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whos running no cat

ChristmasXJ

NAXJA Forum User
Location
christmas,FL
so since i bought my xj there was no downstream o2sensor the one right before the cat. so hell i said why not just cut the cat off and run a flowmaster out the back. i did this and runs fine. i sunk the jeep a while back. i replaced distrubtor/sparkplugs/wires all the fluids. engine runs fine starts fine and runs fine just a little rough idle. the engine is running very rich now looks a deisel some time and smells real like un burnt fuel.

computer brings up 2 codes downstream o2 ya there is no o2 down there just the upstream. the other code is low voltage something.

anyone not running a cat? and had this problem and fixed it
 
Need more info on your XJ... year, motor, etc.

If your rig is OBDII (I'm assuming since you didn't post the year), I would bet that it's in limp mode because of the missing downstream O2 after the cat. You need to fix that and put a catalytic converter back on before you can really tell for sure what's going on. The downstream O2 sensor is important on an OBDII rig and without it the rig will run like poo unless you trick the computer into believing it's there and reading less emissions than the upstream.

Modern catalytic converts on EFI vehicles aren't as restrictive as the old ones used to be on carbed vehicles, and infact may do you more harm than good to remove it in terms of engine power and exhaust flow and backpressure. On top of that, rigs without catayltic converters stink and pollute more than ones with. Just have an exhaust shop put a "high flow" style one on if you're concerned about the flow of the exhaust.
 
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Did you read what I said? You will not get your engine to run well without a cat since your rig is OBDII. Put one on and put the correct downstream O2 on and it will run a million times better than now. It's running rich and smelling like fuel because it doesn't know how to adjust the mixture without the downstream O2 and so it just makes it very rich to make sure you don't blow the motor up.
 
I think you're missing that he said it ran fine without the other O2 sensor and without the cat until he sank it and got everything wet. I don't see how the cat will effect anything other than tailpipe emissions.


Unfortunately, the only way to diagnose the problem is to take it to a shop and get the codes read. An OBDII scanner should easily identify the problem.

BTW, since you asked the question (though it's irrelavant to your problem), I'm not running a cat. I have the cat and a straight section of tubing on flanges so I can bolt the cat back up when I have to get it smogged.
 
ChristmasXJ said:
ahhhhhhhhh but it sounds sooo good


The catalytic convertor plays no part in the way the exhaust sounds. From what I understand, there is no emmisions in Florida, so your not breaking any laws. But it does no harm, and in fact does good....so why not have one on there?
 
AZ XJ said:
The catalytic convertor plays no part in the way the exhaust sounds.

Yes it does!!!

Run a straigt pipe, the run just a cat with no muffler and tell me it doesnt sound different.

I just put a cat on my XJ after running without one for a while with just a flowmaster...made it a good bit quieter(Magnaflow highflow cat) and changed the tone a bit too.

Amazingly enough, i picked up 3 MPG on the highway and now have better low end torque with the cat in place...go figure...
 
In every application that I've ever added cats to, it hasn't made the least bit of difference.
 
AZ XJ said:
The catalytic convertor plays no part in the way the exhaust sounds. From what I understand, there is no emmisions in Florida, so your not breaking any laws. But it does no harm, and in fact does good....so why not have one on there?

Sorry, it makes a huge difference in how it sounds. I run it both ways (read my post above) so I have direct experience. The cat does nothing to enhance performance.......all it does is combust unburnt fuel to lower tailpipe emissions.

A little more backpressure can increase low end, while less backpressure can increase top end.
 
NotMatt said:
Need more info on your XJ... year, motor, etc.

If your rig is OBDII (I'm assuming since you didn't post the year), I would bet that it's in limp mode because of the missing downstream O2 after the cat. You need to fix that and put a catalytic converter back on before you can really tell for sure what's going on. The downstream O2 sensor is important on an OBDII rig and without it the rig will run like poo unless you trick the computer into believing it's there and reading less emissions than the upstream.

Modern catalytic converts on EFI vehicles aren't as restrictive as the old ones used to be on carbed vehicles, and infact may do you more harm than good to remove it in terms of engine power and exhaust flow and backpressure. On top of that, rigs without catayltic converters stink and pollute more than ones with. Just have an exhaust shop put a "high flow" style one on if you're concerned about the flow of the exhaust.


This basically says it all.
I've never messed with an XJ's cat, so who know's, it might be different, but with the applications that I've added cats to, It hasn't made a difference (79 Bronco, big block, no cats w/ Magnaflow. Added 2 cats and made no difference in exhaust sound. Just one vehicle in particular)
 
Goatman, what year is your rig? The problem is, older rigs can work fine without the cat, infact I've run my '87 for quite a while without a cat, but a 96 and later rig is ODBII, and the ECU uses the difference between the O2 sensors (upstream before the cat and downstream after the cat) to adjust the fuel mixture. Since he has no downstream O2 now, the ECU is confused... and even if he added one in, it would still trip the check engine light and the mixture would not be right, because there would be no difference between the two.

I did read that it ran fine before, but what I'm saying is... whether it ran "right" before or not is inconsequential, because it will never run the way it was intended without the secondary O2 sensor at least being there and functional. If he has that, and no cat, it might run a little better, but the ECU is still not going to be able to map the fuel mixture perfectly without it. I do not understand the complexities, all I know is what I have experienced in vehicles with ODBII emissions systems and hacked-up exhausts/gutted cats.

Here's a trick the ricers with ODBII cars use... you might be able to make this work for running without a cat, but I still would highly recommend putting one on. It does not affect the performance of your engine, and the sound change is minimal at most with a muffler on. Wire the secondary O2 sensor in correctly to the wiring harness (plug or re-wire if it's been hacked off), and then cover the tip of it with aluminum foil and zip-tie it somewhere out of the way. Supposedly this will fool the ECU into thinking it's reading the emissions correctly... but I can't vouch for how well it works or if it even works at all.
 
ya pretty much there are no laws in florida lol about lifts or exhuast ah their cracking down on the rice burners for having too much punch in there in subs lol . theres guys running around with 44s sticking half way out the wheel well and sit high as monster trucks.
 
well i think im gonna check my spark plugs and spark again. make sure everything is gettin enough spark to burn the extra fuel lol. ah my xj runs good as hell i can roast my tires and beat most anything off the start lol
 
ChristmasXJ said:
ya pretty much there are no laws in florida lol about lifts or exhuast ah their cracking down on the rice burners for having too much punch in there in subs lol . theres guys running around with 44s sticking half way out the wheel well and sit high as monster trucks.


Yea, I used to Live in Ft. Lauderdale and Coral Springs. I have a cousin in Hollywood who has one of those trucks on 44's that has the majority of the tires sticking out past the fenders lol. Great state if your into muddin and modifying vehicles.
 
NotMatt, your are 100% wrong

the downstream O2 sensor does NOTHING as far as adjusting air/fuel

the upstream does that, its testing raw exhaust, and adjusting the air/fuel as needed

the downstream does NOTHING but meter your cats efficienty, its after the cat, testing the cat, thats it
 
Well I guess we'll have to agree to disagree. I've built lots of exhausts and never had a rig with two O2 sensors run correctly after the second one was either disconnected or removed from the exhaust stream. Maybe it throws the ECU into limp mode when it's not there, I'm not sure. I wouldn't have posted it if I didn't know it to be true.
 
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