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Truck leaning out!

So, here’s the deal. 1991 D150 with a TBI 360. In open loop, it runs great. After it warms up and goes into closed loop under load, part to full throttle, it stumbles and leans out terribly. New O2 sensor, MAP is fine, as is the fuel regulator. Fuel pressure is around 14psi. All electrical connections are good. Not pulling any codes on my scanner. Manifold vacuum is good, timing is spot on and we even ran the propane test. When it began to stumble, we introduced the propane and it smoothed out. Not entirely, but very noticable. So, it’s obviously a lean condition, but why is the computer not getting the correct information when in closed loop?? Any help is greatly appreciated!!

You did all those checks with the air cleaner in, intake duct work installed correctly, right? Because when your car goes into closed loop, it is taking reading from your MAF and other sensors, when it is in open loop, it might be running on a default setting. Running lean usually indicates an intake leak. Keep looking around, ensure you don’t have an intake leak somewhere.