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Owns A Torsen
Joined: Mon Sep 18th, 2017
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Cats are not causing bad idle even if empty. The upstream O sensors are all you really "need" and they are before the cats. 5.0's do not idle bad unless there is something really wrong like a bigger than normal vacuum leak, or electrical malfunction.
I'd start looking for obvious stupid things being you evidently are sharper than average about doing things and figuring things out. Get it in the dark and spray some water on the motor while it is running, look for blue spark traces. Disconnect the power brake line and put your thumb over it. If it is loud you might not be able to hear the vacuum leak.
If you took the intake off while putting the motor in maybe you got a piece of crap under it or screwed one of the gaskets up.
Has to be something simple.
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