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Old 06-23-2019, 02:30 PM
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Low to mid-range hesitation 2.5

Greetings all! New to the forum and new to a TJ. I just purchased a 98 TJ about a month ago. Since then it has been a pain in my side engine wise. This TJ has sat for a long time for the amount of surface rust that was on the suspension and frame. The bucket itself is practically rust free except for some discoloration in the floors. Which I will treat and paint in short order. Btw it's an SE. On to the problem. No power in the beginning and hesitation. Plus old gas. Replaced gas and added seafoam to tank. And a little marvel mystery oil to oil. That helped the power problem a bit. I drove it to work and about a mile from home it started back firing at both ends Bucking and jumping and the cel would flash and go off every time the engine bucked. No codes... A fellow jeeper stopped by and followed me home to make sure I made it.

At that point I replaced the TPS, plugs, wires, cap, rotor button, and coil. Drove it down the street and it started Bucking again without the backfire. At that point I replaced the crank position sensor. That solved that problem. Engine runs a ton better now except for this. Hesitation under load. Almost like it is running out of gas at times. Sometimes you can push through it by getting the rpms above 3k but not always. I have replaced every sensor at this point except to coolant temp sensor and the fuel pump. No codes at all. Oh it it has 50 psi at the rail. That has not been tested under load though. I am at my wit's end. Any ideas??

Thanks!!

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Old 06-23-2019, 03:04 PM   #2
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Old 06-23-2019, 10:38 PM   #3
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Upstream 02 sensor. I'm not sure if you have 1 or 2. I had the exact same symptoms with my 97 and finally figured out it was the upstream o2 sensor.

- Disconnect battery (to delete "learned data")
-Install new upstream NTK 02 sensor(s)
- Reconnect battery and giver her a go.
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Old 06-24-2019, 12:05 AM
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Mine looks like it had been recently replaced but I'll give that a shot. Thanks!
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Old 06-24-2019, 10:04 AM   #5
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Something about the injector harness on the 2.5 makes people tend to reverse injector harness plugs 3 and 4

Fuel entry timing is actually controlled by intake valve opening so swapping injector harness has little adverse effect except something about the injection map leads to a stumble at 2200-2500 rpm that is cured by getting correct harness plug on correct injector

Check the harness wire colors at injector plugs make sure they are on correct injectors


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Old 06-24-2019, 12:46 PM
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If that is the answer I owe you a beer or 12!
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Old 06-24-2019, 08:19 PM   #7
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Hopefully it's an easy fix. Mine ran rough for quite a while then I decided to figure out the issue. She runs like a sewing machine now.
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Old 06-26-2019, 11:42 AM
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Update

Here is what I have done so far. Sunday I replaced the fuel pump. That was fun. Not! It was worth a change whether it fixed it or not as the pressure regulator was bad and the fuel sending unit was bad as well.



Now it starts after a bump of the starter.



Now for the problem. Again! Still has a hesitation. I checked the fuel injector harness and it is in the correct order. So I decided to drive it to work this morning. Drove for about 30 minutes before I started smelling raw gas really bad. All windows were out. Thought I had a gas leak at the pump. Stopped and checked around and the smell was gone lol. Started the jeep back up and the CEL came on. I had my scanner with me and it showed p01071 bank 1 lean. So i turned around and headed back home. And boy was the smell bad. It was the exhaust. I guess it was in limp mode because the exhaust was like raw gas. but the jeep ran better than ever!


Always something. Guess I will swap out the upstream O2 sensor this afternoon.



PS. according to my scanner the short and long term fuel curve was at 32%. Maybe that will shed a little light.



Thanks all!
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Old 06-26-2019, 07:45 PM
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I made a big update earlier and I do not see it. So here we go again. I changed the fuel pump out on Sunday. Fixed the starting issue as the regulator was bad. Also a bad sending unit. The hesitation is still there though. I also checked the fuel injection harness and it was correct.

Decided to drive it to work this morning and started smelling gas really strong. Stopped and checked around looking for leaks and nothing. The smell was the exhaust. Started it back up and it threw a code p0171 bank 1 lean. Light stayed on and I believe I could have lit the exhaust on fire it was so strong. Ran great though. Turned around and brought it home. I also had the scanner hooked up for the drive before the light came on and the long and short term fuel curves were 32%. Not sure if that is good or bad as I haven't looked yet. Guess my next step is to change out the upstream O2 sensor.

Thanks!
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Old 06-28-2019, 06:53 PM
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Just replaced the upstream O2 sensor. This is what came out of the exhaust after cranking it to an idle. Talk about being rich when I parked it. Will drive it shortly and see how it does.
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Old 07-08-2019, 10:18 PM   #11
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Any updates ?
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Old 07-12-2019, 09:52 PM
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Yep. Got it mostly straightened out now. New oxygen sensors up and downstream. And new cat and muffler. Somehow and I'm still trying to figure this out it passed emissions less than 2 months ago. But the cat was completely hollow. Also replaced the fuel pump and pressure regulator. Nothing left to replace lol and it finally runs decent.

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