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Unread 04-06-2015, 03:24 PM   #1
psycokilla
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1992 YJ Wrangler 
 
Join Date: Feb 2015
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yj 4.0 std max speed = 60

I have a yj 4.0 std , it has 31 tires on 15 rim . on a good day 5th gear can go up to 65 but most of the time is 60 . 4th gear feels powerful, but when I shift to 5th it just slowly very slowly picks up mph . what should I do ? thank you

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Unread 04-06-2015, 03:44 PM   #2
mike134
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Do you know your final drive gearing? Has the gearing ever been changed? Can you confirm your speed with a GPS or by counting mile markers? What is is the RPM at 60?

Your 4.0 should be able to get you going faster than that on 31s.

Does it feel like a caliper or rear parking break might be sticking?

any check engine codes?

when was the last time you did a tune up? spark plugs, wires, distributor, fuel filter, air filter?

Any symptoms other than losing power at 60 mph? sputtering, hesitation on acceleration below 60?

What is your fuel economy around town and on the highway?

Any smell of gasoline? any engine smoke (white, blue, black)?

I'm sure we can get this sorted out!
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Unread 04-06-2015, 04:11 PM   #3
psycokilla
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Quote:
Originally Posted by mike134 View Post
Do you know your final drive gearing? Has the gearing ever been changed? Can you confirm your speed with a GPS or by counting mile markers? What is is the RPM at 60?

Your 4.0 should be able to get you going faster than that on 31s.

Does it feel like a caliper or rear parking break might be sticking?

any check engine codes?

when was the last time you did a tune up? spark plugs, wires, distributor, fuel filter, air filter?

Any symptoms other than losing power at 60 mph? sputtering, hesitation on acceleration below 60?

What is your fuel economy around town and on the highway?

Any smell of gasoline? any engine smoke (white, blue, black)?

I'm sure we can get this sorted out!
I bought the yj like a month ago , I ahd not really driven it a lot . the headers had small crack so that was just replaced. I changed the spark plugs and the oil 2 weeks ago but not the distributor nor the wires . the yj had the plates expired from 3 years ago when I got it. I also replaced the fuel pump . ill drive it tomorrow , and post the info on the gps speed , and rpms at 60 on 5th gear and 4th gear . thank you
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Unread 04-06-2015, 04:58 PM   #4
1991YJisLife
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Max speed 95 over here in 5th gear! Haha but you likely still have the stock axle gearing (3.07 or something). 3.73 or 4.10 gearing will give you 5th gear back. Until that point you will probably want to just use 4th on the highway and don't try for 5th gear. I have 31s and 4.56 gearing and it's overkill for the highway but great for offroading.
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Unread 04-06-2015, 05:02 PM   #5
Steve707
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You probably have the stock speedo gear too, that'll throw your speedometer off. That being said, with 31s and the stock gear I was able to get up in speed on mine. I didn't try gaining speed in 5th though, I'd always get to the speed I wanted and then shift up.
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Unread 04-06-2015, 05:53 PM   #6
psycokilla
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it does drive normal 1st - 4th . since it was parked for 3 years , will changing the distributor and wires help with that issue ? I do pick it up to 65 on 4th then shift to 5th and u can tell it has difficulty maintaining the seep at 65 it will slowly go down to 60 sometines. ill check for the rpms tomorrow that I get a chance.
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Unread 04-06-2015, 05:56 PM   #7
mike134
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If your original tires were 205/75/15 upgraded to 31 inch tires and the speedometer was not recalibrated and no regearing was done, then 60 mph on the speedometer will equate to 68.38 mph in real life. It could be even more inaccurate depending on whether the gearing was altered. This is why you need to double check with a GPS before considering repairs. Just let us know how it turns out and we can make suggestions from there.
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Unread 04-06-2015, 06:14 PM   #8
psycokilla
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Quote:
Originally Posted by mike134 View Post
If your original tires were 205/75/15 upgraded to 31 inch tires and the speedometer was not recalibrated and no regearing was done, then 60 mph on the speedometer will equate to 68.38 mph in real life. It could be even more inaccurate depending on whether the gearing was altered. This is why you need to double check with a GPS before considering repairs. Just let us know how it turns out and we can make suggestions from there.

I see what u mean . ill post that info by tomorrow . hope to fix this soon . jeep beach on the island is on the 25th of this month !
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Unread 04-07-2015, 03:20 PM   #9
psycokilla
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I was able to check the speed with the gps app and rpms on the way to work . with the 4.0 and 31 in tires , 5th gear was able to maintain a speed of 66 mph at 18000rpms according to the gps speed app , while the speedometer was reading 62 mph , so the speedometer is off . on 4th gear I drove it 70mph at 24000rpms according to the gps app, while the speedometer marking was 65. so is this normal ? with the 31s ? or what should I do next ?
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Unread 04-07-2015, 03:26 PM   #10
Anticanman
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Next should be a tune up. Are you running doors and top on or off? I've found I can go much faster under certain conditions not including a 20 mph tailwind.
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Unread 04-07-2015, 03:32 PM   #11
Jeepsr4me
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no its not normal.... I routinely break the interstate speed limit... with that very combo
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Unread 04-07-2015, 07:54 PM   #12
The_Goob
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Quote:
Originally Posted by psycokilla View Post
I was able to check the speed with the gps app and rpms on the way to work . with the 4.0 and 31 in tires , 5th gear was able to maintain a speed of 66 mph at 18000rpms according to the gps speed app , while the speedometer was reading 62 mph , so the speedometer is off . on 4th gear I drove it 70mph at 24000rpms according to the gps app, while the speedometer marking was 65. so is this normal ? with the 31s ? or what should I do next ?
stay in 4th. the powerband is 2100-2500 rpms anyway. you will use more fuel trying to lug the engine in 5th than a steady easy maintained 2400 in 4th. use 5th for over 70 if you can get that high.
For what its worth I have to pull 3000 rpms to do 60 mph.(auto 3 speed trans and 35's)
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Unread 04-07-2015, 08:15 PM   #13
geusey
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I don't think this is just a problem of gearing. Seems like there's more going on here.
My 92 4.0 topped out around 65 only when facing a serious headwind (this is west texas, so I do mean SERIOUS headwind), and that was with 3.07 gears and 235/75/15 tires (not too much smaller than your 31s). Normally I could do 80 or 90 no problem. And that was with a warped head and bad rings.
Do a full tune up. Change the distributor cap and rotor, air filter, fuel filter and dump some fuel injector cleaner into the gas tank. If it's been sitting for 3 years it could probably use all that anyway. Make sure the check engine light isn't on, or the bulb isn't out (you should see the check engine light come on when you first start the engine. If not, the bulb is likely dead.)
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Unread 04-07-2015, 08:16 PM   #14
mike134
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definitely doesn't sound normal. You really should be able to get to higher RPMs. Do the basic tune-up like anticanman suggests. Plug wires, distributor cap, fuel filter, air filter.
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Unread 04-07-2015, 09:50 PM   #15
Chrisnvegas
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I have buried the speedometer across the Delamar dry lake bed. 100+ after a about a mile and a half. Dead level. No real wind. Nice hard, flat surface.

The lake bed was an emergency runway for the old X-15 and is over 15,000 feet long.

This Jeep 4.0 set the land speed record for: quarter-mile standing start, kilometer standing start, 1 mile standing start, 10 km standing start, flying kilometer (140.255 mph), and flying mile (141.381 mph) powered by a 4.0 in a Jeep Comanche.

http://www.allpar.com/racing/land-sp...-comanche.html

I have a feeling many of your Jeeps aren't running as good as they should be.

A stock Wrangler with a 4.0 should be able to hit 100 mph level and no wind. (About)

Knock off mph for bigger tires and lift regardless of gearing. Higher body = more wind resistance and higher rolling mass with bigger tires.
No top is worse than hard top. No top + windshield folded (and well secured) is best.
I know, because I have done it.
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