have some idea of where the mixture is, and what the kPa is from reading the
scan tool. From experience you can’t run the same numbers everywhere at idle. On the
high side of idle kPa go richer, i.e.; with an engine that idles at 60 kPa if it is
30 there, at 65-70 it would be 32, and at 50-55 28. Yes, the 30 is a lot but its
easy to go lean, and clean up the idle, then having stall all the time, and having
to wait for the prime shot to burn out.
Next, lean the VE values (Table 3) down till the O2 is swinging (value moving back
and forth through .45 volts), the IAC is less than 10, and you have a min idle
speed. For the initial butterfly opening I cut a strip of paper 1/4" wide and set
the throttle idle setting so the paper is pulls through the butterfly opening with a
minimum of effort and without tearing. Now the engine starts and will idle. This
is a good start.
Light Cruise
This section is on how to do light throttle cruising set ups, using GM ECMs. This
is for an engine using a slightly hotter than stock cam, healthy ignition and assuming
that some roughing out of the fuel timing tables have already been done.
The objectives are: Advance timing as much as possible consistent with minimum
throttle opening; and use just enough fuel to run 128 or slightly lower BL numbers.
For monitoring the engine, a digital volt meter (DVM) is attached to the Throttle
Position Sensor (TPS), a vacuum gauge is hooked up to the manifold (but this is
almost redundant of the TPS), a scan tool is hooked up for looking at the Block
Learn (BL) numbers, and a 10 element LED display is attached to the oxygen sensor.
With this wired up it is easy to now start changing the timing of the 40 kPa areas
of the fuel map by rpm for cruise. On a flat road with the car in top gear, find
the cruise speed at 40 kPa and note the DVM signal from the TPS. Add timing and
repeat the road test at the same TPS value. Observe whether the vacuum value is
above or below the original signal. Repeating this procedure will reach a limit
where the knock sensor will start retarding timing. Next, drop off as much timing
going to the 20 side (overrun), which richens the O2 readings and helps to cut the
fuel on overrun, so the car slows when you take your foot off the gas. On some of
the OEMs you almost have to hit the brakes to slow the car.
From the 40-50 kPa vacuum value, again depending on your car, drop off as little
timing going toward 100 kPa (full throttle), as possible. Run enough fuel in the VE
table so that when pulling slight grades the motor doesn’t chug (love that technical
language). Also use a minimum amount of timing for the WOT spark value.
A suggestion is stay on the low side of the BL 128 at cruise so the engine is
instantly smooth and pulling when the gas is nudged. There is no difference in MPG
between this and 128. If you have the capability for pulse width modulation (PWM)
of the EGR I’d turn this off during the initial tinkering so that you don’t chase
too many items at one time. Just doing the spark and fuel can be an exercise in
patience. Also, with a big cam just off idle and just as you go to light cruise at
say 30-35 mph, you may have a dodo area (in this area of the PROM maps, BLs are
lousy regardless of timing AFR). What is happening is the engine is starting to
wake up, the intake tract length is starting to work, and the engine is reducing its
self EGR. Pulling from 1600 rpm in high gear, smoothly from a roll might be the best
performance test possible. Some engines just don’t like to lug down to 1400.
EEC-IV Technical Notes: Modifications/Upgrades63 last edited: 9/29/98