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Old 07-14-2014, 09:19 PM   #1
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97 Wrangler Sport Down on Power, Milage. (4.0)

Hello All!

After stalking this awesome forum for a while I thought I would make my first post. I need some help. I recently bought a 97 Wrangler with 150k miles and I love it. However, I do think it's down on power, and the gas mileage is worse than it should be. Here are the details.


I have the 4.0 with a 5 speed manual, 31" all terrains with 3.07 gearing. I'm driving VERY fuel conscious. I am getting 11 mpg's and my 5th gear is absolutely useless. It also just seems down on power compared to other jeeps I have driven with absolutely identical drivelines (same engine, transmission, gearing, tires).

So far I have....

-Changed the oil
-Changed the oil filter
-Changed the air filter
-Check the spark plugs (Gaps, build up etc, everything looked good)
-Compression tested (all 160 and up)
-Fuel pressure tested (45 at idle)
-Check the 02 sensors
-Checked the temp sensors

Everything seems to check out.


OBDII isn't throwing codes and she seems to run fine,

Any suggestions???

Thanks

Jon

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Old 07-15-2014, 01:04 AM   #2
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Did you check fluid in diffs, xfer case and xmission also? and do a compression check on the motor, you never know how the previous owner drove it.

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Old 07-15-2014, 01:06 AM   #3
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Sorry missed the compression test.
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Old 07-15-2014, 03:40 AM   #4
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your tires are oversized for your gearing. Before I regeared the 3.07 was anemic with the skinny 28's. For 31's you need 3.73-4.10 and your mileage and "pep" will go up. If you think your going bigger you should take that into consideration when regearing.
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Old 07-15-2014, 10:03 AM   #5
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There is one issue for sure and one possible issue... first, that 3.07 axle ring & pinion gear ratio is killing your power... 3.07 is bad enough with smaller tires and worse with bigger tires.

The potential issue is a bad upstream 02 sensor, the one mounted just below the exhaust header. When they go bad, and they go bad like spark plugs go bad, the upstream 02 sensor can wreck the performance and ruin the MPG by fooling the engine computer into making the air-fuel ratio way too rich. No, that will not always cause a Check Engine light or set a Diagnostic Trouble Code since early TJs don't have as sophisticated of a computer or enough additional 02 sensors like later TJs have that can more easily tell when there is a bad 02 sensor.

I'd swap the upstream 02 sensor (but not the downstream 02 sensor mounted on the catalytic converter) with a new NTK 02 sensor and see what happens. Avoid Bosch, they don't work well in Jeep engines.

And if $$$ isn't an issue, I'd consider regearing your axles to a lower ratio to bring the performance up where the Jeep would be more enjoyable to drive. Or if you have plans for bigger tires, wait until you get them and regear then when the right ratio would be different than it would be for your present 31" tires.

Good luck with it.
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Old 07-15-2014, 10:16 AM   #6
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Hi everyone! Thanks for the responses

How would I go about checking the fluids in the driveline components? Could that really wreck my performance this bad?

I am pretty convinced it's not the gearing. I have a friend with and IDENTICAL driveline and his 5th gear works, and he has way more power and better gas milage.

I am considering an axle swap eventually. Just need to find the time. I'm not going more than 31's

I thought I checked the 02 sensors, but they are bosch so maybe I should just replace it and try again.

Any other suggesions?

Thanks again for all the help

Jon
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Old 07-15-2014, 10:23 AM   #7
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I am pretty convinced it's not the gearing. I have a friend with and IDENTICAL driveline and his 5th gear works, and he has way more power and better gas milage.
5th gear doesn't work well with oversize tires with the 3.07, do you know absolutely for sure his is running a 3.07 axle ratio too? Most TJ's with the 4.0L engine came with 3.73, 3.07 was not ordered by the dealers as often as 3.73 was. Or in the case of '97 TJs, 3.55... 3.73 didn't come along until '98.
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Old 07-15-2014, 10:34 AM   #8
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5th gear doesn't work well with oversize tires with the 3.07, do you know absolutely for sure his is running a 3.07 axle ratio too? Most TJ's with the 4.0L engine came with 3.73, 3.07 was not ordered by the dealers as often as 3.73 was. Or in the case of '97 TJs, 3.55... 3.73 didn't come along until '98.
Yeah 100% sure. We ran his VIN. Same gearing.

I think I'm going to do a vacuum check later to see if the exhaust is restricted.
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Old 07-15-2014, 04:46 PM   #9
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Update: Just checked vacuum. Not really sure where I pulled the line from to be honest, but it sucked air so hopefully that's alright. I got 15 inches at idle, and 18 at 1500 rpm. I feel like that's a little low.
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Old 07-15-2014, 05:55 PM   #10
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Depends where you are hooking up your vac gauge. however as Jerry said about the gearing. 3.07's don't work very well with 31' tires.I changed my 3.07's to 4.10. 4.0L w/auto trans-a world of difference. However if you are convinced your gears are the same as your friends-and you are happy with the way theirs runs-replace the uptream Ox sensor (NTK or NGK brand only). I did and I could tell the difference in performance. BTW I used to do axles for a living. Jerry knows his axle ratios/tires, drivability,engines and especially electronic engine conrtols. He gives good correct information. He has helped me out many times.
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Old 07-15-2014, 07:03 PM   #11
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I am pretty convinced the gearing is identical. I took it to a shop to get checked out, so we'll see what they have to say. I will replace the 02 sensor after that.

As for regearing.

I do plan to do it when I get a bit more cash (student at the moment). From what I've read swapping the gears out is pretty difficult, and requires quite a bit of know how. I'm leaning towards buying some new axles and doing a full swap instead. Any opinions on the matter?

Thanks again everyone for your time and expertise!

Jon
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Old 07-15-2014, 07:37 PM   #12
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Possible plugged catalytic converter also
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Old 07-19-2014, 01:13 PM   #13
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Alright we took it to a shop and they checked out everything. Considering no one can find anything wrong with it I think the regear is the way to go.

I don't trust myself with doing a re gear, how much can I expect to spend? Can anyone recommend a shop in CT?

I know I could handle an axle swap. Anyone have any suggestions for where to look for axles/ have some they would like to sell? I've search craigslist but TJ axles are far and few between. My Wrangler has ABS so that would be required too.

Thanks again everyone
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Old 07-19-2014, 01:21 PM   #14
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Alright we took it to a shop and they checked out everything. Considering no one can find anything wrong with it I think the regear is the way to go.

I don't trust myself with doing a re gear, how much can I expect to spend? Can anyone recommend a shop in CT?

I know I could handle an axle swap. Anyone have any suggestions for where to look for axles/ have some they would like to sell? I've search craigslist but TJ axles are far and few between. My Wrangler has ABS so that would be required too.
Regearing both axles to a lower ratio will produce a dramatic boost in power and performance. Dramatic. Plan on $1200-$1500, though I paid just under $1100 for my last regear job.

I have a shop full of tools, welder, air tools, and I've been wrenching for many years but I wouldn't do a regear job either. It takes special tools, more knowledge than I care to learn, and more time than I would spend on it.
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Old 07-19-2014, 02:34 PM   #15
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Regearing both axles to a lower ratio will produce a dramatic boost in power and performance. Dramatic. Plan on $1200-$1500, though I paid just under $1100 for my last regear job.

I have a shop full of tools, welder, air tools, and I've been wrenching for many years but I wouldn't do a regear job either. It takes special tools, more knowledge than I care to learn, and more time than I would spend on it.
Thanks for the info. Yeah considering the time, tools, and expertise, I just don't want to stress myself out over a job like that.

I was able to track down a dana 35 with 3.73 gearing and abs. It does have 175k miles on it. The guy only wants $100 for it. Is it a good deal? or is that too many miles to make the job worth it?

Thanks again for all the help everyone

Jon
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Old 07-19-2014, 03:32 PM   #16
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For $100 I'd install that D35 until you can regear. Don't forget you won't be able to use 4x4 after installing that 3.73 D35 so long as your front axle still has the 3.07 ratio.
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Old 07-19-2014, 03:45 PM   #17
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One quick easy thing to do if your thinking your cats are plugging is to unhook the pipe from the manifold and wire it to the side. Take it out and drive it and see if that helps. Wont cost you any thing.
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Old 07-19-2014, 03:51 PM   #18
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You can more or less simulate the effects of different gears with smaller diameter tires to get a feel for the effect without spending the money

For a 2wd only test run you just need the smaller diameter tires on the back

For approximately same effect the the ratio of new to old tire diameter should be same as ratio old to new gear ratios or smaller number on top for each ratio

Effect will be slightly more then gear change as the smaller tires are also lighter with a smaller moment of inertia
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Old 07-19-2014, 04:33 PM   #19
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For $100 I'd install that D35 until you can regear. Don't forget you won't be able to use 4x4 after installing that 3.73 D35 so long as your front axle still has the 3.07 ratio.
Haha yeah I know. I was thinking about strapping the transfer handle down so I wouldn't forget. I am going to redo the front when I find a good front axle. I think 3.73 will be good for now. I will update the thread with progress pictures. Picking up the axle tomorrow and will be cleaning and prepping it this week.


I'll try to check out the cat sometime soon as well. I plan on replacing the upstream 02 sensor anyhow considering they are so cheap.

Thanks everyone!

Jon

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