P0301 on a Hot Start

[Bratch]

The LJ has been running great lately but I got a random P0301 on Saturday afternoon.

I was running errands with the wife after getting into town it was stop for 10-15 minutes and then 0.5-1 mile to the next stop and repeat. After a few stops I got a rough idle on start up but it drove fine. Stop or two later, another rough idle on start and it threw a P0301 CEL.

We went home and got out a few hours later for dinner, perfectly smooth idle at start and no problems running 75 MPH down the highway. Start up a few hours later after dinner again perfectly smooth and no power issues at speed.

I’m going to go ahead and put fresh plugs in and check compression while I have the plugs out.

Is there anything I should be looking at that would cause it to happen on a hot start but not on a cold start?

 

[macleanflood]

Hot crankshaft position sensor.

But that's usually P300 and random cylinders.

I'd check the injectors with a screwdriver. #3 is really prone to heat.

-Mac

 

[RMETeeJay]

Is the heat shield foil intact on the top of the intake? The cats get hot and their heat rises.

There is a TSB modification procedure to further insulate when necessary. Your code is more associated with a misfire. Have you checked for other stored codes?

TSB 1803103 - 4.0L ROUGH ENGINE IDLE AFTER RESTART FOLLOWING A HOT SOAK
Date: 09/05/03 (supercedes 1802702 dated 09/06/02)
Model Year(s): 1999-2004

Description: NOTE: This bulletin applies to vehicles equipped with a 4.0L engine. This bulletin involves the installation of a fuel injector insulator sleeve.

Details: Customers may describe a 20 to 30 second rough idle following the restart of a heat soaked engine. This condition may be most noticeable when the engine is restarted following a prior 10 to 20 minute heat soak in hot ambient conditions of approximately 32 C (90ºF) or higher. This condition may be consistent with short city stop-and-go driving trips and can be aggravated by the use of fuel with a high ethanol content. Depending upon various conditions a MIL may occur due to DTC P0303 - Cylinder #3 Misfire.

This condition may be caused by heat from the exhaust manifold that following engine shut down migrates to the area around injector #3 and causes fuel vapor to form within the injector. This in turn may cause a momentary misfire of cylinder #3 until the fuel vapor is cleared of injector #3. The insulator sleeve lowers the injector #3 temperature to a point below which the fuel will not normally vaporize.

If vehicle exhibits a rough idle, and if a misfire of cylinder #3 is observed following a 10 to 20 minute hot soak, perform the Repair Procedure.

Parts required:
56028371AA Ignition Wire Shield

Repair procedure:
1. Cut insulator sleeve, p/n 56028371AA, to make two (2) insulator sleeves about 25-30 mm (1 in.) in length. See diagram.
2. Install one sleeve around injector #3, with the slit on the upward facing side of the injector. Install the other sleeve with the slit on the downward facing side of the injector.
3. Confirm sleeve is flush to intake manifold surface around injector.
4. Check injector #3 wire and ensure that the injector is rotated to a 2 o'clock position.

 

[Bratch]

Is the heat shield foil intact on the top of the intake? The cats get hot and their heat rises.

There is a TSB modification procedure to further insulate when necessary. Your code is more associated with a misfire. Have you checked for other stored codes?

I’ll check out the heat shield when I work on it next weekend.

That TSB describes what happened to me to a T except I got P0301 instead of P0303, ambient temperature was 95* on Saturday afternoon.

On the 3 key turn code check P0301 is the only thing showing.

Thanks

 

[OilSpot74]

Every time I’ve thrown a P03xx code it’s always been a loose plug wire that’s happened at highway speeds. (We have crappy roads in Colorado) Something just a mm enough loose to throw a code.

 

[That'll Buff]

Mine does the same thing. Run it till its hot and if I let it sit for 10-20 min and start it up again it'll idle rough for maybe 30 seconds to a minute. Runs completely fine otherwise. Only recently did it throw the same P0301 code on a trail, but I disconnected the battery and cleared it once I realized it was a false positive. It's too the point now where I'm considering hood louvers to help when driving around the city to allow it to vent out some of that extra heat while its sitting with the engine off. Since I wheel with a big (and often slow) group that stops often, when I'm out on trails and stop I'll lift the hood. Can't tell if it's fixed it but it doesn't hurt either.

 

[ItchyDong]

If it’s truly a repeatable P0301 and only on a hot start, it’s probably a leaky injector.

You shut the jeep off, it leaks into the cylinder and vaporizes. Now that cylinder is way above stoich. If you give it enough time, it dissipates past the rings and condenses as the engine cools. But if you start it immediately, it has a hard time catching spark because it’s way too rich.

Try swapping the injector with another cylinder and see if the misfire follows.

You can also look at fuel trims. The offending bank will have negative STFT’s as the computer tries to correct for a rich condition. They’ll even out after it idles for a minute.

 

[rouxbicon]

If it’s truly a repeatable P0301 and only on a hot start, it’s probably a leaky injector.

You shut the jeep off, it leaks into the cylinder and vaporizes. Now that cylinder is way above stoich. If you give it enough time, it dissipates past the rings and condenses as the engine cools. But if you start it immediately, it has a hard time catching spark because it’s way too rich.

Try swapping the injector with another cylinder and see if the misfire follows.

You can also look at fuel trims. The offending bank will have negative STFT’s as the computer tries to correct for a rich condition. They’ll even out after it idles for a minute.

I'd bet on fuel boiling in the rail and primarily affecting cyl #1 in the OP's case. The TSB specifically references #3, but seems likely that other injectors/cylinders could be affected under certain circumstances. I have had a P0304 with the exact same symptoms, i.e. couple minutes of rough idle after a hot restart, but smoothed right out after those couple minutes, and always smooth after a cold start. I eventually installed the Design Engineering injector/rail insulation kit, which helped a lot

 

[ItchyDong]

I'd bet on fuel boiling in the rail and primarily affecting cyl #1 in the OP's case. The TSB specifically references #3, but seems likely that other injectors/cylinders could be affected under certain circumstances. I have had a P0304 with the exact same symptoms, i.e. couple minutes of rough idle after a hot restart, but smoothed right out after those couple minutes, and always smooth after a cold start. I eventually installed the Design Engineering injector/rail insulation kit, which helped a lot

Very likely the case. Just haven’t seen it on one as often. It’s usually 300, 303, and 304. Swapping injectors is an easy troubleshooting step before spending money on a kit.

 

[Irun]

I just went through this. Check, spark, fuel, and the mechanicals (compression and leak down). If the plugs and coil pack are good, pull the injector rail and swap injectors #1 and #2. If that does not work, then do the compression and leak down test. If you're changing the plugs, that's the time to do it. Lastly, you do have an '06, so you can reach out to @Wranglerfix, for a potential PCM issue.

 

[Bratch]

I just went through this. Check, spark, fuel, and the mechanicals (compression and leak down). If the plugs and coil pack are good, pull the injector rail and swap injectors #1 and #2. If that does not work, then do the compression and leak down test. If you're changing the plugs, that's the time to do it. Lastly, you do have an '06, so you can reach out to @Wranglerfix, for a potential PCM issue.

Mine is a 2004 not a ‘06.

I’ve ordered some of the insulator sleeves referenced in the TSB. I plan to check the insulation shield and install the sleeves.

I got an ODB reader to see the fuel trims.

I’m going to go ahead and swap plugs, they are 9 years old and have 50k on them. I’ll check compression while swapping them.

I’ll also check the injector to make sure they sound good.

I think it is the situation from the TSB, the quick on and off is a pretty uncommon driving style for us so I don’t think I’ll learn much from swapping injectors unless I purposefully recreate the situation the next time we get hot.

I’ll update with my findings.

Thanks everyone

 

[Irun]

Mine is a 2004 not a ‘06.

I’ve ordered some of the insulator sleeves referenced in the TSB. I plan to check the insulation shield and install the sleeves.

I got an ODB reader to see the fuel trims.

I’m going to go ahead and swap plugs, they are 9 years old and have 50k on them. I’ll check compression while swapping them.

I’ll also check the injector to make sure they sound good.

I think it is the situation from the TSB, the quick on and off is a pretty uncommon driving style for us so I don’t think I’ll learn much from swapping injectors unless I purposefully recreate the situation the next time we get hot.

I’ll update with my findings.

Thanks everyone

Have you checked the cam sync signal? I've had issues with that before on my '04.

 

[Wranglerfix]

I can test your ecm if you’d like to send it into me. I almost never see a misfire code in a computer. It can come down to brand of sensor, causing the problem.

 

[Stumpjumper_AZ]

I had this heat soak, rough running issue on my 2005 LJ, found bare copper on wires at bend going into crankshaft sensor. Taped up the three wires using electrical tape and it’s been running smoothly since.

 

[Bratch]

I can test your ecm if you’d like to send it into me. I almost never see a misfire code in a computer. It can come down to brand of sensor, causing the problem.

Good to know. Thanks.

If I can’t get any traction on this I’ll reach out.