ECM Tolerances? EEPROM PCM in LT1

[peter paul fenske]

Hi Larry

Not sure. Please take this with a grain of salt

1.      An 8051 PCM can have different cals.
2.      O2 sensors do not have identical chars. Only at 14.7
3.      You may have injector variance from bank to bank.
4.      Exhuast temps may be different from bank to bank

later:peter


At 09:43 AM 8/20/98 -0500, you wrote:
>Guys:
>
>I have a 1994 LT1 Camaro, that has a bunch of the basic bolt on
>modifications. However, I have always felt the car was a bit weak for
>what I had, but I couldn't figure out what was up. I ran a couple of
>Diacom runs and have a knock retard problem (10 degrees of retard) at
>pretty much any RPM over 4000 to about 5500 or so. The effect of this
>retard was obvious on the dyno runs I made....the power/torque curves
>went up smoothly until about 4000 rpm, where they got very choppy, and
>the rate of increase fell off dramatically. So, I thought I had a
>problem with low octane fuel. Since I was running 93 anyhow, I popped a
>can of 104 boost in, to no avail. So, I felt I would have to just live
>with it.
>
>Well, I recently was able to get a hold of another PCM from a friends
>car (Dave), so I decided to do a bit of experimenting. First, I swapped
>knock modules between the computers. While I still had knock, it was
>shorter in duration and only peaked around 8 degrees or so. So, I
>figured I'd try the whole PCM next. I swapped it in, and, until it
>learned my car, I got some retard, but now usually around 6 degrees,
>with an 8 here and there. Well, once the computer learned my car ( a
>couple of miles of driving), I subsequently ran my FIRST Diacom run from
>a stop through third gear (@ WOT), with NO knock at all. I was
>shocked...and pretty happy (and still need to talk to Dave about this).
>In normal driving since then, I still get a few spikes every once in a
>while, but they are two-three frames tops, and peak at about 6 degrees.
>The car feels MUCH stronger.
>
>Since the car is identical, uses all of the same sensors, and has the
>same operating characteristics...what in the heck is going on? Well, I
>thought I would do some analysis of all of my Diacom runs. After
>spending a few hours last night with a database, and Excel, I have some
>interesting observations...and need some further opinions.
>
>First, all analysis was done as follows: I screened all of the Diacom
>data to only provide data points that were made with throttle positions
>of 90% or greater...so that I was in WOT mode. I then did an average of
>each run for the range of RPM. So, what I ended up with was a data
>table, by rpm, with all of the Diacom values averaged at that speed. I
>then graphed them all (each measurement seperately, but all runs on the
>same graph). 
>
>Results:
>
>First, both the left and right O2 readings were remarkably different
>between the two ECM's (I'll refer to them as the LK...original one, and
>DW...Dave's). The LK had both O2's in the 920-940mv range.
>Interestingly, the curves started out around 900 at  lower RPM's, but
>curved up as RPM climbed. The DW curves were in the 880-900mv range and
>were flat from low to high rpm.
>
>Second, with the SAME engine and associated hardware, the airflow (MAF)
>rates were slightly higher with the DW than the LK. 245 gps peak with
>the LK and about 260 with the DW. There were some unusual peaks with the
>DW that were not in the LK...peaks in the 310-315 GPS range.
>
>Injector pulse widths were on average lower with the DW than the
>LK...confirmed by the lower O2 readings. However, the curves followed
>the same paths from low to high RPM...which doesn't explain the rise in
>the O2 curves of the LK.
>
>Finally, the battery voltage measure in the DW was consistently .2 - .3
>volts lower than the LK. This surprised me, since I think the low
>battery compensation in the PCM actually raises the pulse width for
>lower voltages. Granted, the actual measurements were 13.1 in the DW vs
>13.4 in the LK, so I don't know if this is a factor or not. I suspect
>they are both in the normal operating window of the PCM, so no
>adjustment was needed.
>
>So...what is going on here? Since the car was the same, and the ONLY
>thing that changed was the PCM, I assume something is up here. What?
>Both the LK and DW PCM's are 94/95 with the current GM programming. I
>can see driven devices (injectors, advance, etc) being different between
>computers if they have different programs, but why are the sensors
>behaving differently, as far as the PCM is concerned? They obviously
>aren't, since they are the same sensors. My first guess at this was
>maybe the production tolerances in the PCM, especially the internal
>voltage regulator. Since the PCM provides 5v and 12v regulated voltages
>to sensors (and a 450mv reference bias to the O2's), maybe that could be
>it. However, the TPS voltage at WOT between the LK and the DW PCM's was
>identical...4.9 volts. Maybe there is a circuit in the PCM that measures
>voltage (must be, since it determined +bat), and there is a bias in it?
>Obviously it is reading my battery voltage as lower with the DW than
>LK...but would this carry over to all of the sensor readings as well?
>Maybe it is interpreting a "real" 13.5 volts at the battery low, so it
>is interpreting a real 5.2 volts as 5 volts reference? I'm lost...
>
>Any ideas? Need more info?
>
>Thanks!
>
>Larry Kurek
>
>