05' Grand Caravan Bad MPG After Cat Co Problem

Hollowed out the catalytic converter cause the van was shifting at 4000 rpm or around there. After that the mpg was great, all problems solved.. then mpg drops but no power loss or any other problems. Just terrible mpg ($$). Could it be that the down stream o2 sensor is outta whack because the hollowing out of the cat? Or is the sensor screwed cause there is too much air flow with no resistance/ back pressure from the cat? Its very aggravating getting 11 miles to the gallon as everyone can imagine haha. Thanks in advance guys. Btw if you say its illegal i checked the laws in my area, no emission codes. Free for all:)
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Put a legal cat on it pronto. It's a federal requirement.

What is causing this prblem though? And that is what its looking like i have to do.

I can only guess and say the engine management system was designed with a cat in the system. Remove it and the decreased exhaust back pressure changes the predetermined system reaction to loads and driver inputs. Anyway, you're violating federal law without a cat. That said, will you get caught?

Post cat oxygen sensor is used for fuel trim also; it is not just there for cat efficiency. The cat is judged on it's ability to store oxygen. So the computer is getting very inaccurate readings from that rear sensor. Here's a case study.
http://www.hondacarforum.com/honda-3/22414-ping-tigger-case-study-rear-o2-sensor-fuel-trim.html

It really makes sense now, i appreciate the thread link. Im gonna try that sensore seeing as when it got maintenanced, the o2 sensor was swinging around under the van. After putting it back in, thats when it didnt work. Anyways gonna go get a new sensor. Thanks and ill let you know what happened

The link from jpb53 is very interesting and definitely news to me. In this case, however, I don't think it applies. First off I agree with RIP that you should put a cat back in and I strongly urge that you do.

In the interest of finding out what is happening here, I believe that without the cat the downstream O2 sensor would, if anything sense a richer exhaust and, if what they say in the link is accurate, it would LEAN the mixture, not make it richer. That is why I believe the above link does not apply here.

I'm think that the lack of backpressure is making the upstream sensor read leaner thus making the mixture richer.

Without a cat it will, if it hasn't done so already, throw a check engine light when it senses little or no difference between the readings from the upstream & downstream sensors. It may not matter to you much if you don't have OBDII Emissions testing where you are, but it would bother me. Something else may happen down the road and you won't know because your CEL would have been on already.

Greatly appreciate all the feedback guys, im gonna get a new cat. Best option i feel now haha
But thanks a lot. Its been a bunch of help.