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Old August 20th, 2020, 19:28
alexporter255 alexporter255 is offline
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Odd starting/idling issue. Maybe a vacuum leak?

Alright, so I've searched this forum trying to find someone with the same issue as me, but to no avail. So I made an account so I could ask myself.

I've got a '99 xj sport, 4.0L I6. Earlier this month, I was having rough idle issues, and it was having a hard time starting. Replaced my o2 sensors, spark plugs, wires, distributor cap, rotor, and ignition coil. Solved the issue for about a day.

Got a check engine light code p0171 lean fuel condition. Saw that this might be the exhaust manifold. Sure enough, manifold was cracked. So I replaced that, and the jeep had some trouble starting, but after 2 or 3 tries it finally started. (Thinking that's a problem with the valve in the fuel pump, maybe?) Ran fine pretty good the rest of the day.

The next day I was at a red light on a hill, really rough idle out of nowhere, then the jeep died. Luckily I got it pushed into a parking lot, but couldn't get it started for a while. Eventually it would start, but idle real low, like 2-300. I would try giving it some gas, and it would just die.

I'm probably just gonna have it towed to a shop, but before I do I was wondering if anyone here might have any ideas.

Thanks
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Old August 20th, 2020, 19:36
alexporter255 alexporter255 is offline
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Re: Odd starting/idling issue. Maybe a vacuum leak?

Forgot to mention I also replaced the cups, tps, and iac
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Old August 20th, 2020, 23:17
lawsoncl lawsoncl is offline
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Re: Odd starting/idling issue. Maybe a vacuum leak?

I hate blindly replacing sensors without testing them as it can cause more problems. Those original mopar sensors are better quality than the cheap chinese sourced parts. Lots of stories about replacing the CPS with cheap parts only to have it dead out of the box or lasting less than a year.

Borrow a fuel pressure tester from Autozone and confirm you have fuel pressure. Have a good look at the wiring around the manifold and where it goes from the engine to the firewall. It's not uncommon to have the harness to the o2 sensor melted against the manifold or chaffed.
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Old August 21st, 2020, 03:48
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Tim_MN Tim_MN is offline
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Re: Odd starting/idling issue. Maybe a vacuum leak?

Installed NTK O2 sensors and NOT Bosch ? 2x on fuel pressure testing and only using genuine Jeep engine sensors.



For 1996+ Jeeps, the OBD-II engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator is not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM or poor battery charging. Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem.

A faulty Coolant Temperature Sensor may also cause similar symptoms. Testing of the CTS is recommended as part of the diagnostics. A poor ground at the ignition coil can also cause similar symptoms.

The reboot symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. (3) The ECU will usually relearn the idle settings after a short period of driving and the engine will start and idle normally.
Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the ground wires at the coil, and the ground wires from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

If the symptoms reoccur, you need to have your battery and/or the alternator Load Tested at a repair shop or auto parts store. This is usually a free service they offer to bring in potential customers. Handheld testers are very inaccurate and will usually pass marginal alternators. Even with low battery voltage, the engine will start, but the ECU can re-boot multiple times and a new battery may be required.

Cleaning the Idle Air Controller (IAC), regular tune-ups, and using fuel injector cleaner should be part of your routine preventative maintenance for smooth starts.
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