HORRIBLE gas mileage after deleting cat....

[llorracs93s10]

We deleted the cat on my 90 tbi 5.7 silverado a few weekends ago and I have now had three fill ups and I have averaged 12.5-13.5 mpg. Before the cat delete, I was averaging 15-15.5mpg. Obviously its not as bad as some others but the dramatic drop is just terrible IMO. I have pedaled it quite a bit but even with a 40 min highway drive and cruising on county back roads, the most I have averaged was the 13.5mpg. Any thoughts?

And FYI, before the cat delete, if I was to pedal it and rev it, ect, it would still average 14+ mpg. So its really not atleast imo having to do with my right foot.

 

[Cooter]

I just got mine removed off my 88 Silverado and the mileage is around 16 now. And I drive it like it's stolen too lol

 

[ChrisAU]

Is the idle high? Running rich now?

 

[///RAGMC]

Try disconecting the battery to reset the ECU and see what happens

 

[kn1ght]

Have you checked it for an error code? Also, the only reason your economy would go down, is if it started running rich, because of a bad MAS sensor, or an issue with the oxygen/air reading. Because when you cut that out, it frees up air flow, and in turn runs a little better. You sure you're not giving it more gas cause you like the louder sound?... lol. Reset it, see what you get.

 

[llorracs93s10]

Have you checked it for an error code? Also, the only reason your economy would go down, is if it started running rich, because of a bad MAS sensor, or an issue with the oxygen/air reading. Because when you cut that out, it frees up air flow, and in turn runs a little better. You sure you're not giving it more gas cause you like the louder sound?... lol. Reset it, see what you get.

Honestly, it really is driven the same, atleast now a few weeks after the cat delete. I duno what is going on.

 

[llorracs93s10]

Try disconecting the battery to reset the ECU and see what happens

We had the battery disconnected after the cab corners were done and that is when we did the exhaust as well. I duno whats going on!!

Is the idle high? Running rich now?

Not really, I mean when first fired up, it usually idled higher but my boss said that is normal. After that, it has a nice idle before and after cat delete.