05-05-2015, 07:46 AM
Join Date: Apr 2010
Total Cats: 0
Correct voltage reported to PCM by o2 sensor, is PCM bad ?
This is a stock PCM/ECU on a 94 Miata.
According to the wideband gauge, the car when cranked cold will idle at about 14.7 for about 20-25 seconds, then immediately start dropping to full rich (10 on AFR gauge).
Car has a Rotrex with a piggyback fuel card, running light green top Supra injectors (about 315cc). The injectors were sent off to be cleaned by marren injector service about 6 months ago and have had no issues. I currently have the fuel card disconnected.
Car has been running great until this last event, which came on suddenly.
Need help determining if the PCM is bad before I place an order for one.
I replaced the ECT, o2 sensor, all vacuum lines, checked fuel pressure, fuel hold pressure. The resistance values on the injectors, o2 sensor, ECT, MAF, TPS are all good. The voltage values being reported to the PCM by the MAF, ECT and o2 sensor appears to be correct (except for the o2 sensor, see below).
1- When it idles near stoich for 20-25 seconds, then goes rich, is it going into closed loop ?
2- I check the voltage of the o2 sensor (backprobe the wiring harness at PCM) and it is reporting .9 to 1v, which makes sense because it is running rich. Does this indicate that the PCM is not trying to adjust the air/fuel mixture ?
Any help would be appreciated,