PI closed loop algorithm for 1227747 ECU?

[Scott Lundahl <scott@lumenetics.com>]

Hello everyone.  I'm new to the list and this is my first post.  I'm doing 
an FI addition to a 1980 Triumph TR8.  This is the car with the GM 215 (3.5 
liter) cid aluminum block V8 derived Rover engine.  OEM fueling for this 
(49 State) car was via twin Zenith carbs in the usual British style.  The 
California version had Lucas (really Bosch L Jetronic) port fuel injection.

On my car, the original Zeniths were later replaced by a good old low tech 
Holley 4 barrel and Edelbrock Performer manifold.  I added a tame to medium 
cam and headers to perk things up a little.  This setup ran OK, but lacked 
many of the usual amenities such as a choke that worked properly, idle 
speed control (yes this car has A/C) and so on.  It was also OPEC's dream car.

So I decided to add non-eom FI to the car (the eom systems have no 
performance potential and are incredibly expensive).  I chose a TBI system 
using a 1227747 (C3 series) ECU because it interfaced easily to the current 
setup in the car.  The C3s are also easy to manipulate via EPROM (which is 
good because I have burned many so far).  This system is driven by the 
stock (non-ESC) distributor via a buffer.  A heated O2 sensor is on the 
header collector (4-2-1).

Anyway, I have the system in and dialed in to the point where it will run 
with perfection when I set the ECU to run in Open Loop mode.  It will run 
well with the fuel map set for slightly lean or slightly rich.  However, 
when I set it to run Closed Loop, the system develops a light throttle 
cruise surging (very annoying) and backfire on decel (I can almost 
eliminated the backfire via the decel params).  It also wants to oscillate 
at idle.  The BLMs tend to wander all over the place.

Since things seem to run OK in Open Loop when fueling is slightly rich or 
slightly lean, running at stochiometric does not seem to be the problem 
(some engines just don't like to run there).   There must be something 
about how C.L. operation is accomplished that is causing an issue (check my 
logic here).  Therefore, I suspect that the PI parameters (gain factors, 
time delays, etc) may not be correct for this engine (this is somewhat 
plausible when you consider that this ECU was originally setup to operate 
with an engine 50% larger or more than mine).  Thus it operates at a point 
that is close to instability in C.L.  So, I have two questions.

Does anyone have any suggestions / experience / solutions with a similar 
symptoms?

Does anyone have information on how the PI algorithm operates on this 
ECU?  I have seen such details of how the spark control algorithm works.  I 
also have the detailed hack of the type $42 (this is an incredible piece of 
good work) but the comments in the PI section aren't quite detailed enough 
to reverse engineer what's going on.

Any help/suggestions would be greatly appreciated.

Thanks,

Scott



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