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Old 08-07-2010, 04:19 PM   #1
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Is my 4.0 Jeep a dog are are they all lacking power?

It's got 124,000 miles on it and is a 5 speed with stock tires, I'm not sure what gears are in it. When I am pulling the slightest of hills it just seems short on power where you have to downshift if you want to speed up. Yesterday I was doing about 70 up a slight grade and someone was tailgating me so I wanted to speed up and change to a slower lane, I floored it and you literally couldn't tell any difference. It seems to take an extra long time to increase RPM's when you floor it. I just replaced the spark plugs a couple of weeks ago.

I had two older Commanche's with 4.0 and 5 speed but I don't remember them being this slolw to respond.
Thx,
Bob

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Old 08-07-2010, 04:24 PM   #2
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I would think with that amount of miles it should still feel pretty powerful. maybe time for a tune up?

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Old 08-07-2010, 04:32 PM   #3
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i was just gonna say to get a tune up also. i hit the 100,00 mile mark and replaced the distributor cap and rotor along with plug wires and diamond fire e3 spark plugs, and it made a noticeable change in driving behavior, gained some power back and helped with gas millage by 2 or 3 mpg. the cap rotor wires and plugs that i replaced were all original and never replaced till now, all the spark plugs were black and had the spark contact burned away. most of the points on the cap were worn away and the rotor was off-set be about a 1/16 of an inch and making full contact to 2 of the points and not much of a spark between the other points. the only thing i need to do now is the o2 sensor.
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Old 08-07-2010, 04:33 PM   #4
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mine has 128k and it does the same thing, you'll just have to get used to it, downshifting isnt all that hard.
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Old 08-07-2010, 04:41 PM   #5
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Yeah, going up a hill in 5th isnt gonna happen.
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Old 08-07-2010, 04:44 PM   #6
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Originally Posted by fonecop1 View Post
It's got 124,000 miles on it and is a 5 speed with stock tires, I'm not sure what gears are in it. When I am pulling the slightest of hills it just seems short on power where you have to downshift if you want to speed up. Yesterday I was doing about 70 up a slight grade and someone was tailgating me so I wanted to speed up and change to a slower lane, I floored it and you literally couldn't tell any difference. It seems to take an extra long time to increase RPMs when you floor it. I just replaced the spark plugs a couple of weeks ago.

I had two older Commanches with 4.0 and 5 speed but I don't remember them being this slow to respond.
Bob, what size tires are you running and what ratio is in your axles, 3.07 or 3.73?
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Old 08-07-2010, 04:58 PM   #7
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Bob, what size tires are you running and what ratio is in your axles, 3.07 or 3.73?
You probably saw this coming, but how can I tell what gear ratio it is? The tires are 225/75/15's. There is a metal tag on the Dana 35 with some numbers on it but nothing looked like a gear ratio.
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Old 08-07-2010, 05:02 PM   #8
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The metal tag will state the gear ratio just as Jerry mentioned, 3.07 or 3.73 or 3.55 etc.
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Old 08-07-2010, 05:04 PM   #9
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Originally Posted by Jma20a View Post
i was just gonna say to get a tune up also. i hit the 100,00 mile mark and replaced the distributor cap and rotor along with plug wires and diamond fire e3 spark plugs, and it made a noticeable change in driving behavior, gained some power back and helped with gas millage by 2 or 3 mpg. the cap rotor wires and plugs that i replaced were all original and never replaced till now, all the spark plugs were black and had the spark contact burned away. most of the points on the cap were worn away and the rotor was off-set be about a 1/16 of an inch and making full contact to 2 of the points and not much of a spark between the other points. the only thing i need to do now is the o2 sensor.
You have never done a tune up and your at 100,000 miles? Shit your lucky it runs at all
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Old 08-07-2010, 05:05 PM   #10
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how a bout a Edge trail jammer kit? really wakes up the 4.0!
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Old 08-07-2010, 05:08 PM   #11
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how a bout a Edge trail jammer kit? really wakes up the 4.0!
Only when it is also properly geared. Adding an Edge kit to an undergeared Jeep where the engine is lugging from too big of a tire for the axle ratio is like putting lipstick on a pig.
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Old 08-07-2010, 05:29 PM   #12
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"Only when it is also properly geared. Adding an Edge kit to an undergeared Jeep where the engine is lugging from too big of a tire for the axle ratio is like putting lipstick on a pig. "

just depends on how big of a gap you have with your tire and axle ratio. problem is that trail jammer kit last i looked was pretty pricey may as well do a gear swap.
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Old 08-07-2010, 05:33 PM   #13
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you are right they are pricey! I want to do a gear change also
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Old 08-07-2010, 05:33 PM   #14
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Only when it is also properly geared. Adding an Edge kit to an undergeared Jeep where the engine is lugging from too big of a tire for the axle ratio is like putting lipstick on a pig.
Ooohhh.....I like lipstick on pigs.
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Old 08-07-2010, 05:39 PM   #15
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Survey Says: 3:07 gears. I used some WD40 to clean the tag and I could make it out. No wonder it won't get out of it's own way, that's a pretty tall gear! I am planning to pull my single axle 5X9 trailer with a 650 and a 250 four wheeler on it, I'm afraid that will bring it to it's knees. Also on the highway The other day I got because I couldn't get over quick enough, the guy thought I was doing it on purpose he didn't know I had it floored.

Also I think these inline 6 cylinders like to be cranked up to higher RPM's to make their power. Does anybody know what the peak HP/Torque RPM is for this engine?
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Old 08-07-2010, 05:44 PM   #16
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You have never done a tune up and your at 100,000 miles? Shit your lucky it runs at all
yeah i know, i purchased the jeep from my dad in '02 with 30,000, he never beat on the thing or took it off road.

sad story is that i had my '87 grand national parked in front of my house because we had our driveway re paved, and some drunk 16 year old hit it, according to the cops, dong about 45mph and totaled it. that thing was my baby, it had less than 20,000 original miles, it was bone stock exept for the turbo, original owner who i got it from didn't let the turbo cool after a long ride and it froze up in him. purchased it with money i inherited from my grandfather who passed away.

i still had my '73 cj-5 running at the time so that got me around for a few years and than a blew 2 piston rings. at that time my dad had back surgery and couldn't climb into the jeep no more so i bought it from him than.
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Old 08-07-2010, 05:48 PM   #17
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I'm not sure about the last part of your question, but your tires are pretty much stock, and IMO even with the 3:07 gears you shouldn't have a problem towing that trailer with the 4.0.

I have the same gearing in my jeep with slightly taller tires (P235's) and I haven't towed with it but it certainly seems capable. You may not hit 5th gear but you don't want to go much over 60mph towing anyway.

And the guy who flipped you off was a (fill in the blank)
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Old 08-09-2010, 07:09 PM   #18
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Also I think these inline 6 cylinders like to be cranked up to higher RPM's to make their power. Does anybody know what the peak HP/Torque RPM is for this engine?
The spec sheet for my 03 4L says: 190 HP @4600 rpm and 235 ft lbs @3200 rpm stock.
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Old 08-09-2010, 07:40 PM   #19
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Also I think these inline 6 cylinders like to be cranked up to higher RPM's to make their power. Does anybody know what the peak HP/Torque RPM is for this engine?
I don't have the chart here with me but the torque curve is pretty flat and it gets there at pretty low rpms. While you of course shouldn't lug it, it doesn't need high rpms either. In fact, it's an old OHV (overhead valve) engine design which isn't exactly known for high revving. It's more of a stump-puller that seldom needs high rpms unless it is severely undergeared.
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Old 08-10-2010, 04:37 AM   #20
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Just putting my 2 cents in, I've got a 2000 tj, 4.0L with 117,000 miles (no rotor cap on the 2000 models) auto 3.73 gears, 30x9.50x15 tires, it does slow a little on the hills, but give it a little fuel and it runs like a raped ape. it gets about 16mpg all the time, doesn't matter city or highway.

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