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I have a auto meter pro comp air fuel ratio gauge in my 86 na (260,000 miles). I installed about a year ago and it works fine.
I have done a couple of 100 mph passes on the freeway when coming home from work and I noticed around 95 to 100 the gauge reads almost lean on the gauge which is the red area. From 80 to 100 with the pedal down it reads rich but once 95 and 100 comes around the meter goes lean. What would cause this to happen ? Thanks for any help. k&N air filter with three inch pipe and a obx muffler.
 

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well when I'm at 95mph in 5th gear, my RPMs are at about 3700. Does this same phenomenon happen in every gear at the same spot on the RPM band? If youre racing, and you're in 4th gear, you should be getting pretty close to 5000RPMs which is about where the engine starts getting choked for air due to the stock intake manifold.

If you're really pushing it in 4th gear and it starts going lean after 5200 RPMs, its probably because the injectors are running their highest at that point so anywhere beyond that point you're running into fuel capacity issues. We only got dinky little 160cc injectors in our NAs, and the intake manifold won't allow the engine to breathe properly after about 5,200RPMs.
 
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