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A1aman New Reader
8/27/20 1:28 p.m.

When I was driving my 3.8L Project Car Mustang I noticed that it was running in a closed loop but Short and Long fuel trim for my Bank 1 O2 sensor was reading -20.31%for STFT and -14.84% for LTFT. Of course, my Bank 2 O2 sensors were in good percentages (both under -5%) I noticed this when I either came to a stop or when I was at idle. When I increased the speed to around 1500 RPMs the Bank 1 Fuel Trims corrected themselves. I just replaced both upstream o2 sensors and I don't have a catty converter. I am wondering if this is normal or if this is caused by a vacuum leak?

Patientzero HalfDork
8/27/20 3:37 p.m.

Yes, running that far off from bank to bank would indicate another issue. I'd start checking for vacuum leaks, exhaust leaks, and pull the spark plugs to see how they look. You could also swap injectors from side to side to see if that is causing the issue.

Since it corrected itself with RPM, I would lean toward an exhaust leak ahead of the 02 sensor. Possibly leaking or cracked exhaust manifold.