Misfire prolem

hey everybody before you pounce on me, I did A LOT of research for what to do and I still can't solve my problem.

I own a 2001 jeep cherokee I6 and since i have owned it I have had a misfire on cyl 3. I did what most people do and replaced the plugs (i did it with bosch +4s) cleared the code and well within 100 miles check engine light on again. Hooked up the good ole' scan tool and received:
P0300 (random misfire)
P0303(cylinder 3 misfire)
P0304(cylinder 4 misfire)

from that I thought I would check the coil pack because cylinders 3-4 are on the same coil I hooked up my ohm meter and it read .77 ohms and from alldata that's in specs

I looked up TSBs for this problem and there was one. First said to wrap the 3rd injector with heat wrap because of heat soak. Does this actually help or is that a waste of my time?

Second TSB said for me to clean my exhaust valves. Now if it were my exhaust valves wouldn't it be all of them that would be the problem not just cylinders 3 and 4?

I would GREATLY appreciate any help to fix this annoying problem.

Thanks,
Outlander

Side note: my RPM's fluctuate at idle it minimal but the guage isn't steady could the misfire cause this to happen?

You might try a 20 mile run up the interstate at a higher than normal RPM. A lot of short trips can allow carbon build up on the exhaust valves, they don't rotate well at lower RPM's. If you typically drive a lot of short trips a bonzai run up the interstate in third gear may help clean things out some.
I know it works wonders for the wife's XJ when it gets a misfire.

I have a 2000 and I have found that I can mitigate it quite a bit by simply giving my jeep a few extra minutes to cool down before starting it up after i've been hammering on it.

You could have a nasty case of heat soak. It is the most common on the 00-01. The TSB from Jeep (wrapping the injector) doesn't help much for most people. Options for you include an e-fan timer that keeps the e-fan on for a predetermined time after you turn off the key and also cutting louvers into the hood. Both of these things reduce engine bay temperatures, which is the root cause of "heat soak".

Try a set of NGK spark plugs. There have been more than a few reports of mysterious misfires with Bosch spark plugs on the 4.0. These misfires cleared up when switching spark plug brands. The failure mechanism isn't understood, and to be fair it doesn't happen for all owners, but it does for enough so that I won't install Bosch plugs, nor will I recommend them (and I am a BIG Bosch fan for just about everything else......) I like Champion copper plugs for 84-99 4.0, but the NGK is the best fit for the 00-01 ignition system in my opinion.

If this were my XJ, I'd start with a new seat of NGK plugs, then see what you are left with and consider that e-fan timer (SEARCH; there is a great thread on this on this forum).

If after all of the above and you still have the misfires, check your fuel pressure with a gauge. You should have 49 psi, plus or minus 5 psi. The P0300 (random misfire) can have a common cause like low fuel pressure.

Good luck.

thanks for the helps guys. i have noticed something today that I'm very surprised I havent noticed before. My e-fan doesn't work. At all. So this could be a big problem.

birchlakeXJ what type of NGKs do you recommend? From all the reading I have done I have knocked it down to Bosch +2's, champion truck plugs, or autolite platnuim series. But have heard little for NGK. So when it comes to plugs I have no idea anymore what to use.

Skip the Bosch and get the NGKs for the 00-01. No reports of problems with the NGKs, but can't say that about Bosch.

I don't think they make the Champion truck plug any longer. The regular Champion copper plug will work okay, but the NGK seems to hold up better with your type of ignition over the life of the plug. (no longer than 30k)

And definitely resolve that e-fan problem! That is likely a root cause here.

so NGK platinum's will work? does the e-fan run all the time? or does it have a temp sensor switch that activates it?

did you try changing the wires? if they are bad you will definitely get a misfire if they are arcing out somewhere.

its a coil pack. but yea im going to be replacing the coil pack anyway because I got an awesome deal on a new one plus everyone I talked to said its my coil so why not.

birchlakeXJ the fan has been fixed so I'm going to wire it so the fan is run off of an on/off switch kind of like what you suggested. Thanks again.

Just got a thought, would heat wrapping a good portion of the fuel rail be bad?

Outlander560 said:

so NGK platinum's will work?

Click to expand...

Skip the platinum. Go with the basic NGK V-Power (copper) for any 00-01. Use the SEARCH function and you'll find plenty of others that go this route for their 00-01. Chrysler actually switched from Champion to NGK for their 4.0 OEM plug; can't recall what year....may have been 2002?

Copper is actually a better conductor than platinum; a true platinum plug can give you extended time between changes, but that's the only advantage for the 4.0 engine.

I go with copper plugs, and change them every 30k. Money well spent.

Your E fan won't work in the 2001 unless the HVAC coolant pressure switch is triggered AND/OR the jeep reaches 220 There were some changes in the 2000/01 year model e-fan controls.

Ditch the platinum +4s, I've had misfires with those. NGK is better on the 2000/01 ignition system.

thanks for the help guys. ordering a new coil and I'm going to pick up the NGK v-power plugs. thanks for the help.

goodburton thanks for the info.

Outlander

Replace the Bosch crap plug with some NGK V-powers and use some sort of insulator on the injectors.

Talyn: if i was to buy heat wrap and wrap the fuel rail and injectors would that cause and problems?

also i did a fuel pressure test on my fuel rail I was at 49 psi. BUT when I shut the vehicle off and 20 minutes passed the pressure only dropped 11 psi. is the valve in the pump the problem? should the pump be replaced?

Mate, you say you reasearched but put Bosch plugs in?
NGK's and make sure you gap and torque them correctly.
Your issues will dissapear

LOL no i researched after I put the Bosch plugs in. before that I was just doing what my brother did to his 96 xj. I'm going to get the NGK's.

BUT I was misfiring before I even put the bosch plugs in so I think it's something a little more in depth then just plugs

Thanks for takin the heat...now that begs the question, what plugs were in it before?
I had Bosch Platinums in my '93 and they were the best plugs ever...just not in an '00-'01
Good Luck

I would not wrap the fuel rail if you get a fuel leak you may not notice it till its 2 late. If you do have some carbon issues try useing bg intake cleaner it works well.

sbxj: from what I remember when I took the old plugs out, they were champions. Most likely the ones from the factory they were old and had a decent amount of rust on them.

quadlessone: good idea i'll just do what the TSB tells me buy a lil bit of heat wrap and give it a shot.

When I get the $$ and the time I will buy those hood vents and install the h.o cooling fan I have laying around hope fully all this will help me out with this dang problem

After I installed the hood vents on my 2001 I never had the misfire problem again.