Open loop after vehicle is drives for 20+ mins.

[cody dutton]

I have a 2000 integra gs-r, it has the b18c1 vtec engine. Car is stock except headers and a short ram intake. Car runs great during normal operation up until a certain point. Well after it reaches operating temp when i stop at a stop sign or a drive thru for lunch the car goes into open loop and begins to run like garbage. I monitored the ecu through a obd2 scan tool with live data. ECT temp reading shows within spec at 178 f at the time of fault. O2 sensor voltage is within spec and averages in the middle at about .4v – .5v. The car will be running fine, the idle will drop, and the scan tool shows “Open Loop Fault” in the o2 reading category. The odd part is that both ECT and O2 sensors are new, ECT is new honda and O2 is a new Denso unit. The issue is definitely not related to the ECU because changing ECU’s wont affect this issue. I have a obd1 P72 ecu and the OE obd2 P72, both are stock acura integra gs-r ecu’s and are unmodified. When the car acts up i can shut it down and swap them with no change to the issue. I have not checked the wiring to the o2 with a DVOM but visual inspection shows good quality. i have checked ECT wiring with DVOM and its within spec but neither of these have been checked when the issue is happening. The car is lowered so i cannot get to the o2 to check it when the issue is happening. Any help or prior experiences with this kind of issue would be helpful. I am at my wits end and about ready to buy a new engine harness. Also no check engine lights relevant to this situation are occurring. The obd2 ecu has CEL for downstream o2 because there is no cat on the car(i bought it like this) and the obd1 ecu has a CEL for IACV which is a whole other problem i need to work out.

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