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Unread 06-29-2013, 06:28 AM   #1
marshweb
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94 yj wrangler 4.0 not going over 65

I just bought my first jeep and I have never had a problem like this with any other vehicle I have owned but the jeep runs amazing in all of my lower gears and everything runs great until I get on the high way in 5th gear it runs fine untill about 55mph then slowly climbs to 65mph and holds there. The rpms hold at about 2k. I have 31/10.5r15s on there but I have no idea if that makes a difference. Please help

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Unread 06-29-2013, 11:16 AM   #2
XJ93
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Have you tried holding your shifts longer? I have never shifted into 5th until 55 or faster on any vehicle I've had.
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Unread 06-29-2013, 11:27 AM   #3
acat
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That might be all it has with 31s and 3.07 gears. Especially if your 4.0 is a little tired. You may be able to get a little more out if it in forth gear. Jeeps are horribly unaerodynamic which is a big part of the problem at highway speeds. Find a set of 4.10 gears out of a 4 cylinder yj and swap the axles. I did this on 31s and loved it. Still works fine on 33s.
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Unread 06-29-2013, 01:42 PM   #4
marshweb
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I will try not shifting until later I have never owned a standard so not fully used to the full timing yet. About how much does it cost to change gears out like tgat? Also I heard it could be my overdrive not kicking in which is supposed to in 5th gear or that it could just be a modulator
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Unread 06-29-2013, 02:57 PM   #5
Xpress
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Buddies Jeep does the same, same gears and tire size too, 5 speed. It'll do 65mph cruising on the straight and level in 5th gear, otherwise it's gutless. Keep it in 4th if you're driving over highways that are anything but perfectly flat and level.

BTW, your speedo is off by about 11%. At 65mph you're really doing about 72mph or so.
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Unread 06-29-2013, 06:20 PM   #6
marshweb
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Well its saying im doing about 55. But I knew it was off almost 10mph
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Unread 06-29-2013, 10:00 PM   #7
Raindem
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Quote:
Originally Posted by acat View Post
That might be all it has with 31s and 3.07 gears. Especially if your 4.0 is a little tired. You may be able to get a little more out if it in forth gear. Jeeps are horribly unaerodynamic which is a big part of the problem at highway speeds. Find a set of 4.10 gears out of a 4 cylinder yj and swap the axles. I did this on 31s and loved it. Still works fine on 33s.
Right on the money. When I went to 31's with my stock 3.07 axle I totally lost 5th gear, even on a level road. I swapped to 3.73 and that seemed to put it back to factory performance. I wouldn't go lower than 3.73 if you plan to do a lot of highway driving.
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Unread 06-29-2013, 10:58 PM   #8
Anticanman
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4th gear in the AX-15 transmission is 1:1. Keep it in 4th for a while. 3500 rpm isn't gonna kill anything.
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Unread 06-30-2013, 11:27 AM   #9
bowtech
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All the above. Remember you are driving a vehicle that will cut through the air stream with the efficiency of a brick. I run a 2.5L I4 with 4.88 gears pushing 35's. Top end is about 65mph. Jeeps are not speed destined vehicles.
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Unread 06-30-2013, 02:34 PM   #10
LJameson
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I'm not convinced its his tire size, I think there's something wrong with his engine. Do a tune up on it for starters...

When I had my LJ, it would go 80+ mph without breaking a sweat. That was with 31s, a 42rle with its steep overdrive and a higher curb weight.
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Unread 06-30-2013, 04:12 PM   #11
YJ475
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Make sure your air filter is clean, and that you have Champion spark plugs in. Those might make you much faster. My 4.0 had no problem hitting and keeping at 65 with 33s and 3.07 gearing. I run 4.10 gearing now, and it's very spunky. As to the cost, gearing is expensive (about $700, depending on your area). Instead, try to find some stock axles from a 4 cyl, as they come from the factory with 4.10 gearing. It's an easy swap, that can be done in one Saturday, and should cost about half as much as gearing.
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Unread 06-30-2013, 04:32 PM   #12
Reno72
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I keep hearing from some the 4.0 is gutless but that is not what I've found. I'm running 32x11.5 with 3.73 gears and I can hit 70 before the on-ramp ends. Cruising at 75 in 5th is no problem, and it takes a long climb before I need to shift down. I've towed a 4x8 cargo trailer filled with camping gear and wood to 9000 feet without problems. I think you have a motor problem or are shifting way to early.
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Unread 06-30-2013, 11:24 PM   #13
jred
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Check your fuel filter. I had a similar problem with a bike. Didn't make any sense to me, I could hold high RPMs in lower gears, but 5th was just a dog. I called myself checking the filter, but all the blockage was on the back side. It was letting *almost* enough fuel through
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Unread 07-01-2013, 07:47 AM   #14
XJ93
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O.P. whoever told you that your o.d. isn't kicking in doesn't understand manual transmissions. When you put it into 5th you are in overdrive. There is no separate unit for the o.d. and nothing to "kick in".
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Unread 07-01-2013, 09:11 AM   #15
Xpress
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Quote:
Originally Posted by Anticanman View Post
4th gear in the AX-15 transmission is 1:1. Keep it in 4th for a while. 3500 rpm isn't gonna kill anything.
With stock gears, 31's, and the AX15, he'd be doing something closer to 85mph at 3500rpm....
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-1989 Jeep Wrangler 2.5L, soon to be 5.2L
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-2004 Kawasaki KLR650 sunny day driver
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"A well regulated Militia, being necessary to the security of a free State, the [B][I]RIGHT[/I][/B] of the people to keep and bear Arms, [B][I]SHALL NOT BE INFRINGED[/I][/B]."
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