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Could a crack that size effect mileage noticably?


Yes. Extra air enters the exhaust manifold and the o2 sensor reads it as a lean condition. The ECU takes the bogus signal from the sensor and tries to correct by adding fuel.

Overly rich mixtures can damage/plug catalytic converters, so it's possible you will need to replace yours. I wouldn't automatically condemn yours, but be aware of the possibility.

Frank

'89 [color:"white"]G-Raider[color:"white"] [color:"black"]Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 172k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, Lifeline AGM, Rust-Oleum