99 4.0L, Weak crank, low idle.

99 XJ 4.0L auto 135k miles.

Had the fuel pump replaced 2 weeks ago, and at the same time had 4-hole injectors installed. Ran okay till last weekend. I disconnected the battery to reset the ECU (so it "re-learns" the fuel mapping).

It ran okay on Sunday after that, but Monday morning when I go to start it up, it cranks over VERY weakly (even got the dead battery clicking at one point). Subsequent cranks, it seemed to get stronger, but until I gave it some throttle, it didn't actually catch.

Once it caught it ran smooth, but the idle would drop to ~300rpm and stall unless I kept giving it throttle.

After a while, it idled on it's own okay (just left it for the day and took my bike to work).

Monday afternoon, it was the same thing... very slow/weak cranking, but when I'd give it throttle, it would catch and run, but the idle would occasionally drop low and it stalled once.

No CEL, scanned for codes and none are showing.

Never had any issues with her before now. I'm thinking IAC?

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.

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.

Tim_MN said:

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.

Click to expand...

That sounds like it exactly. So how long should I expect this "reboot" to last?

When will it go back to normal? Do I just need to start driving it (2-footing the pedals) for a while in order for it to relearn the idle settings?

I have heard that it can take up to an hour of driving before the ECU re-learns enough to start and drive normally.

I'd also be concerned by the "extremely slow cranking", thinking it may be resulting with numerous re-boots of the ECU. Sounds like old/weak battery and/or poor battery cable connections may be the root problem.

Flyfisher said:

I'd also be concerned by the "extremely slow cranking", thinking it may be resulting with numerous re-boots of the ECU. Sounds like old/weak battery and/or poor battery cable connections may be the root problem.

Click to expand...

I don't know what it was, but it would crank slow, but when you gave it gas, it would start just fine.

Also: driving around for a while seems to have fixed the idle issue.

Thanks.

Low voltage during cranking will corrupt the idle position memory in the ECU.

This will cause stalling until the ECU re-learns hot in-gear idle position.

Can also be corrected by disconnection the battery positive terminal and touching it to ground for 5 to 10 seconds. This resets the memory to a pre-set. (reattaching to a poor battery will cause the memory to corrupt again at start up)

After about 10 minutes or less of driving, the idle setting are re-learned. You may indeed have to two-foot drive for at least a few minutes. If the symptoms persist, you need to diagnose and fix the cause.

You need to check cranking voltage at the starter and the battery. Volt meters are cheap if you don't have one. Check voltage while cranking directly at the battery, should flutter around 10 volts, if you are down in the single digits, and the battery is fully charged, it is likely you have a battery problem. Now check voltage from the positive battery terminal to the positive terminal on the starter, and crank the engine, should be around 0.5 or less. On the negative side, put the positive end on the negative battery terminal and the negative end on the housing of the starter, then crank, voltage should be 0.3 volts or so. If the voltages are good then I would assume that starter is bad.

So, a couple of people on here have the right idea, but I'm going to add a few points that might help people better understand the JTEC idle control "reset" issue. Yes, if the voltage drops too low during cranking the IAC step offset can reset, however, if your throttle is clean and everything is up to spec (see: http://www.naxja.org/forum/showthread.php?t=1121429 ), a normal reset won't cause a stalling issue. What happens is that if the IAC step offset is lost due to low cranking voltage, it's very typical for the ecu to not complete the IAC zeroing process. Basically you wind up in a situation where the ecu thinks the IAC is at 30, 40 , 50 steps, but really it's at 0 steps or some value considerably lower than what it's trying to command. I've seen this on many Chrysler vehicles that use JTEC. Yes, if you run the car long enough it will relearn, it does this while idling by the way, not while driving . For the ecu to start learning the idle steps, the engine temperature must rise and stay above 150 degrees.

Thanks hex.

Went out this morning to go to work and it did the same thing again when I tried to start.

I'll check the battery with a multimeter.

Ran into an issue when I reconnected the battery last time too: I forgot that a previous owner had installed an alarm. Reconnecting the battery set off the alarm. Luckily I remembered that there was an override button, so when I turn the ignition to on and press the button, it silences the alarm. Would like to just remove the alarm altogether, but since I don't know anything about it, I'm worried about damaging/disabling critical systems if I tried.


It seems to relearn the idle after a bit of driving, but after sitting overnight, it has forgotten again. You think that using the throttle to get the engine running, then just driving it for a few days will eventually get it to relearn for good?

I've owned enough unreliable vehicles that I'm pretty good at two-footing the throttle and brake to keep the engine running.




edit: went out to start it again, and the cranking was even weaker.... got the clicks multiple times before I got it to start. Looking like it could be a battery issue.... :flamemad:

My 99 was doing the same thing, the battery would show ~12.5-12.8 volts, but when cranking it was dipping down to 4 volts. After replacing the bad battery (5 year old optima) the jeep cranks over much faster and has no issues idling.

wolf2600 said:

It seems to relearn the idle after a bit of driving, but after sitting overnight, it has forgotten again.

Click to expand...

If it needs to relearn the idle every morning I think it's getting reset every night, probably dying battery or loose terminal/ground. Once it learns the idle it should stay until it resets again.

peterjford said:

If it needs to relearn the idle every morning I think it's getting reset every night, probably dying battery or loose terminal/ground. Once it learns the idle it should stay until it resets again.

Click to expand...

Yeah, I think it might be the battery. Hooked up a charger, went to "full charge" after about 5 minutes, then dropped back to empty. Then back to full, and back to empty every 5-10 seconds.

Got it running/idling, and I'm getting 13.85v, so the alternator seems good. Just time for a new battery, I guess.


Gonna be fun heading back from Autozone on the motorcycle with a battery in my backpack. :gag:

Battery seems to have done it. Put it in, cranked right over, idled perfectly.

Great to hear you got it fixed. Be thankful you at least have the motorcycle, I once had no vehicle and a battery to replace, had to ask a neighbor for a ride.