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Old 10-25-2012, 12:39 AM   #1
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2004 TJ Loss of Power

Jeep
2004 TJ
4.0 Inline 6
3.73 Gears
35" Tires
5 Speed

Now that the tech specs are out of the way. I have noticed a severe loss of power on acceleration. This is not due to the Tire / Gear ratio. I have been running this setup for a year. This loss of power is more of a bogging down when you open up the throttle.

If I am light on the throttle engine runs and sounds fine. If I am heavy on the throttle there is a drop in acceleration and the engine sounds like it is starving for fuel. It will do this under load and under no load. The best way to describe the sound is low and throaty. I do not have a check engine light and I am not throwing any codes.

1. Poor fuel delivery - fuel pump or clogged fuel filter.
2. Poor air delivery- I doubt this one
3. Poor exhaust flow - Might be (I hope not as I have the three cat setup)

Any other ideas besides this?

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Old 10-25-2012, 09:26 AM   #2
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I'm having the same issue with 1998 wrangler. I don't have a lift or big tires but I get the same feeling. When it's idling or I'm driving around slowly it drives and runs fine but if I try to accelerate with any urgency it just feels like I'm not getting full performance.

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Old 10-25-2012, 12:37 PM   #3
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If your fuel economy has taken a similar downturn, my first guess would be a bad 02 sensor. When an 02 sensor goes bad, and they do all go bad eventually, it can fool the computer into making the fuel mixture too rich will will hurt both performance and mpg. A bad 02 sensor won't alway trigger a Check Engine Light either, the computer won't always be able to figure out if it's receiving bad info from a defective 02 sensor. If you replace any 02 sensors, I would avoid Bosch which don't seen to work well in our Jeeps. RockAuto Auto Parts is a good source, look for OE replacements which will have the right connectors to match up with the Jeep. Not all 02 sensors come with the right connections, some don't even come with connectors at all.
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Old 10-25-2012, 12:48 PM   #4
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^ he's right but also keep in mind that up streem to the O2 sensors are your spark plugs. A fouling plug will throw off an O2 sensor... When was the last time you changed spark plugs? I'd go after that first.
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Old 10-25-2012, 01:24 PM   #5
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Quote:
Originally Posted by Jerry Bransford View Post
If your fuel economy has taken a similar downturn, my first guess would be a bad 02 sensor. When an 02 sensor goes bad, and they do all go bad eventually, it can fool the computer into making the fuel mixture too rich will will hurt both performance and mpg. A bad 02 sensor won't alway trigger a Check Engine Light either, the computer won't always be able to figure out if it's receiving bad info from a defective 02 sensor. If you replace any 02 sensors, I would avoid Bosch which don't seen to work well in our Jeeps. RockAuto Auto Parts is a good source, look for OE replacements which will have the right connectors to match up with the Jeep. Not all 02 sensors come with the right connections, some don't even come with connectors at all.
I can't speak for the OP but in my case I just replaced my upstream O2 sensor with an NGK sensor. I didn't change the downstream O2 sensor, could that cause my issue?

Quote:
Originally Posted by sevenservices View Post
^ he's right but also keep in mind that up streem to the O2 sensors are your spark plugs. A fouling plug will throw off an O2 sensor... When was the last time you changed spark plugs? I'd go after that first.
Personally I have not changed the plugs as I've only had it for less than 6 months. I do have 6 brand new plugs ready to go in so I will try that this weekend.
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Old 10-25-2012, 02:03 PM   #6
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Originally Posted by cballou View Post
I can't speak for the OP but in my case I just replaced my upstream O2 sensor with an NGK sensor. I didn't change the downstream O2 sensor, could that cause my issue?
In early TJs, the downstream 02 sensor's duty was limited to checking whether or not the single catalytic converter used then was operational. In newer TJs, after they added the additional pre-catalytic cats, the downstream 02 sensor's duties were expanded to also take on part of the responsibility for setting the air-fuel mixture that was previously handled by the upstream 02 sensor. Depending on your TJ's model year, and I'm not sure what year the downstream 02 sensor took on that expanded role, it may also be a good idea to replace that one too.
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Old 10-30-2012, 06:59 AM   #7
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I have an '06 that I just bought a few weeks back. I am having much the same symptoms and it did throw a sensor code (#1 sensor/#1 bank). Replaced it with OEM and still same symptoms. Idles great and drives great, just way down on power. Replaced plugs with Autolite AP985's (thanks Jerry), air filter, cleaned TB, AIC, etc. It runs great, but just has the feel of a banana up the tailpipe.

Tomorrow going to the dealer for a "diagnostic". I am now suspecting cat(s) and hope that's the problem, at 74k miles and still under the emissions warranty.

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