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Old 04-22-2013, 04:02 PM   #1
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4.0 tj runs rough

Hello all, I am new to the site, i drive a 97 4.0 tj. I bought the truck about a month ago and it needs a little bit of love. It has 98000 original miles but it seems to run a little rough, it accelerates slower than similar built jeeps that I have ridden in and it seems a little grouchy. I know I have an exhaust leak would that cause noticeably slower acceleration? Also I am considering doing plugs and wires even though it idles fine. Suggestions?

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Old 04-22-2013, 04:07 PM   #2
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air filter, fluid changes, exhaust

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Old 04-22-2013, 04:49 PM   #3
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Originally Posted by jhorton5 View Post
Hello all, I am new to the site, i drive a 97 4.0 tj. I bought the truck about a month ago and it needs a little bit of love. It has 98000 original miles but it seems to run a little rough, it accelerates slower than similar built jeeps that I have ridden in and it seems a little grouchy. I know I have an exhaust leak would that cause noticeably slower acceleration? Also I am considering doing plugs and wires even though it idles fine. Suggestions?
I would start with a tune up like you said. A lot of times the vehicle can idle fine and wont really act up until you put the engine under a load. That would be a good place to start.

Is the Check Engine Light on?

How big is the exhaust leak? I really dont see this being the reason that it is running rough. My uncle has a slight exhaust leak and has no effect on his.
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Old 04-22-2013, 04:54 PM   #4
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A rough running engine can be one or more of the following... worn out spark plugs, worn or cracked distributor cap, worn distributor rotor, cracked/bad ignition wiring, a bad upstream 02 sensor, or even a bad TPS (throttle position sensor) or dirty IAC (Idle Air Controller).

With that many miles, I'd start by replacing the spark plugs, distributor cap, and rotor. Three good spark plug choices for your '97 would be the Autolite AP985 or Champion 3034, both of which are platinum-tipped plugs good for 100k miles. Or the iridium-tipped Autolite XP985 which is good for somewhere close to 200k miles.

For the distributor cap & rotor, most come with aluminum contacts... but a better longer-lasting way to go would be to find them with brass contacts. Upgraded distributor caps & rotors with brass contacts are not always easy to find but they are worth a little extra effort to find.

A new air filter is probably a good idea too. If those normal maintenance items don't fix the rough idle, then it will be time to talk about the TPS, IAC, & upstream 02 sensor. Actually, your upstream 02 sensor is ripe for replacement too, they do go bad & need replacing like spark plugs do though not as often. 02 sensors start becoming likely replacement candidates once they get 75-80K miles on them. The downstream 02 sensor is mounted on your catalytic converter & it only monitors your cat for proper operation, it won't cause a rough idle on early model TJs like yours is.
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Old 04-22-2013, 05:15 PM   #5
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Jerry pretty much said it - though I personally have found the best plugs for the 4.0's are the cheap copper plugs. (Every other car I've owned I've used better plugs, but that's just what I've found for the jeep)

If the check engine light isn't on, then you need to do manual testing. Your engine needs three things: Spark, fuel, and air. You can'teasily test air except for looking at the filter. They have to be pretty darn dirty to cause an issue like this.

Spark: Plugs, obviously. If they look to be in good shape you can test by holding a plug up to the block. If there is a spark, it's getting the proper signal. If not, then a wire or the cap/rotor could be at fault.
To test fuel, an easy way is to start unplugging injector harness one at a time. Each one should make it run even worse when removed. If one makes no difference, could be a faulty injector.

I'd also consider replacing the fuel filter (cheap).

I'm not sure about that year TJ, but many cars ignore o2 sensors until the engine is warmed up and changes "loops". If the car is idling poorly right from being started, I'd hesitate to say it was an o2 sensor.
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Old 04-22-2013, 06:37 PM   #6
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I'd also consider replacing the fuel filter (cheap).
Not cheap for the TJ. The TJ's fuel filter is inside the fuel pressure regulator and you have to drop the gas tank to replace it. I've never seen the FPR (which contains the fuel filter) selling for less than about $80. Jeep designed the fuel filter to be a lifetime filter, it has a positively huge filtration area. It looks like a big cylinder-shaped piece of white felt.
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Old 04-22-2013, 06:46 PM   #7
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Not cheap for the TJ. The TJ's fuel filter is inside the fuel pressure regulator and you have to drop the gas tank to replace it. I've never seen the FPR (which contains the fuel filter) selling for less than about $80. Jeep designed the fuel filter to be a lifetime filter, it has a positively huge filtration area. It looks like a big cylinder-shaped piece of white felt.
Oh crap - you're right. Totally didn't remember.

Ok, well the OP's better choice then would be to test the fuel pressure. I had a WJ a couple of years ago where I just couldn't track down a reason it was running rough. Turns out the pressure coming from the tank was about 2/3 of what it was supposed to be. But since this requires a gauge he probably doesn't have, it's pretty low on the list of the diagnostic process.
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Old 04-24-2013, 03:24 PM   #8
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awesome thanks guys! I think Ill go after the pugs and wires next. I just did an air and fuel filter so I should be good there.
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Old 04-24-2013, 03:25 PM   #9
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also fuel filter was 13 bucks.
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Old 04-28-2013, 02:35 PM   #10
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Thanks

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