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Old 06-26-2012, 01:55 PM   #1
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acceleration question

Okay here is my issue….
Just purchased a 1999 4.0L, and the guy told me most sensors have been replaced recently and the computer reset. He said it would run a little rough for around 150 miles until the sensors and computer start to sync again.
Issue- it loses power during acceleration, at which point I need to hold the clutch in and let off the gas for a few second and then apply the gas pedal again. Sometimes I will continue to sputter or it will accelerate normally. It’s really intermittent and some days it’s almost to frustrating to drive it. I scanned it and it shows up with no codes and o2 sensors are ok (which I thought might be the issue). Oh, also when the sputtering occurs I don’t receive any check engine light.
Thoughts? What would cause it to do this and yet will not throw a check engine light and no codes?
Any ideas would be greatly appreciated!

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Old 06-26-2012, 02:46 PM   #2
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I have the same problem with mine, i was told that it wasn't holding fuel pressure when I cut the engine and then started it back up later. It's the pressure valve above the fuel pump. It's a cheap part but a PITA to get to

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Old 06-26-2012, 02:55 PM   #3
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Mine will persist intermittently for miles. Does yours only act up on start up?
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Old 06-26-2012, 02:56 PM   #4
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I had a similar issue recently as mine was bucking and surging going down the road with lack of power. Finally it threw a code which was the throttle positioning sensor TPS. Pretty easy fix and now is running great with power and better fuel mileage. It took weeks to throw a code on mine.
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Old 06-26-2012, 05:21 PM   #5
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Thanks Cap- now the question is, do I wait for a code or just replace the TPS. Did you replace it yourself?
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Old 06-26-2012, 07:19 PM   #6
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Thanks Cap- now the question is, do I wait for a code or just replace the TPS. Did you replace it yourself?
I had the same problem. At a certain point I would lose power then it would kick back in. About a week later it finally threw the code so I replaced it. You may want to wait for the code to be sure.. But keep in mind it's dangerous to drive while it's failing. I was going home one day and all of a sudden I lost all power as I was switching lanes and almost got rear ended. This was followed by the code being thrown. And to answer your question, it's a ~$50, five minute fix.
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Old 06-26-2012, 08:04 PM   #7
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Reissigree thanks for the info, and yes its dangerous, especially when trying to go uphill with no no power and a line of cars behind you. Do I need to disconnect the battery or anything? Or, just swap it out?
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Old 06-26-2012, 08:14 PM   #8
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The computer does need a little time to reset its tables with new sensors but not 150 miles worth. I disconnect my battery all the time which forces my computer to relearn everything but it doesn't take more than 10-20 miles or so at the most to do that. I'm thinking something else is going on.

He might have installed the incorrect spark plugs, the throttle body might need cleaning, the rotor might be bad, the distributor cap might be bad, maybe the ignition wiring is falling apart. If you replace the spark plugs, three good safe good performing plugs are the Autolite AP-985, Champion 3034, or Autolight XP985. THe first two are platinums, the last one is iridium. If you replace the rotor and distributor cap, try to find them with brass contacts. Cheaper rotors and distributor caps use aluminum contacts which don't last as long.
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Old 06-27-2012, 10:55 AM   #9
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Let us know what happens.
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Old 06-27-2012, 11:14 AM   #10
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I plan on just replacing the TPS and see if that fixes the problem....i will let you know how it works out.
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Old 06-27-2012, 03:42 PM   #11
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Okay, just replaced the TPS and halfway down my street, loss of power, sputtering and bucking. Im at a loss. I will give it few days and see what happens.
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Old 06-27-2012, 03:58 PM   #12
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Fuel pump? You are describing just about every symptom my old car had when fuel pump was on its way out. Mechanics didn't believe me until they ran a pressure test on the lines while running and it finally did the same to them.

Replaced and was good to go.
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Old 06-27-2012, 04:05 PM   #13
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I'm still thinking it might be the downstream o2 sensor. Everything else pretty much has recently been replaced but that. I'm just surprised that everything is checking out fine when scanned.
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Old 06-27-2012, 04:14 PM   #14
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I doubt the o2 sensor would be the issue. They either work or dont. Just a voltage reading as far as the computer is concerned, but I'm not an expert.

I've had my fair share of faulty o2 sensors and the only emptoms I got with them was either very rich or lean mixtures where you could smell it. Good luck with your search. I hate finding gremlins.
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Old 06-27-2012, 07:56 PM   #15
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I'm still thinking it might be the downstream o2 sensor. Everything else pretty much has recently been replaced but that. I'm just surprised that everything is checking out fine when scanned.
The downstream 02 sensor has nothing to do with the engine's performance, fuel mixture, etc. on early TJs, that job belongs to the upstream 02 sensor. The only job of the downstream 02 sensor on early TJs like yours is strictly to monitor how well the catalytic converter is performing and give you a Check Engine light if the cat is having a problem.
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Old 06-27-2012, 10:01 PM   #16
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Should I replace both or just the upstream?
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Old 06-27-2012, 10:38 PM   #17
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MAKE SURE YOU PUT IT ON THE RIGHT WAY. When I first put mine on, I was driving down my street thinking "yes, I finally fixed it" Then it started sputtering again. It DOES go on two ways and that can make the difference. Of course it took me a mechanic trip to figure that out.
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Old 06-27-2012, 10:56 PM   #18
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Don't keep throwing parts at it find out what's wrong first. I'd start with a fuel pressure gauge...you should be able to rent one from chinazone. I haven't looked on mine but if there isn't a schrader valve on the fuel rail then put a temporary T fitting in the fuel line...or use whatever the guage comes with. The fuel pump is more than likely your problem.
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Old 06-29-2012, 08:51 AM   #19
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reissigree- how did you put yours in the wrong way? The upstream and down stream are different part # but they should simply screw in. And regarding the downstream o2 not effecting engine behavior, it certainly can with mid to late 90 jeep wranglers.
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Old 06-29-2012, 10:06 AM   #20
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Quote:
Originally Posted by primer500
reissigree- how did you put yours in the wrong way? The upstream and down stream are different part # but they should simply screw in. And regarding the downstream o2 not effecting engine behavior, it certainly can with mid to late 90 jeep wranglers.
He's talking about the TPS.
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Old 06-29-2012, 10:56 AM   #21
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Well sorry about the bad info on the TPS. I would go back to the basics as Jerry Bransford states. Are you getting good spark to the cylinders. Are you getting good fuel, meaning sufficient amounts and sufficient pressure. Check for restrictions in the air intake side. Has this thing ever ran correctly since you've owned it?
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Old 06-30-2012, 02:35 PM   #22
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Okay, so it's been a few days with the new o2's and it's running solid. I'm sure it was the upstream o2 causing the issue, but replaced them both because they are the only two sensors that were original.

Thanks for all the help.....You guys rock!

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