Rough Idle, P0301, blinking CEL In 2001 XJ 4.0L

[driggs]

I've got a 2001 XJ with 4.0 I6 engine with 215k miles on it; it recently developed an engine problem that quickly got worse to the point that I'm not able to drive it. It started as just a rough idle, that I suspected was due to early wear-out of spark plugs. The rough idle quickly got worse, idling at higher RPMs, until the engine threw P0301 (cylinder 1 misfire) one afternoon - time to do maintenance!

What I've done:

1. 1/2 can SeaFoam into throttle body, let sit 5 minutes, smoke it out
2. 1/2 can SeaFoam into gas tank with fuel light on, so ~5gal in tank, drove high RPM in 1st gear for a few miles to try and clean fuel injectors
3. Replaced spark plugs - OEM NGK ZFR5N, old ones (same plugs) had 22k on them and all came out gapped at 0.045 and "normal" wear; I ensured the new plugs were gapped at 0.035, as were the old set when installed

What I've still got:

When I start it up cold (it's below freezing here right now!), idle is high ~1.1k RPM, slowly drops down to normal range ~700RPM as the engine gets up to operating temperature, though operating temperature has been about 197deg this winter. The numbers all come from an ODB-II display. As it warms up and idle RPM goes toward normal, idle gets more and more rough, in a cyclic "beat" pattern... engine throws P0301, and then within a couple minutes the "check engine" light starts flashing!

I've been pouring over the forums looking for ways to narrow it down, but I'm not sure where to start... swap fuel injector 1? Replace coil rail? Replace cam position sensor? Clean/replace throttle position sensor?

Possibly related, possible "red herrings":

• Just last week, my previously-cracked oil pressure sending unit finally bit the dust; oil pressure gauge dropped to zero; I replaced the sending unit with Mopar OEM unit and all seems well. This was just a few days before the rough idle started.
• Like many '01 XJs, mine suffers from the godawful "heat soak" P0303 rough idle, despite installing the stupid insulator on injector #3. It does so bad enough that I always turn the key to prime the fuel line with pressure before starting the jeep, because it seemed like it could be leaking pressure to allow it to happen so easily. Note that this heat soak issue is NOT what this thread is about however.
• It also threw P0455 (evap system large leak) twice in the last 2000 miles, but I wrote it off as gas cap looseness

Can you help an inexperienced Jeep mechanic figure out how to solve this problem? THANKS!

 

[driggs]

As it warms up and idle RPM goes toward normal, idle gets more and more rough, in a cyclic "beat" pattern... engine throws P0301, and then within a couple minutes the "check engine" light starts flashing!

I'd like to clarify that the rough "idle" isn't just while idling, it can be felt under acceleration also.

 

[birchlakeXJ]

For a single cylinder misfire, you gotta concentrate on things "specific" to that cylinder.

With a new spark plug in there (how did the old plug from that cylinder look?), the next thing would be to try swapping a fuel injector to a different cylinder (don't use #3) and see if the misfire follows. If it does, you know which fuel injector is bad and swapping in a complete new set would make some sense.

After that, a compression test on all cylinders would make sense and look for any abnormal variations. The spec compression for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

A cracked cylinder head can create some pretty strange misfires. And you have the "infamous" 0331 cylinder head which has a known casting defect between cylinders 3-4, which is where it normally fails. Not all of these heads fail, but there are quite a number of them that do. The #1 symptom of a cracked 0331 head is "unexplained coolant loss". How is the coolant level in the coolant reservoir?

Cleaning both the idle air control and throttle body may resolve the higher idle problem. Start with a cleaning for that issue.

 

[98XJROKS]

I just went through this with my wifes 01'. I had a few issued to deal with, which caused me a lot of confusion. No matter what I did seemed to fix the problem for more than just a few days.

I did a number of things and none of which really fixed the over all problem.

Started with cel, flashing cel. Codes were 300, 301, 304, 306 etc.

To make a LONG story short, here is what I did to fix the problem.

1) Pulled the new plugs (Bosch Platinums) and replaced them with the factory Champion coppers gapped at .35. Returned the crap Bosch plugs to O'reilly's.

This took care of the random misfire's as well as 301 & 304. But 306 still exhisted and was very periodic. I also got an Open circuit injector #6, can't remember the code.

So, I switched the #6 Injector with the #4 and the misfire followed the injector.

2) I Replaced the #4 injector and viola. It's fixed.

There are a number of things that you can do before you change out sensors. such as; check the camshaft position by removing the sensor and doing the "toothpick test" to make sure it's lined up etc.
Check the sensors with an ohm meter to make sure they're in range.



Hope this helps.

 

[driggs]

To follow up, after my initial SeaFoam followed by changing the spark plugs...

I had a mechanic swap fuel injectors, he claimed that the cylinder misfire followed the #1 injector so he replaced it with a new part... the XJ threw a P0301 (cylinder #1 misfire) before I made it out of his parking lot. Ugh. Had him swap the coil rail out with my spare, no change.

I began to fear a cracked head and took the XJ home rather than have him waste any more time scratching his head. I was prepared to start checking/replacing the cam and crank position sensors when I remembered reading about TSB 09-003-03:

This [cylinder misfire] condition may occur when the engine is not allowed to run at engine RPM’s that are greater than 3,200 RPM. At 3,200 RPM or higher the engine exhaust valves will rotate if not impeded by high carbon deposits. Low engine RPM’s and high carbon deposits are associated with short trip driving where the vehicle engine is not allowed to fully warm to normal engine operating temperatures. Cold ambient temperatures will increase engine warm-up time and add to the opportunity of carbon deposit build-up on the stem of the engine exhaust valve.

OK, I've got high mileage and this is the dead of winter, perhaps my cylinder 1 exhaust valve is gummed up with carbon. The TSB calls for the use of Mopar Combustion Chamber Cleaner, but it wasn't convenient to hit up the dealership to see if they had any in stock; instead, I picked up a can of SeaFoam Spray to try out. This is like regular SeaFoam, but in an "aerosol" can with a special U-shaped straw that lets you spray it into the throttle body in "atomized" form.

I warmed the engine up to operating temperature, pulled the air duct off the throttle body, and adjusted the U-shaped straw to hook over the lip of the throttle body and spray directly down into the open butterfly. I had a helper keep the engine running at 1500 RPM while I unloaded the can into the TB. About halfway through the can, and very suddenly and obviously, my sick-sounding engine suddenly stopped misfiring and became much quieter... HOLY CRAP, this stuff works! I emptied the remainder of the can into it, killed the engine, and let it heat soak for 2 hours, tapping the ignition once or twice to move the pistons around. I also used some throttle body cleaner and a toothbrush to clean my filthy TB before starting up, since the SeaFoam Spray already started the job. It coughed and sputtered when starting up, but didn't produce the same "mushroom cloud" that a normal SeaFoam job does. I drove it around hard, giving ~20sec bursts in the 3500 - 4000 RPM range with rests between.

It's been almost an entire week now, and my XJ has not thrown another P0301, and the engine actually feels like it's running well! Thanks for all the advice and info, !

 

[joe_peters]

I hope you got it resolved.

FTW:

1. Chryco recommends the NGK V-Fire plugs with the distributorless ignitions in the 4.0 engine.

2. That POS Chryco wrap for the number 3 plug is about as effective as wrapping a Taco Bell burrito shell around it. Get a programmable timer: http://www.at-fairfax.com/P1786-ELK-960.htm and install it set to run your auxiliary fan for 5 minutes after engine shut down. No more heat soak. Search for Muddeprived, he has a couple EPIC threads on his heat soak and the timer.

3. There also is an issue with "alignment" of the synch sensor in the distributor OR the oil pump drive for the distributorless engines. Search "toothpick alignment" for the procedure.

Good luck to all.

 

[Kejtar]

Deja vu...... I just went through that.
Here's things to look at:
1. Check compression
2. Move plugs around
3. Get a noid light to check injector for firing signal at the injector
4. Check for spark (at cylinder 1) (not sure how to do that on distributorless)
5. Pop the valve cover off and see if the cylinder 1 has the same movement on the springs as the rest (do that with no wire on the plugs or injectors)

Hopefully you'll strike gold and one of the above is going to take you to to a solution cause for me... it ended up being an engine swap.