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Old 08-24-2014, 09:37 AM   #1
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Having an annoying (sometimes serious) issue lately..

Hey guys,

I have a 97 SE 5-spd with 140k on the clock, and I recently started having some idle/running issues.
My Jeep starts perfectly everytime, but after running for about 30 seconds to a minute it starts to idle roughly, sometimes almost cutting off, and I have to give it some gas to keep it running.

That had been going on for about 6 months or so, and recently it began running rough while driving.
When driving it acts as if it starving for fuel and sputters and acts like it wants to cut off. I actually had it stall out on me sitting at a red light yesterday morning.

It seems to only sputter continuously up to 2k RPM, but runs fine as long as I'm over that.
I noticed this because if I shift into the next gear and the RPMs fall below 2k, it will sputter continuously until the RPMs reach above that.

The engine is not throwing any codes, but after reading about O2 sensor issues, I thought that may be causing the rough idle, so I changed it out with a NTK O2 sensor, and the problem hasn't changed (in fact it may have gotten worse), but it runs the same whether it is unplugged or not, so I don't believe that's the issue.

I have also recently changed the plugs, wires, and distributor, so I don't think those are the culprit either.

I thought maybe I had trash or a blockage in the fuel system so I am running 93 octane and some fuel treatment through it right now, but with little effect thus far.

If anyone could help me troubleshoot I would really appreciate it. With winter quickly approaching I want to get it resolved.

Thanks a lot!

German

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Old 08-24-2014, 11:19 AM   #2
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No one?

I'm leaning towards the FPR being on it's way out, or maybe a bad fuel injector.

Any input, or ways to diag. this without having to replace it would be awesome.

If it makes any difference, I'm fairly certain I have an exhaust leak around the header/down-pipe connection.

Any advice would be greatly appreciated!

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Old 08-24-2014, 11:47 AM   #3
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Check fuel rail pressure
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Old 08-24-2014, 11:56 AM   #4
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I have the same year with 154,000 on it. I was experiencing the same problem. I took the throttle body off, cleaned it and the AIC with throttle body cleaner. Problem solved!! Runs perfect now.
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Old 08-24-2014, 12:23 PM   #5
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An engine that runs fine when cold but runs roughly after it warms up makes me think bad upstream 02 sensor.

The 02 sensor isn't referenced by the engine computer when the engine is cold, only once the engine has warmed up enough. I'd replace your upstream 02 sensor with a new NTK or NGK, don't use a Bosch. The upstream 02 sensor is the one just below your exhaust manifold. The downstream 02 sensor mounted on the catalytic converter would not cause this issue so I wouldn't bother replacing it. Its job is solely to check the catalytic converter for proper operation.
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Old 08-24-2014, 01:23 PM   #6
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Desertraider - I recently (within the last 5k miles) cleaned the throttle body and cleaned all the sensors related to it, but I will definitely check to make sure it isn't gunked up again.

Jerry - I just replaced the upstream O2 sensor with a NTK sensor (2 days ago), and it almost seemed to make it run worse (after flashing the ECU as well).
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Old 08-25-2014, 07:54 AM   #7
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Anymore ideas? I'm nervous about driving now because of this. On startup this morning it ran for a second then cut off, had to wait and crank it again for it to stay running.

I guess I'm going to break down and bring it to a shop if I can't figure it out.
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Old 08-25-2014, 12:26 PM   #8
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Check fuel rail pressure
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Old 08-26-2014, 07:49 AM   #9
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Hey Digger, how would I go about checking it? My 2.5L doesn't have the check valve on the fuel rail so I'm guessing it's going to be way more involved?
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Old 08-26-2014, 09:12 PM   #10
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If you don't have a Schrader valve on the rail they make a short hose that can go on in series where flex hose connected to rail as it has one matching male and one matching female fitting and an attachment point for a pressure gauge
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Old 08-27-2014, 06:46 AM   #11
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Thanks. I will try that.
Am I wrong to think it's not fuel related? Usually if a fuel pump or something is going out, the more you put the engine under load, the more it will tax the fuel components, making it act up even more.
But in my case, as long as I'm driving at 2k+ RPM my jeep runs perfectly.. Almost sounds like a MAP or IAC or something?
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Old 08-27-2014, 07:33 AM   #12
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Agreed not typical but worth a check


Other possibilities that would tend to be more problematic at lower rpm

Large vacuum leak as most significant effect on lower rpm where less air via throttle body and higher manifold vacuum making any vacuum leak both flow more air and be a bigger percent of total ingested air

Bad TPS with signal indicating throttle body more open then it really is

Small injector leak where the lower the rpm the more significant the leak is relative to amount of fuel being injected normally. Here would expect code indicating miss in that cylinder and sooty spark plug in affected cylinder
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Old 08-27-2014, 07:54 AM   #13
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I'll definitely check it out.

This morning it was having a hard time idling but leveled out around 750 RPM (which is it's normal idle), but after driving 25 mins to work it wasn't having any more problems.

Would the vacuum leak or bad TPS affect it after it has warmed up completely?

Also, would any of that cause it to run rich? I have some sort of exhaust leak near the headers and can smell a significant amount of gas.

I am running 93 octane right now with some fuel system cleaner, to help clean out any gunk.

Thanks for all the help Digger!
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Old 08-27-2014, 08:23 AM   #14
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Once warmed up it switches to closed loop ( rather quickly after only a few minutes at most)
In closed loop it used upstream o2 sensor feedback to correct mixture issues which would tend to mitigate a vacuum leak effect and might improve and other mixture related issues including proper fuel rail pressure
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Old 08-27-2014, 08:28 AM   #15
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Gotcha. So definitely check fuel rail pressure and search for vacuum leaks.

I forgot to mention that the other day while troubleshooting, I removed the air intake system and with the throttle body unrestricted it seemed to idle and run a little better.
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Old 08-27-2014, 08:30 AM   #16
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Mine lost power and MPG when the exhaust manifold cracked. That's also a common problem with Jeeps
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Old 08-27-2014, 08:32 AM   #17
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Putting all the info together I would blame it all on significant exhaust leak before upstream o2 sensor

It would have no ill effect until switching to closed loop ( so would run normally for first minute or so if cold and even less when restarted when warm) once in closed loop at lower rpm the exhaust leak lets in outside air seen as excess O2 by upstream sensor which adds fuel making run like crap and run rich. But at higher rpm the leak is less significant and exhaust pressure rises so little outside air drawn in to mess up upstream reading and runs pretty normal
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Old 08-27-2014, 08:32 AM   #18
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Yeah I have been thinking my manifold is cracked, I tightened all the bolts yesterday with no change in the leak so I'm thinking it might be cracked. I'm not sure if that could cause my problem though?
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Old 08-27-2014, 08:36 AM   #19
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So do you think replacing the exhaust manifold would be good investment? It's rusted to heck and looks pretty bad (as does the rest of my exhaust).
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Old 08-27-2014, 09:31 AM   #20
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One interesting free experiment unplug up stream o2 sensor and ignore the check engine light but see if it runs better

If it does then yes repair of exhaust is absolutely what I would do next
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Old 08-27-2014, 09:40 AM   #21
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Alright I'll do that this afternoon after work. If it runs worse with the O2 sensor unplugged, what would that mean?
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Old 08-27-2014, 09:50 AM   #22
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It would still be a mixture issue but the closed loop fuel trim adjustments would be helping rather than worsening the mixture issue so we could not blame it on a false lean signal from air with oxygen entering exhaust after engine and before upstream sensor
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Old 08-27-2014, 05:10 PM   #23
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So I unplugged the upstream O2 sensor, and low-and-behold, it ran much better. So my next step is replacing the exhaust manifold. Thanks for all the help troubleshooting Digger. I'll be ordering my new manifold and hopefully installing it this weekend.
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Old 09-26-2014, 09:17 AM   #24
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Did replacing the manifold do it for you? I'm having similar issues and starting with upstream O2 sensor this weekend and going from there.
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Old 09-27-2014, 04:09 PM   #25
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I had the same issue with my 97 SE. No CEL.
I replaced the tbs and it ran the same. I then cleaned the IAC and it idled HORRIBLE!! worse than before. So I replaced the IAC and it fixed the idle but then again after it warmed up it would sputter and idle rough. So I disconnected the upstream O2 sensor and the CEL is on but the jeep runs great. I ordered a new O2 sensor from rock auto for $30 and its like new again.

It also wouldn't hurt to change the plugs, wires, cap and rotor while your at it.

just my .02 hope it helps.

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