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Unread 01-20-2014, 04:23 PM   #1
mdmbkr
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1999 XJ Cherokee 
 
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Location: Pembroke Pines, FL
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O2 Sensor Heater

For years my XJ has run rough and produced low power immediately after startup, particularly a hot start.

If you give it enough gas, you'll hear a "pop" (not a backfire) and feel it in the gas pedal, and the issue suddenly and magically clears.

Likewise if the vehicle stops moving the engine miraculously snaps to a smooth idle. If you're moving at all it will want to die.

I didn't really know where to start with this but the O2 sensor heater hypothesis made a lot of sense: if the sensor doesn't heat properly, it gives the PCM false data, which messes up the mixture. If you floor it, the PCM goes to open loop; and if you stop rolling, the PCM goes into idle mode.

What really threw me on this is that no codes were thrown. I replaced the O2 sensor a year or two ago as preventative maintenance but there were no codes before or since.

At any rate, to attempt to diagnose it, I unplugged the O2 sensor and went for a drive. Voila, symptoms are gone.

Now the question is, do I have a bad sensor, bad wiring harness, or what, and what's the smart way to figure it all out?

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Unread 01-20-2014, 05:32 PM   #2
CJ7-Tim
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What brand of O2 sensor did you install ? Direct fit or universal fit ? Which O2 sensor did you unplug ? Have you done any continuity testing on the O2 sensor wires ? Have you tested the O2 sensors ? Have you tested the fuel pressure ? Are all the tune-up parts fresh ? Have you tested or replaced any of the other sensors ?
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Unread 01-20-2014, 05:54 PM   #3
mdmbkr
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Hi, good questions.

I believe it's a Bosch sensor. It is a direct fit. It's the front sensor that I replaced, and the front sensor is the one I unplugged.

I haven't done any testing on the wires yet. Between which points should I test?

I do have fairly fresh plugs, wires, cap, and rotor. I also have new injectors, new crankshaft position sensor, new TPS, new MAP sensor, clean IAC, clean air filter, new cat (magnaflow catback), etc. The motor makes good power at full throttle throughout the RPM range. The odometer reads around 240k miles, and it's a 1999 XJ 4x4 with NP242, AW4, 8.25 rear end, and a rubicon express 3.5" lift with new spring packs and xfer case drop.

I recently replaced the motor mounts and exhaust manifold, which was severely cracked. I was hopeful that the new manifold would fix this problem but it did not. Although it runs a lot better now, it has become harder to start on a hot start.

Have not tested fuel pressure, because I don't have that equipment, but I don't suspect the fuel pump since it's clearly able to deliver adequate fuel flow under full throttle.

One other issue that I'm a little concerned about is the wiring to the camshaft position sensor in the distributor. Two of the wires leading into the plug on the harness side are exposed and there's nothing stopping them from shorting to each other. However, they are physically separated at this time and I would hope that I'd get a code if they were indeed shorted. I'm a little reluctant to deal with the issue because the only way to fix it I can think of would be to cut the harness and splice in new wires.

I've been seeing around 13 mpg on average, up to 18 mpg at best.
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Unread 01-20-2014, 05:59 PM   #4
CJ7-Tim
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Fix any known issues, such as bare wires. Check/test the O2 sensor fuses.

It has been said by other Jeepers that Bosch O2 sensors do not play well with Jeeps. NTK/NGK were installed by the Jeep factroy, and are available everywhere.
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Unread 01-20-2014, 06:16 PM   #5
mdmbkr
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OK, thank you. I will verify the progeny of the current sensor and switch to NGK if needed, and check the fuses for the heater circuit. I have the Haynes book so hopefully there's a little bit of useful info in there.

Thanks again for your help, I'll post back.
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Unread 01-24-2014, 01:10 PM   #6
mdmbkr
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Current sensor is indeed a Bosch. Will update when NGK/NTK replacement is installed.
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Unread 01-24-2014, 01:18 PM   #7
CCKen
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Check the O2S heaters fuse in the PDC.

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Unread 01-24-2014, 01:29 PM   #8
mdmbkr
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Fuse checked good. Thanks for the diagram, earlier I checked every fuse in the box since they weren't labeled.

By the way, I was not getting any OBD codes. I would hope that if the heater fuse blew I'd get a heater code.
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Unread 01-25-2014, 05:42 PM   #9
mdmbkr
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I plugged the Bosch sensor back in and cleared the OBD codes to see what would happen. The symptoms returned.

I have a new NTK sensor and will report back on its performance once I'm done upgrading the steering box.
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