Tim,
I don't think that messing with the variable potentiometer is the answer. Altering that resistance would most likely affect the AFR across the board. I don't know what the Authority chip does but the stock DME has a MAP table with Deg C as the axis and the values are % fuel adjustment, I'd guess they are simply using that table. In stock 89 chip table looks like this:
Deg C = % fuel adjustment = IAT Resistance value
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-32C = +9.38% = 26,000ohms
0C = +6.25% = 5,900ohms
20C = 0% = 2,500ohms
60C = -5.47% = 600ohms
As you change resistance it selects from this table the amount of fuel change. So at 20C (2500ohms) it does no correction. And the table is interpolated between values. Meaning that at 10C it will calculate a point between the 0C and 20C so that at 10C the fuel adjustment will be about +3.125%.
Most likely (but not 100% positive) the Authority chip left this table as is and as you alter the potentiometer it makes use of this table to set the base mixture. If you have a WBO2 you could prove the theory by simply altering the resistance to the above values and observing the results on the WBO2.
But your issue is something else, as your AFRs seem to only be lean at low load and are OK at WOT. It could be that the MAF sensor is old and somehow compromised? Have you closely inspected the MAF? If it's a Hot Wire sensor it could simply be dirty?
READ READ READ
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You also said your are lean when 'in closed loop'. Unplug the O2 sensor and leave it unplugged, with it unplugged what happens to the AFR at idle? If the sensor is faulty and is always sending out a voltage >0.6vdc it will be sending a false rich signal to the DME and then the DME will lean the mixture. But if the sensor is bad and is always send >0.6vdc the O2 signal processing logic will simply keep leaning the mixture! I have seen bad O2 sensors cause this. Furthermore: within the DME it ignores the O2 signal under these 2 conditions:
1 - RPMs greater than 5120RPM
2 - Inj pulse widths greater than 4.8ms (moderate to heavy load, WOT has PW about 8ms, Cruise has PW about 2-3ms, idle PW about 1.2ms)
Since under moderate to heavy loads the O2 signal is ignored then a bad O2 sensor would go un-noticed at moderate to heavy loads but would lean the hell out of the low load conditions. It's just a educated guess that you may have a bad O2 sensor, unplug it and take it for a ride, does the lean condition change?
Quote:
Originally Posted by jungspike
Thanks for the help Sal.
I did verify WOT operation with a volt meter, so I saw the resistance change from infinity to zero, but I will crack it open and clean it. Can't hurt.
What you said about the O2 sensor impacting my dead cylinder test makes sense and I should have thought about that. Darn it!
Since my car is only lean at partial throttle and idle, my (current) problem is the lean AFR while the DME is in closed loop. I guess my real question is: Can I use the variable resister on the Autothority MAF (which simulates the intake air temperature signal) to adjust my AFR. This resister can be used to lean/rich mixture. However, I am afraid to adjust it because I do not want to mess up my full throttle AFR which is finally at 13.0. I have read on this site some guys changing the resistance and then getting misfires/detonation at the top end. Should I treat it like the mixture adjustment on the stock flapper box? I have it set at 2200 Ohms, but some have set it to 900 Ohms.
Tim
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