Hello. I know this is off topic from blazers. But my dad has a 91 GMC Topkick work truck with a 366 TBI engine. Anyways the truck has no power after it warms up. I checked for codes and got a code 44- meaning lean exhaust detected, but uses a ton of fuel. Now I noticed both exhaust manifolds have cracks in them. Would this be enough to fool the oxygen sensor into thinking it is running lean and cause the ECM to richen the mixture up when it really makes it too rich? Any input would be appriciated.

 

Most definitely! This is a very large problem on the Series 1 3800 exhaust manifolds. As they age, the welds on the fabricated manifolds crack and cause this exact problem.

You'll want to get those manifolds patched up or straightup replaced.