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Start and Idle intermittent issues


NAXJA Forum User
Hello. I have an 01 Cherokee Xj sport (4.0) with some starting and idling issues. They are intermittent but always seem to present when my daughter has the car at school and it has sat for 6+ hours. When it happens I can usually coax the car to start but it will stall without using the gas pedal.
I recently replaced the oil cap which got rid of the check engine light but then this issue presented the following day. The car is reluctant to start but will if given some gas but doesn’t want to idle initially. It usually clears up and runs and restarts fine but not always. I replaced the idle control valve and have ordered a TPS. I searched and saw the fuel pump check valve can be a culprit but that’s not an easy fix from what I see.
Curiously, there is a gray plug towards the front of the manifold but I have searched and been in the Haynes manual and can’t determine what it is. (temp sensor??) unplugging it triggers a check engine but seems to solve the idle issue.
Any thoughts appreciated.
Faulty tps, ox sensors, coil rail, and heat soak can all cause that. The intake air temperature sensor can also cause that on later mopars, not sure about xjs.
"gray plug" doesn't exactly narrow things down, but operating on principles I would be inclined to follow those wires to find out which sensor you are unplugging. If you have a sensor which is giving bad information to the computer that could be your problem. Alternatively, get the codes which are being triggered by the unplugging. The codes will likely tell you which sensor is being affected. If you can look at live data for that sensor you might be able to get an additional insight into the problems.
Pictures of "gray plug"?

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 may sometimes start and idle normally.

Begin with basic trouble shooting of the start and charge systems. Remove, inspect, 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, wire brush until clean and 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 maintenance for good starting.
I appreciate the replies. I am starting to wonder if it’s a battery issue. As was outlined above, there is always enough to crank it but then it needs to run for a few minutes and then all is normal. I replaced the tps today but disconnected the battery so it was learning again when driven.
Update in case it helps someone with similar issues. I was sceptical about it being battery related but I had it tested. The voltage was fine but the cranking amps were weak and dipping a lot despite it always starting the car. The new battery has been in for a couple of weeks and appears to have solved this issue.