Mileage Tanked After Manifold Swap

[ek49-ret]

I have an '87 Pioneer with the 4.0L 6 engine; I recently changed the exhaust man. with a new stock model w/o the side tube. My mileage has drastically dropped from the standard 15-20 mpg to 6.

I changed nothing else, and since this was a relatively simple swap-out, I'm puzzled.

Does the external tube matter, or if not, what do I need to troubleshoot?

[Pete M]

I split your post into its own topic so more eyes will see it. :thumbsup:

[nitroxsteve]

Did you damage the o2 sensor in the process?

[cz777]

maybe flow is not good ...welding on flanges inside ''leaking '' and o2 maybe bad  

[91Pioneer]

So did you block off the EGR tube? That's the tube that your original manifold had a spot for.