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Old 10-09-2010, 11:55 PM   #1
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97 Sport will not go faster then 60mph

I have seen old posts on the net regarding this problem but there was never a resolution posted. Anyway... My 97 Sport 4.0 will run out of power at 60mph in 4th gear and I actually loose speed shifting to 5th. I hooked a buddys snap-on code scanner up to the Jeep and the only code I am getting is a bad downstream O2 sensor. I will replace this but have never heard of that causing my issue. I did a fuel pressure test and I have a constant 47psi at the rail so the fuel is getting to the injectors. The tires are a little bigger then stock but only enough to throw the speedo off a couple of MPH as I used gps to double check. Any ideas what could be going on here?

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Old 10-10-2010, 12:05 AM   #2
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What gearing do you have? Your Jeep may just not have enough power to overcome the wind resistance of the vehicle at speeds greater than 60mph...especially in 5th gear.

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Old 10-10-2010, 12:13 AM   #3
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From what I understand your gearing and your tire size can greatly affect your top end speed. I had a 94 yj 4.0 six cylinder that had 40% more horsepower than the 2.5L 4 cyl that I am running now. Because of its 35" tires and low gear ratios the 6cyl could never hit 70mph while my current ride can exceed 80 without a sweat.
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Old 10-10-2010, 12:41 AM   #4
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How long has it been since a tune up? Is your air filter dirty? There is so many things that can cause this problem. Gears can be a problem, depending on what gears you have. Have you checked for vacuum leaks from the hoses?
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Old 10-10-2010, 12:02 PM   #5
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I did put a new cap & rotor in along with plugs

The air filter is new. Throttle position sensor is right on. All injectors are firing properly I haven't gotten to checking the gearing yet. Just seems really doggy!
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Old 10-10-2010, 12:08 PM   #6
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What's ur rig look like. My bro had this problem. Turned out it was just too dang heavy. We took his swinging spare off ( 200 plus tire) and it ran like a new jeep
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Old 10-10-2010, 12:51 PM   #7
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Try driving it after disconnecting the upstream O2 sensor. ECM might not be sending enough fuel to the engine because of bad O2.
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Old 10-10-2010, 01:17 PM   #8
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You never answered the gears/tire size question.
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Old 10-10-2010, 01:23 PM   #9
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Your symptoms sound exactly like your catalytic converter is clogged. Have a pressure test done on your catalytic converter by a muffler shop. They go bad and the ceramic elements can break up which will clog it enough that the engine can't pump its exaust gasses through which significantly reduces higher rpm power.

Go to a small non-chain muffler shop to have it tested as they will do that kind of a test without trying to rip you off like Midas likely would. Your symptoms certainly point to that clogged cat possibility.

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