TJ slows down at highway speeds and feels like it's bogging

John Wolf

John Wolf

TJ Enthusiast
Joined
Oct 8, 2018
Messages
117
Location
Eastern NC
99 TJ 4.0 5sp. Once hit highway speeds, jeep slows down, feels like it bogs down. Does this whether on cruise or not. Weird since it runs well around town, rather quick too. Since I've owned it for 3 years, replaced O2 sensors (NGK), new mopar IAC valve, just changed plugs and wires recently (AL iridiums), new vac lines. Running 32" Falken WPAT3 with a 2.5" lift. Again, cranks and runs like a champ in town but after been at highway speed, it slows down and fluctuates speed, won't keep up with cruise setting. I live in flat land, no hills. It's more of a nuisance than anything but if anyone can shed some light, it would be cool to fix it. Thanks!!
 
John Wolf said:
99 TJ 4.0 5sp. Once hit highway speeds, jeep slows down, feels like it bogs down. Does this whether on cruise or not. Weird since it runs well around town, rather quick too. Since I've owned it for 3 years, replaced O2 sensors (NGK), new mopar IAC valve, just changed plugs and wires recently (AL iridiums), new vac lines. Running 32" Falken WPAT3 with a 2.5" lift. Again, cranks and runs like a champ in town but after been at highway speed, it slows down and fluctuates speed, won't keep up with cruise setting. I live in flat land, no hills. It's more of a nuisance than anything but if anyone can shed some light, it would be cool to fix it. Thanks!!
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I'm guessing it's not regeared? I would definitely see this being normal if you have 3.07s with a lift and tires.
 
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if you modify and install a corvette front clip and sweep back your windshield this will help alot. were driving a brick down the hwy and the slightest breeze will slow us down.
 
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Rubicon88 said:
These posts are impossible.

No mention if mods were done and this started...if it's always done it...etc?.!
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I guess you dint read the part about the 2.5" lift and 32s, and all the other stuff I did to it. If there were other mods I would have listed them.
 
So is the engine surging? Like the tach moving up and down without your input?

I wouldn't drive something uncontrollable.
 
tr21triton said:
if you modify and install a corvette front clip and sweep back your windshield this will help alot. were driving a brick down the hwy and the slightest breeze will slow us down.
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Haha, yep, this is what I've figured too. Having had a YJ and a CJ7, all inline 6s, none of them go fast but they all go through!!! (y)
 
I'm guessing the 3.07 gears. That is what mine has and I feel the same way. At speed if you "punch it" it don't do much.My son had a 99 with 3.73 and it felt much different. I plan to re gear at some point...
 
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Apparition said:
So is the engine surging? Like the tach moving up and down without your input?

I wouldn't drive something uncontrollable.
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No it just slowly drops speed once i get it to 65 or so, especially on cruise, then it will slowly gain it back. Never hear a thing. I know I have the tire and stock gearing issue but that really doesn't bother me since it's a 5sp. I really was wondering if anyone heard of this that had a cat or vacuum issue. Wondering about my vacuum reservoir canister.
 
Jerry Bransford said:
The only thing that comes to mind is perhaps the catalytic converter is starting to break up/come apart inside. Like the ceramic is breaking up, moving around, and is restricting the exhaust while you're having the problem.
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Yes, this I wondered too. I have no idea how old the cat is either but it does run great at lower speed and through the gears, even when hot. Could the vacuum reservoir have this affect?
 
John Wolf said:
No it just slowly drops speed once i get it to 65 or so, especially on cruise, then it will slowly gain it back. Never hear a thing. I know I have the tire and stock gearing issue but that really doesn't bother me since it's a 5sp. I really was wondering if anyone heard of this that had a cat or vacuum issue. Wondering about my vacuum reservoir canister.
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When my cats failed it caused the engine to misfire, couldn't drive over 50-55 mph without throwing the misfire code. This doesn't sound like that.
 
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BLindzy said:
I'm guessing the 3.07 gears. That is what mine has and I feel the same way. At speed if you "punch it" it don't do much.My son had a 99 with 3.73 and it felt much different. I plan to re gear at some point...
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Yep, 307s with the heavier tires. It jumps through lower gears but 5th is a struggle, lol. If that's just the way it is I can live with it.
 
John Wolf said:
Yes, this I wondered too. I have no idea how old the cat is either but it does run great at lower speed and through the gears, even when hot. Could the vacuum reservoir have this affect?
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How many miles on the jeep? Cat failures seem to be 100k-120k
 
John Wolf said:
Haha, yep, this is what I've figured too. Having had a YJ and a CJ7, all inline 6s, none of them go fast but they all go through!!! (y)
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I've got the exact same setup as you 4.0 5sp. with 2in lift on 32's and 3.07. i live in the mountains of n ga. but go to middle ga alot. even on flat hwy. running 70+ my speed fluctuates alot. the po put the 32's BFG MT's on it and when they wear out i'll go to factory sized hankook dynapro mt's . had them on my last 97 and loved them, it had a 4 cyl. and was a real dog up here in the hills.
 
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