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XJ wouldn't run on a hot start unless I reset the ECM


NAXJA Forum User
A week ago, my XJ (2001, 4.0, AT, 4WD) started running very rough on *every* hot start - so rough I could barely coax it through a parking lot. It did not go away if I gave it gas, like it would with heat soak. However, if I disconnected the battery for long enough to reset the ECM (~5 min) it started right up, no problem. I had no problems whatsoever on cold starts. It threw a few codes during this time, all misfires, multiple cyl and then #4. The problem occurred after a 700 mile drive to Nashville. Before that drive, we hadn’t been putting many miles on it, maybe 50 a week. We made it back to DC fine - I avoided hot starts when possible and used the disconnect trick twice. Other than this problem, the jeep was running fine. Three days later, the problem disappeared. It’s back to running like it should.

Background: bought it about 18 months ago. I had some intermittent issues with misfires, rough hot starts, and occasional stalls. The stall issue magically resolved itself last year. The rough hot starts seemed to be related to heat soak and went away when I installed some heat shielding on the rail and injector #4 this summer - but it’s hard to say exactly, because the issues have been so intermittent. Some of the misfire codes do not seem to be related to heat soak, and come up even when there isn’t a rough start or noticeable issue.

Relevant work done: did a compression test recently and it passed with flying colors. In the last year I’ve replaced the plugs (Autolite Copper Core) and cleaned the TB and IAC.

As best as I can guess, the problem is emanating from the ECM (otherwise, why would resetting fix it?) But reman’d ones go for $250 online, used on ebay is $100, and the junkyard isn’t a great option either (nearest pick and pull with a 2001 is an hour away, and there’s no guarantee the ECM would match or even still be in the vehicle).

But I'm not discounting the chance it could be something else entirely, which is why I'm a little wary spending money + time on the assumption it's the ECM. From what I've read, people tend to blame the ECM when they can't otherwise diagnosis a problem. And this Jeep has had enough misfire issues prior to make me think that the recent issue could be connected.

So while it’s working right now, I’d be nervous to take it on any major road trip until I figure out what was (is?) going wrong. But I have no idea how to diagnose a problem that isn’t happening anymore.

Apologies for the novel, I just wanted to put all the info up front.

I’ve searched the forums and google to see if anyone else was able to fix hot start problem with an ECM reset, but didn’t find anything. Anyone got any ideas?
… people tend to blame the ECM when they can't otherwise diagnosis a problem


Have you tested the fuel pressure, especially when the symptoms are occurring ? Have you cleaned and snugged all the grounds and cleaned all the wire harness wire plugs ? How old are the O2 sensors, and do you have any Live Data from an OBD-II trouble code reading device ?

ECM failure should ONLY be considered after a thorough and in-depth diagnostics and trouble shooting of the more common, the cheaper, and the more likely suspects. Thermal failure of the Crankshaft Position Sensor is a primary suspect. Is the CPS a genuine Jeep part ?

Re-booting the ECM causes it to ignore the OBD sensor data ( both good and bad data ),and revert to pre-programmed parameters. It will run on the pre-programmed data until enough live data is saved.
I think you have just a hot restart problem just like mine had I made a heat shield that is large then the ones they sell and it did fix my hot restart problem . the fuel will boil in the fuel rail around injector #3 & maybe #4 if you look at the exhaust manifold and how close it is to some injectors you can see how you need a heat shield to stop the heat from getting to the injectors and fuel rail . By you opening the hood and sitting there I think a lot of the heat out of the engine and it cooled down so it could not misfire .

I haven't tested fuel pressure - the problem happened over Xmas in TN and I wasn't able to replicate when I got back to DC with my tools. I do have a OBDII reader that shows live data.

I spot checked for loose wires and grounds, but I more thorough check is probably in order.

What I read online about O2 sensors didn't seem to match my symptoms - from what I saw it was typical "rough running, bad mileage" not my situation where I can't get over 10 mph. I looked into replacing them preemptively, but the ~$150 price tag (for both sides, downstream) put a hold on that. No idea if they're original, the PO has not much in the way of repair history. I see that you can test them though, I'd add that to the to do list.

I actually have a OEM CPS on hand that I've been meaning to install as preventative maintenance. But from what I read they tend to fail catastrophically. One minute you're fine, the next the car stalls and won't restart.

Thanks for the info on the pre-set parameters. Seems like it might not be a hot start issue, per se, but rather a "miles driven after reset" issue. I'm trying to remember if I had a failure when the engine was cold but I had driven a lot since the last reset, and I'm not sure I did.

tech: that's heat soak. I'm familiar with that probably, but it's not what was happening with this jeep.
on a related note - if i buy a 1 year membership, can i edit my posts? i seem to recall reading that earlier but didn't see it mentioned on the membership page.
I had this exact same issue a few months ago, and the IAC was the problem.

A little history on the IAC, 2000 XJ Sport, original IAC started acting up. I didn't bother cleaning it because, well, it was about 20 years old and I hate cutting corners. Got a new one, but it was a $7 one from China (yes, a cut corner, but I was broke and Amazon delivered the next day). That one lasted about 2 months. Got another one for $7 and that one lasted about a month. The second one was where I was having the same problem as you. It got to the point where it wouldn't start at all unless I had the throttle open, but would still stall, and resetting the ECU didn't fix it anymore. That took a few days to happen after the initial symptoms. Oh, I almost forgot, it started, but ran at a super hight idle if I took the IAC out of the TB.

I decided that I didn't want to screw around with this anymore and sprung for the Mopar IAC. Started the first try (after resetting the ECU) without a problem and is still running like a champ.

Maybe pop the IAC out of the TB and give it a go. If it starts, then you know what your problem is.

I hope this helps.