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· Registered
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I have a couple questions that I am hoping someone can offer some insight on. I recently did the gt2835 twins on my 94. I am running the MAP-ECU. Have had the setup on for about three months with no CEL.... About a week or two ago, the CEL came on. I pulled the codes and got a 25 (a/f lean) and a code 47 (sub throttle sensor) but the 47 problem is in another thread.
My questions are this... If the factory ecu senses that the system is lean, will it bump up the fuel? I ask this because when the CEL is on, the car is super rich. I am idling at a 9.80 AFR. Without the CEL, I am idling around 11.0... My other question is that do you think that a faulty primary o2 could cause this problem? I am running no cats and my rear o2 is tied up under the car. Just as a precaution, I bought two new sensors and will be putting them in tomorrow. But just thought I would try to get some insight into this. Thanks in advance
 

· Alpine Hardtopper
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I don't remember exactly, but I believe the OBD I cars still use the rear O2 sensor for something like long term trim. Having it tied up and not in the exhaust stream puts it in the presence of oxygen all the time. Over time the ECU is going to try to richen up the mixture because it's sensing that it's too lean (from the one not in the exhaust system). Eventually it's going to reach the limit it's alowed to richen up, then throw a code that says it's still too lean, even though it's at max rich trim.

Since you're OBD I, I would definitely recommend putting that O2 sensor back in the exhaust. Even though that means you'll have to get a bung welded up in there. Alternatively you could just get one of the O2 simulators and plug that in instead.
 
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