Yes, a cracked exhaust manifold will result in excessive gas
consumption. The easiest way to test if theres a crack is listening to
the sound of the engine. First, start off by making sure that you
engine is at full operating engine temperatures. Then pop open your
hood and manually apply the throttle from the throttle body. If you
hear any abnormal tone in your exhaust then you can be sure that there
is a leak. I hope this helps you.

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