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Super low MPGs/catalytic converter question

C

coldoath

NAXJA Forum User
Location
West Texas
So the cat was already deleted when I bought my "98 XJ just a few months ago. When I had my Flowmaster put on they just replaced the straight pipe that was in place of the cat because the piece that was there was a POS weld job. I just lifted the jeep with a 3.5" RE and it's sitting on 33" 12.50 km2s. Just got my gauges fixed and found out I'm getting like 9mpg! So I am going to replace the spark plugs and the spark plug wires tonight in hopes of upping the MPGs. However I was wondering if I put the cat back on and put the 02 sensor back in place if this could possibly help up my MPGs? I'm running stock gears which are 3.55s. I've read threads where several guys are still getting 16mpg with 33s and stock 3.55 gears. If I can get 16mpg I'll be happy. Already replaced the air filter btw. Any other ideas?

BTW I really like this cat: http://performance-curve.com/425250-25catalyticconvertero2.aspx

Thanks in advance for the help!!
 
I have that same CAT and it does its job. The only o2 that does anything is the front one.

Have you factored in the tire size difference? I know mine auto w/ 3.55's and 31's my mileage is about 14% off from stock. Meaning for every 100 miles driven actual is 114 miles. That would make some difference in your mpg factor. Although you would still be under 13mpg.

Do the tune up, run some fuel system cleaner through her and chage the fuel filter. See where that puts you.
 
whitexj98 said:
And true the downstream o2 sensor just monitors catalytic efficiency, so you probably have a check engine light, that alone would make me put a cat back on.
Click to expand...

x2 on putting the cat back in. If not for legal and air quality reasons, this is likely your main cause of lousy mileage right now.

Having said that, it's possible to have a cat or downstream O2 fail and not throw a code. Happened to me when I still lived in CA; getting a smog fail for a dead downstream sensor with no CEL on was a lovely little surprise at smog time.
 
whitexj98 said:
Yeah I would do the regular plugs, wires, cap & rotor and sea foam it. There’s no external filter on a 98 so you can't do that. And true the downstream o2 sensor just monitors catalytic efficiency, so you probably have a check engine light, that alone would make me put a cat back on.
Click to expand...
if your going to sea foam it, id do it BEFORE the new cat goes in. ive seen instances of all that carbon being blown out (the sea foam is doing its job) and clogging up the cat.




to the OP, im on 32s with 3.07s. your mileage sounds about right. take into account driving style and conditions as well. i know mine FALLS horribly in the winter because im constantly spinning the tires in the slush and over reving. its a jeep, not a honda.
 
I'm getting 12 in mine right now. Specs in sig.
On 30s and a 3" boost, I got 18.
 
casm said:
x2 on putting the cat back in. If not for legal and air quality reasons, this is likely your main cause of lousy mileage right now.

Having said that, it's possible to have a cat or downstream O2 fail and not throw a code. Happened to me when I still lived in CA; getting a smog fail for a dead downstream sensor with no CEL on was a lovely little surprise at smog time.
Click to expand...

Why would this be the source of my poor MPGs? JW cause other guys say that the sensor after the cat doesn't do anything really other than throw a CEL when it's not there.

Oh also, anybody know what code that 2nd O2 will throw? Cause I do have a CEL and I'm going to go to Autozone or whomever will check the code for me to see what it's throwing.
 
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Without the upstream o2 sensor you are likely in full-rich mode. The downstream o2 sensor just provides the PCM with data on the CATs performance.

You can have a bad/failing upstream o2 sensor screwing you out of mileage and causing driveability issues and NEVER get a CEL or code.

Without the CAT you are in violation of federal emissions laws, regardless of your state's position on the issue. "Give a Hoot--Don't Pollute".
 
joe_peters said:
Without the upstream o2 sensor you are likely in full-rich mode. The downstream o2 sensor just provides the PCM with data on the CATs performance.

You can have a bad/failing upstream o2 sensor screwing you out of mileage and causing driveability issues and NEVER get a CEL or code.

Without the CAT you are in violation of federal emissions laws, regardless of your state's position on the issue. "Give a Hoot--Don't Pollute".
Click to expand...


Yea, I notice my Jeep smells REALLY rich, . So perhaps replace the upstream O2?
 
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Thanks for your awsome support of the 4X4 community.
 
Last edited by a moderator:
Do you like 4-wheeling? I'd imagine you do. Do you like being part of the problem? Idiots who thumb their nose at federal emissions laws and pollution will be the ones who give enviro-nazis the ammo needed to get us kicked out of public lands. You will assist them in passing laws to severely restrict/eliminate vehicular modifications. Congrats!

Also, nominated
 
RCP Phx said:
Thanks for your awsome support of the 4X4 community.
Click to expand...

Darky said:
Do you like 4-wheeling? I'd imagine you do. Do you like being part of the problem? Idiots who thumb their nose at federal emissions laws and pollution will be the ones who give enviro-nazis the ammo needed to get us kicked out of public lands. You will assist them in passing laws to severely restrict/eliminate vehicular modifications. Congrats!

Also, nominated
Click to expand...

bigalpha said:
(n)
Click to expand...


I agree with all of you! We are trying to keep places to wheel open, not get them shut down! Leave the slandering of federal laws off this board coldoath, thanks!
 
Ok, regardless of this federal law discussion which pertains not the the OP, I believe I have found several things that I will replace and appreciate the help from all of you. Apologies for stepping on toes in regards to feelings about CATs. My bad.

I don't wish to be a jerk or anything and I love all the help I get from the community. Once again thanks for the help! I won't let this happen again!!!
 
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coldoath said:
Ok, regardless of this federal law discussion which pertains not the the OP, I believe I have found several things that I will replace and appreciate the help from all of you. Apologies for stepping on toes in regards to feelings about CATs. My bad.

I don't wish to be a jerk or anything and I love all the help I get from the community. Once again thanks for the help! I won't let this happen again!!!
Click to expand...

:thumbup: Nice response! Glad you didn't go off the deep end about it like we have seen so many do!
 
FlexdXJ said:
:thumbup: Nice response! Glad you didn't go off the deep end about it like we have seen so many do!
Click to expand...

Well that would be just plain dumb! :p Gotta keep my NAXJA: friendz!!!! :cheers:

Back on topic....

Just bought Champion copper plugs x6, distrib cap, distrib rotor, and plug wires. Got the heater in the garage warming up and will knock those suckers out tonight! 44MPG here I come!!!!
 
coldoath said:
Well that would be just plain dumb! :p Gotta keep my NAXJA: friendz!!!! :cheers:

Back on topic....

Just bought Champion copper plugs x6, distrib cap, distrib rotor, and plug wires. Got the heater in the garage warming up and will knock those suckers out tonight! 44MPG here I come!!!!
Click to expand...

Must smoke some good stuff in OK! :D :D
 
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