My 88 duke was getting 28 MPG normally then it went to 21 with an occasional SES code 44 at idle in a stop light.

Code 44 => Lean exhaust, O2 measured too low

I found a small crack in the exhaust manifold, I mean small. Hairline! I thought no, really. But yes, that was it.
I went to the local LKQ and they had one 1989 Chevy Celebrity with a Tech4 in it. The manifold is an exact fit.
My gas mileage is now back to 28 MPG @ 70MPH highway w/an automatic.

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Project Genisis Lo Budget 3800SC swap
12.840@104.8 MPH Intense-Racing 1.9 rockers, 3" exhaust, 3.4 pulley, ZZP tune and 18 year old tires.

88 Coupe under construction

88 formula 3.4L 4t60 swap SOLD

Thanks for posting! I have an '87 Duke, but this information is helpful to everyone.

How did you get the rusty manifold bolts off?

They all came out ok except for one. The three in the middle are 15MM and one almost broke my Harbor freight wrench so I used my craftsman wrench. It was close.