hard start when cold, rough and low idle

Here's what it is.

1999 Cherokee 184K miles AW4 NP231 Meyer plow, air shocks on front and rear.

I started it when it was 33 degrees out. It cranked a while and then idled at about 7-800 Rpms. I had to give it a bit of throttle to keep it from dieing and to start. If you give it gas and let off it will die. Once it is warm it runs good. I know I have the common exhaust manifold crack. I have checked nothing yet as time. I will be changing the plugs, and oil this weekend.

Also never seen the check engine light on ever. Even when I just turn the ignition on.

When it's warmed up I'm getting 20-21mpg going 55mph. I drive it like a grandpa.

I sure don't know much about a 99 XJ, but if it was my 96 or 88 I'd try spraying some spray oil into the IAC inlet with the motor running and/or checking my battery or alternator output.

My IAC troubles seem to be gunk in the IAC or some kind of charging system issue and low voltage. Both seem to be worse in cold weather.

mine is also a 99 XJ with similar issues without the stalling/off . just runs a little ruff until it's 170deg or warmer.
I haven't been able to track down the culprit either...
I too would like to hear some thoughts on this

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.



Your 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 relearn the idle settings after a short period of driving and the engine will start and idle normally.

Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem. 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 and replace them as needed. Do the same for the grounding wires from the battery and engine to the Cherokee's frame/body.

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. 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.

Try unplugging your front o2 sensor before a cold start.

dobesrgrt said:

Try unplugging your front o2 sensor before a cold start.

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That won't do anything as its not reading the front O2 sensors until about a minute or two after cold start. However, a faulty O2 sensor could contribute to future cold start problems.

Ok an update.

It was 25F here and had to go help a friend @5AM. Jumped in the Jeep and it started right away and had a normal 1000-1100 RPM for about a minute or two and idled back to a nice smooth 850.

If it happens again I will look further into it.

Time for a new battery, mine was doing the same thing with the old battery.

I have a brand new battery less than a month old. Voltage readings at the battery 12.8vdc. Engine running it is 14.3-14.6vdc with no load. I may be buying the battery cables from 5-90.

Talyn said:

That won't do anything as its not reading the front O2 sensors until about a minute or two after cold start. However, a faulty O2 sensor could contribute to future cold start problems.

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FWIW, I reread the OP post #1, and I disagree. It is worth a try IMHO. Also, OBD-II tends to switch to and use the O2 sensors just second after it starts since the O2 sensor is electrically heated. This test is free, damages nothing, can be done in a few minutes and CAN be very revealing (if test equipment is not handy) if the there is any kind of trouble with the O2 sensor system, wiring, heater circuit, heater relay, exhaust leak screwing with the O2 sensor reading!!!! Also you plug it back in while the engine is running to see the immediate, and delayed effects!!!! I think it is a often overlooked diagnostic procedure.

If you have a multimeter I'd go check the voltage at the ECU while cranking and see if it drops below ~9 volts. A battery may have 12.8 volts at rest, but if it doesn't have enough CCAs the voltage will drop too low and the ecu will reset and loose the IAC position. Also one thing to take into consideration is that the chemical reaction in the battery slows down as it gets colder yielding less amps, so be sure to run this test when it's cold out.