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Another Random Misfire Thread 2001 XJ

Mgrantham

NAXJA Forum User
Location
Flagstaff, AZ
Hey guys. Having some trouble understanding what's going on with my jeep.

2001 XJ, 4.0, 4WD, 33" tires, about 7" of lift.

Today driving up from Phoenix to Flagstaff my check engine light came on. Pretty soon it started blinking at me. :shocked: The jeep is running fine, but I've never seen the blinking light before. I've read its the end of the world. I've heard its just showing an active misfire is happening. Got it coded at Autozone, it came back with 4 codes: P0300, P0303, P0304, P0305. I apparently have random misfires on cylinder 3, 4, and 5.

So here's the history. It has had random misfires before. Originally I believed it only happened at high RPMs and would reset the code. Sometimes it would come back quickly and other times it would come back a few months later. Recently it has been running slightly hotter than normal. A few weeks ago the radiator literally exploded. The theory is that there was a pinhole leak in the plastic railing that grew, dropped the pressure in the radiator, until the heat went catastrophic and exploded the side of an already defective radiator. Put a new radiator in, its been running just like before which was just slightly hotter than normal. Hotter than normal is just a tick above 210. Threw a new t-stat in and its running the same temp. Just slightly above normal.

So, assuming my cooling system is fine I have been hunting down the other reason my system has been running slightly hot. My thoughts are along the lines of faulty injectors making some cylinders run lean and causing heat up. Would this also cause my misfires?

I was supposed to drive this beast to Montana from Arizona tomorrow. So needless to say, I'm not going anywhere until I figure this out. The engine is running smoothly, even when the check engine light was blinking. I'm pretty stumped on this.

As a side note, I already have the foil under my injectors which I believe is the fix for heat soak and done by a previous owner.

So, any ideas?
 
Update: Changed the plugs to some autolites, added some seafoam to the tank, changed the ignition coil and reset the code. I drove it for about 100 miles and it coded with P0300, P0304 and P0305 about half way through.

Any ideas? I really need to fix this.
 
Ensure the intake/exhaust manifold bolts are tight.

I will check that.

In the meantime, here's what I've found. Opened up the butterfly valve at the air intake and it was caked with sludge. Cleaned it out with some cleaner, and still plan to do some sea foam to it.

Found a fix from the dealership that explains what issues I've been having can be a result from carbon deposits on the exhaust valve. It said to open up the rocker cover, pull off the covers and look for a bullseye wear patter on the exhaust valve tip. The bullseye shows that at high RPMs the valve spins 90* like it was designed to do. If it wasn't it would show a straight line or dot, or some other wear mark that isnt a bullseye.
Anyways, I opened it up. Checked a few valves and it looks all good.

The problem is that I can't tell when I've fixed it. It is intermittant and seems to only happen under load, like going up a hill. So I normally don't find out it has been fixed until I'm half way to Flagstaff from Phoenix.

I will keep updating this. Even though only one person has commented so far, it seems like a fairly common problem with few answers.
 
I can't comment, but I'm watching this thread closely since mine does it sometimes as well. I've yet to pull codes, but mines an 01 limited, so I'm anxious to hear what you find out.

I'm basically in the middle of a 100% new cooling system, I've also done a new ignition system from one end to the other, so far, its running good, but it still does it pretty intermitantly so I still haven't fixed it.
 
If you find a P0300 random misfire code, it means the misfire is random and is moving around from cylinder to cylinder. The cause here would likely be something that upsets the engine's air/fuel mixture, such as a major vacuum leak, or unusually low fuel pressure (weak pump or faulty pressure regulator).

1. Perform a comprehensive search and resolve ANY vacuum leaks

2. Check fuel pressure. Must be done with a gauge. For an 01, you should have 49 psi, plus or minus 5 psi

3. That overheat is a wildcard here. The 01 from the factory came with the infamous 0331 cylinder head, which often cracks when the engine is overheated. A cracked head can lead to misfires. Are you seeing a drop in the coolant in the reservoir? The #1 symptom of a cracked 0331 head is "unexplained coolant loss"

4. Don't drive it with a flashing check engine light. The "flashing" indicates a serious misfire, which if left long enough, will destroy your catalytic converter.
 
Ok lets see if I can respond to some of these replies.

DerMeister, there is 105k miles on it. I haven't checked into re-indexing my CPS but I am familiar with changing them from and older jeep and have begun thinking that perhaps CPS is an issue here. I will look at your link.

BirchlakeXJ, I can't seem to find any vacuum leaks. I dont have a vacuum tester or a fuel pressure regulator. As far as not driving it during a flashing CEL, does this mean not to drive only when its flashing or if it flashed then turned solid? This thing runs perfectly fine other than running a little hot. If it didnt have a CEL I would never know anything was wrong with it. No rough idle, no loss of power, etc. I didn't see a crack when I had the rocker arm cover off. No loss of coolant, no milkshake in the oil. Without doing a compression test I am fairly confident I dont have a cracked head.

Here's my update for what I've done since my previous post.

Added some sea foam into the vacuum lines via the brake booster. Didn't really produce a lot of smoke. Hoping I had cleaned her up and betting on dirty exhaust valves I pushed her up the hill towards Flagstaff to try and get a code. Sure enough 300, 303, and 306 popped up. Interestingly it seems to have happend twice when my temp gage was reading around 220. I watch it climb from 210 as I go up the hill and right before the dial hits the next line up it codes.

Decided I would bite the bullet and get all six injectors. #3 was cracked and another one was starting to crack.

While I had my fuel rod off with the injectors still connected to the fuel rail we jumped my injectors from the battery and they all squirted. Opened the gas cap, opened the pressure valve, pulled off one injector, put the cap back on, turned the key on and almost no fuel came pouring out. Is this a pressure issue? Or does the fuel pump register that there was no longer pressure in the line and kills the pump as a safety issue?
 
Update:

Rented a fuel pressure regulator. It doesn't seem to do much by just turning the key on. But I can start it and it jumps up to 50. Turn it off. It bleeds off to about 40. Turn the key on without starting and it jumps up to 50 and begins bleeding off on its own. After letting it run and turning it off it takes about 4 minutes to bleed off to below 30.

Also, trying to track down the heating issue which may or may not be related we checked the AC pressure and it was in the alert range. Relieved some pressure to the safe zone.

This thing has a new radiator, new t-stat, good on fluid. Runs fine without ac. But heats up with ac. Is my compressor clogged up? Maybe it had air in it and bleeding off the extra pressure helped with that... dont know.

Also discovered that my battery is at 13.75 with no AC. 13.65 with AC. 13.55 when electric fan kicked on. Alternator may be about to give up the ghost...

Any of this stuff ringing any bells?
 
Id say you're issue is due to heat soak since it only happens when it gets hot.
Get your temps under control and the misfires will probably go away.
Hows your header?
Got any cracks?

Did you ensure you've got a good seal for your intake/exhaust?

At this point, its just about worth getting a new gasket.
Might as well pick up an APN header while you're at it...
 
I was under the impression that heat soak is only the rough idle during start up when engine is hot. Does heat soak also include highway driving? I also already have the heat foil but not the injector sleeves.

I can't tell if the header is cracked, however when I did some seafoam I didn't notice any smoke coming out the engine compartment like I did on my brother's obvious exhaust leak in his car.

No I haven't checked that there is a good seal to intake or exhaust yet. Honestly with all the other things I've been doing getting into there to tighten it up looks daunting. Is it easy?

I just dumped probably $400+ into this "fix", at this point I'm thinking I may just go for it and see how she does. Not sure I want to pick up a header just yet. $$$. However, depending on how bad the crack is, if there is one, I would just weld it. So its probably just a project of taking it off.

I'm debating taking her up the hill again and seeing if she tosses another code. I'm really hoping the injectors fixed this. Anyone know how much a cracked injector messes with the ECU?

I agree that getting my temps under control will probably prevent another code. Im about to go check timing to see if I can figure out why its heating up.

Recently changed cooling parts:
T-stat, radiator, fluid.

Cooling parts that seem ok:
Fan clutch seems ok as far as I can tell. Some resitance when warm.
Water pump isnt weeping.
No visible water leaks.
Cap is right LBS but I may change it again anyways.

E-fan seems to pop on when it heats up. However it also comes on when the AC is on but wont necassarily stay on. I can only find contradictory comments about when the e-fan is supposed to come on and when its not.
 
Before you waste any more time, check the pressure in your fuel rail.

(A) Should jump to around 40 psi when you turn-on the key (pump only runs for about a second, so several key on/off cycles may be required)

(B) Start the engine. Should be just over 30 psi running. Should jump toward 40 when you goose the throttle - feature of (C) below.

(C) Remove the vacuum line from the fuel pressure regulator near the front of the engine. Should read close to 40 psi.

If (C) does not cause a higher fuel pressure, and (A) is okay, then suspect a plugged fuel filter or a weak pump.

If (A) or (B) are not good, replace the fuel pump.

Starving your engine is a good way to get a random misfire.

Good luck.
 
Crawldaddy, I did check the fuel pressure. Its a few comments back. It was about 40 with key on. With the engine running we were at 48 or 50. As far as I know the fuel pressure regulator is on the fuel pump along with the fuel filter in the gas tank.

Would a dying alternator give up the same symptoms as a dying fuel pump? If the alternator isnt giving up the goods, will the pump still pump just as hard?
 
Dredging up this old thread, but did the OP ever get this issue resolved? I'm just now getting the P0300 code on my '01 XJ and searching thru the posts to get some guidance in what to look for.

Thanks.
 
Dredging up this old thread, but did the OP ever get this issue resolved? I'm just now getting the P0300 code on my '01 XJ and searching thru the posts to get some guidance in what to look for.

Thanks.

I too am getting P0300 coodes, all I did was verify them and clear them so far they have not returned. I am interested in what others have found on this issue.
 
For some extra info and clarification:

Random 300 codes are not uncommon. Unless you have poor engine performance, poor gas mpg's, a flashing CEL, or excessive CEL Code occurrences, they can usually be ignored.

Cleaning your throttle body and idle air controller and occasionally using fuel injector cleaner is part of your routine preventive maintenance.

Fuel injectors can and should be tested for correct ohms impedance (and if needed for flow rate and spray pattern).

2000-01 have cast iron exhaust manifolds that do not crack, and the APN header does not fit them.

Misfires can be be electrical or fuel related, or both. Later models Cherokees run best on Champion Copper or basic NGK sparks plugs. 2000-01 will run well on these also, but seem to run better on AutoLite APP985 plugs. The 00-01 spark plug rail can be tested, and in rare cases may need to be replaced.

The colling system is more than just the radiator, if yours has experienced a failure or has been neglected, rebuild the entire cooling system. A water pump with no pumping fins will not leak, nor will it pump much water. Temperatures should be tested and confirmed with a no-touch IR temperature sensor gun.
 
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