Recently got a 2000 4.0L XJ with 240k and been battling with a cylinder 1 misfire. The jeep hates to sit at idle, it roughs a little rough and after like 5 minutes it will throw a cylinder 1 mis code. The weird thing is at 1k rpm the engine is smooth and all other rpms, never misfires while driving it only at stop lights or running while parked. Sometimes it’s a slight misfire to throw no code and other times it can even flash the light and start missing hard
Started off with the simple and replaced the plugs with NGKs as this has the coil pack in it. (Before it had autolites, bosch, and champions all mismatched in the cylinders) Problem still persisted so then I changed the whole coil pack and still nothing changed. Pulled #1 plug and it looked good but swapped it with #2 and insider remained in 1. So I took the throttle body out of another jeep that has good sensors and cleaned it completely free of carbon including the IAC, put it in and no change at all. The last thing i did was put Bosch ev6 4 hole injectors in and cleaned out the fuel rail. Thought for sure this would fix it because #1 injector had gunk all over it and was hard to come out and the rail was all dirty. But even after all that it will still misfire occasionally, the injectors have definitely helped the problem. Tested out long term and short term fuel trim values and they were ok. When I did the valve cover gasket I checked to see if the head was em cracked and it was not, the springs look great, and the motor itself is very clean on the inside for its age
Im lost on where to go next because it’s so inconsistent, I’ll drive on the highway for an hour and come off and the thing will idle extremely smooth and other times it throws the code with a hard cylinder 1 misfire_-It has a mind of its of its own really. If it was a burnt valve or low compression wouldn’t it be way more consistent. Could it be carbon buildup or something else i’m stuck here?

Next up would be a compression test. Can't rule out something internal. The compression spec for the 4.0 engine is 120-150 psi with no more than a 30 psi variation between cylinders. Not saying that is what the problem is, but I am saying that it should be ruled in or out at this point.

Even though your fuel trims look okay, still wouldn't be a bad idea to test fuel pressure with a gauge. 5 minutes of your time. You need 49 psi, plus or minus 5 psi for your 2000.

Fuel checked out ok

Compression was
1-60, 2-90, 3-67, 4-95, 5-100, 6-95
Putting oil in 1 & 3 did not change it
It is still running great but if I leave it idling for a long time it will throw a code

Although allwithin a similar range, that's quite sub par along all cylinders. So it drives fine otherwise!?

Yeah still drives fine, just has a choppy idle, i’m pretty sure the motor is just tired it has 245k and the oil pressure is around 13 once warmed up at idle which is pretty low too
Just don’t know why the misfire is so random at times, it literally just goes away with higher rpm
Tried with two compression testers aswell because i didn’t think it could run with compression that low but now I guess now it’s time to do a leak down test

Yeah it seems tired alright. Leakdown might confirm the end of life.
Cylinder 1 has compression half of what it should be at a minimum, adding to anything else that might be an other cause.

Since fuel pressure is ok and new injectors took the injector out of equation; have you checked for consistent spark at #1?

Since fuel pressure is ok and new injectors took the injector out of equation; have you checked for consistent spark at #1?[/QUOTE]

Yeah fuel is definitely out of equation i replaced rusty fuel rail and lines as well, plus took fuel samples and it checked out good. I have not checked for consistent spark though, I just replaced the coil pack with a brand new one. I don’t know how to check for consistent spark on one with a coil pack though, could possibly be cutting spark or weird electrical problem because one time it felt like the cylinder went dead and the code flashed but hasn’t done that in awhile.
Just skeptical about my low compression because yes it’s low but why would it be so inconsistent.

Inconsistent or intermittent problems are more often than not electrical rather than mechanical. Especially intermittent can be challenging to diagnose.
SAme as arrhythmias ( also electrical problem) that resolved by the time people arrive in the ER, but that's outside the scope of this hahah)

I'm not exactly familiar with the coil pack, but the testing ( plug removed from engine block but otherwise connected to coilpack. Than grounding the tip of the spark plug) should be a similar procedure as with the earlier (distributor) year XJ's.

I did a quick google and it comes up with many results so have a look.

It's important to check because that's still not excluded.

Um, folks?

His engine is shot. Those compression numbers tell the story. Any time spent troubleshooting the misfire is wasted time.

Yeah I know Mark and I think the OP too. But out of shear joy on completing a certain byway of the diagnostic process

Plus OP states it otherwise drives fine. Let's delay the inevitable by perhaps a few more trips.

I know it’s been 2 years but I have finally found the cause of this. After countless trips offroad, up and won the east coast, the gas mileage was awful and it started running rougher at idle. Took the head off and found Cylinder 1 Intake valve was not seating sometimes because of a massive chunk of hardened carbon on the valve seat and a section of the valve. Exhaust valves 1-6 were all badly pitted. Headgasket between 1 & 2 was on its last legs. After new valves and cleaning up the seats at home it runs fine. Hopefully it will make it past 300k now.
bc

Garrett, thanks for the update.
I read through this thread without paying attention to the dates and was about to give some advice when I saw your post. LOL.

I'll tell you a story.
A friend of mine had this exact issue on his 99.
He did what you first did, then changed valve springs with the head on the vehicle because that was an issue with that vintage.
No joy.
I had him run some BG44K through the fuel system per instruction and still no joy.

What fixed it was when he listened to some old time mechanics advice, mine, and ran the engine at 2000 RPM and dripped water into the throttle body to get the carbon out. It has run fine ever since.

Chysler TSB 0900303
4.0L MULTIPLE CYLINDER MISFIRE
Date: 08/01/03
Model Year(s): 1999-2004

Description: NOTE: THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH A 4.0L ENGINE. This bulletin involves inspection of all engine exhaust valves and a decarbonizing procedure if necessary.

Details: The customer may experience an incident of engine misfire during certain vehicle operating conditions. The misfire may occur when the vehicle is operated between 50 - 70 MPH and under light loading conditions, e.g. slight uphill road grades. This condition may occur at all ambient conditions, but is more noticeable when ambient conditions are less than 0 C (32 F).

If the vehicle is equipped with On-Board Diagnostic (OBD), a MIL illumination may also have occurred due to Diagnostic Trouble Code (DTC) P0300 - Multiple Cylinder Misfire. Various single cylinder misfire DTC's may also be present. If the frequency of misfire is high the Powertrain Control Module (PCM) may place the engine in Limp-In mode.

The misfire condition may be caused by one or more engine exhaust valves that are slow to close. Late closure of an exhaust valve may be the result of no valve rotation and associated build up of carbon on the exhaust valve stem.

This 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.


Jerry Bransford a senior member of JeepForum posted the following:

Decarbonizing an engine can often be accomplished in 5-10 minutes for free using nothing more than 12-16 ounces of water. With the engine running and the air intake removed from the throttle body, SLOWLY trickle the water into the throttle body, keeping the rpms up a tad with the throttle linkage. Do it slowly enough that it will take several minutes to drain the container. No this will not (!) cause hydrolocking unless you dump the water in too quickly.

The water turns to steam and it creates millions of little shock waves that break up carbon deposits which are then sent out via the exhaust system. This is done all the time on engines that tend to develop carbon deposits when they're driven too conservatively. For example, Corvette mechanics do this all the time on engines that are babied too much by their owners.

Done properly this will not harm the engine in the least and it's an age-old technique that can leave an engine sparkling clean internally. Engines have actually used water injection since before WWII to increase power and the mechanics quickly learned that it keeps the engines carbon free, or gets rid of existing carbon deposits once modified for water injection.

All you have to do is trickle it in slowly enough that it will take probably 2-3 minutes to drain the container... a very slow constant tiny trickle of plain tap water.

Does this decarbonization deposit all the crud on the plugs or is it usually passed out the exhaust? How much effect on the cats?

Right out the exhaust. As for cats, any ones nearby usually flee immediately. they hate the loud noise.

The carbon pieces are very small and likely pass through. Don't overthink this. It flat out works.