Help. Exhaust prob?

Allo, allo.
Aight, so I went home from school for the first time this year this weekend. My exhaust came in the mail the day I left for school, so it's been sitting in my room for two months. Sos I gets to putting it on on Friday finally. I'd been excited about this for two months. I put on a brand new bosch O2 sensor, carsound hi-flow cat, Rusty's 2.5" catback with flowmaster 50 series deltaflow. (maybe 40 series, not sure :doh: ) Anyway, my old exhaust went as follows, bad O2 sensor with 88k miles, crapped out cat, and a rusty muffler with a leak. Love the sound, it's mean as hell, but not too loud at all. But the reason I got it was for power (and emmissions), not sound. And therein lies the problem. After install, I disconnected and drained battery, started it back up and let it idle for bout 15 minutes to let the computer learn all the new sheat. Driving impressions: less power down low, midrange and even high up. I put about 150 miles on it this weekend, does the muffler or cat need a break in period? Will one of them wake up after 300 miles or something? With the state of the exhaust I replaced, I figured for at least a small difference. Sorry bout da long post, thx for any help. :bawl:

I don't understand? Why did you drain the battery? Did I not read your post good enough? What corrilation is there between the battery and the exahust?

Well, I didn't actually drain the batery, I mean I let all the juice out of the car by unhooking the neg cable and waiting 10 or 15 minutes. Then when I hooked it back up I let it idle for another ten or fifteen. You should do this with any air or fuel related mod you do on a FI engine. It resets the computer and when u hook it back up, the ECU relearns the vehicles characteristics. Or something like that. And hey! Why a RWD Jeep? Cuz idoit ricers expect a mustang to be quick, but theyre not. Ricers don't expect an XJ to be quick.

First, idling for 15 minutes is not going to allow the computer to relearn the new performance parameters. You have to drive it for awhile. However, you have now done that.

Unfortunately, you didn't do enough research. The stock exhaust is 2-1/4 inch. It has been pretty well documented on these forums that the 2-1/2 inch cat-back systems result in better top end performance, but with a signficant DECREASE in low-end torque and throttle response.

Sad to say, it appears you got what you paid for -- but it wasn't what you expected.

Eagle, thx for the responce, but I know that a 2.5 inch catback will result in some small sacrifice in low and mid end, for the benefit of high end. Thats why I included in my post that I felt as if I lost power low, mid AND high up. I couldn't afford any of the 2.25 inch systems. My rigs a RWD street queen, and doesn't see any off-highway action, so I figured this to be alright. Also, I beleive the stock piping to be 2 inches. Most importantly, I'm getting off the main question, shouldn't I see an increase everywhere by replacing a crappy O2 sensor, a plugged up cat, and a rusty muffler that has developed a hole? No matter if I lose a horse or two at 2k, overall, with that setup compared to the old one, i should be quicker at most RPMs. Finally, not saying you did or pointing you out Eagle, but for any responces, I'm not a child and have researched the XJ and the 4.0 for years, so plz, no one patronize me. I can understand simple automotive concepts. Jeez, that sounds much crueler then I meant it to, sorry. But thank you for any more responses.

The crossover pipe from the stock exhaust manifold to the cat, did you replace it or is it still stock? If not this is the most restrictive point in the system and will kill your performance.

P.S. not to start a flame thread but chill with the "ricers" BS. If you race in JeepSpeed then you should know to respect the rally cars we share the course with.

The ECU will take more than a few minutes of idling to remap the offset from the baseline fuel and advance curves (except for the curves at the fixed idle rpm). Go out and drive the XJ like you expect it to run (flog it, WOT it, and practice freeway speed passing). The ECU will remap the learned offset from the baseline loop programming for each rpm and throttle position, and each MAP input reading. The more variety of engine load exercise, the more the ECU will auto-tune to your usage.

Check the plugs for color, check for vacuum leaks, review the ignition system, and the wiring, including testing the new O2 sensor. The new O2 sensor may be bad out of the box, and knocking off a hose or wire during work will keep even the best mechanics in confusion.

If the airflow improvement with the new exhaust is significant (replacing a plugged cat, etc.) then the WOT air/fuel mixture may be too lean (slowing the vehicle down). I do not expect this to be the problem, but it's something to consider (find a dyno, or buy a G-Tech, and build a MAP adjuster for accurate testing).

Thanks Ed, I did do all types of driving, and the ECU should be pretty well schooled by now. As to the other possible problems, I'm back at school now, so unfortunatley I can't really diagnose anything for a while now. (I love her too much to bring her down to campus.) As to it running lean, I don't think that it was running too lean. Mileage stayed the same, and the engine stayed as chilly as ever with the 180 degree thermostat.
Kreature, yup you're right about the pipe between the header and the cat. I got it estimated at a Midas that specializes in exhaust work, and I didnt have the time or the funds to get that taken care of at the time, but its a future project. And, sorry, I need to clarify, I didn't mean "Japanese" or Asian vehicles by the ricers comment. My bad, when I say ricer I mean anyone, with any kind of vehicle(Honda, Hynduai, J-body, Neon, etc, ect) that has seen F&F one too many times and considers a 17 second car fast because of a loud exhaust or a body kit or neon. I love many Japanese, Korean, American and German cars, and have loads of respect for the ralley cars all these nations produce. Didn't mean to sound ignorant, thx for checking me though.

Count on Ed, the voice of reason. Good thoughts.

The stock exhaust on the 4.0L is 2-1/4", not 2". Mine's an '88 so it has a different injection and computer sustem than yours, and I can tell you from when I ran one that a 2-1/2" system did nothing for me anywhere in my driving range. I have a 5-speed with stock 3.07 gears, so I never go much (if any) above 3000 RPM -- I don't know how hard you flog on yours. When that system finally burned out I replaced it with a stock 2-1/4" cat-back and felt like I had just gotten a new engine.

The other thing I noticed with mine (which Ed's post reminded me of) is that when my O2 sensor went bad the result was that it ran rich. If the same might have been true for yours, replacing the O2 sensor would have leaned out the mixture, and the new open exhaust would lean it out still more. Therefore, as Ed suggests, check the plugs and try to establish some kind of baseline for your air/fuel mix.

Also, I just noticed your comment about a 180-degree thermostat. That's not what the factory calls for. It should be a 195. Yes, I know a lot of people run 180s and claim it either helps them or at least doesn't hurt. But you're trying to trouble-shoot a problem, and that's one more item that's out of spec. If the plugs appear normal, I would change to a 195 degree t-stat and see if it helps.

I'm happy that I stuck with a 2.25" system on my Jeep. I have a 5-speed and 3.07 gears so at my typical 75-85mph highway cruising speeds, the engine's only turning over at 2200-2500rpm. A larger 2.5" system would almost certainly be detrimental to performance at these speeds.
Like Eagle, I spend 99% of the time with the engine below 2500rpm and only occasionally make a foray towards the rev-limiter either when I'm in a hurry or when I want to hear the engine sing. It pulls hard enough at the top end as it is so I don't need to sacrifice low rev torque for more top end pull by installing a 2.5" system. I only plan to do that when it's stroker time.

JeepSpeed said:

I didn't mean "Japanese" or Asian vehicles by the ricers comment. My bad, when I say ricer I mean anyone, with any kind of vehicle(Honda, Hynduai, J-body, Neon, etc, ect) that has seen F&F one too many times and considers a 17 second car fast

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It's cool. Hope I didn't come off too abrasive, I wrote it as I was dealing with girlfriend issues! :smsoap:

Anyway I remember when I lost my exhaust past the cat, the performance dropped across the entire rpm range just like you mentioned. Once you do the crossover pipe I'm sure everything will fall into place with back-pressure and you should have a noticeable power improvement.

My understanding is that the back pressure needs to be loaded from the rear of the exhaust (the muffler) rather then the first bend out of the engine. This way the entire exhaust system acts as a sort of baffle and allows the back-pressure to stabilize rather then fluctuate throughout the firing order.

Check out www.go.jeep-xj.info/ under exhaust mods.. i think he's got the torque related to diameter issue worked out pretty well.