PO300, PO302 and PO303 codes - JeepForum.com
 
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post #1 of 10 Old 10-15-2021, 07:54 PM Thread Starter
xj2000cherokee
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PO300, PO302 and PO303 codes

Hello Everyone. I got misfire codes PO300, 302 and 303 today. 300 says there are multiple misfires whereas 302 and 303 are specific to the cylinders. My question, though, is this: Is the cylinder # that misfires the number of the actual cylinder when counting back from the front of the car, or is it the cylinder number in the firing order of the engine?

Cylinder numbers from front to back: 1-2-3-4-5-6
Firing order: 1-5-3-6-2-4

As you can see, cylinder #3 is the same either way, but what about cylinder #2? Is it the second cylinder back, or is it #2 in the firing order (which would be the 5th cylinder back)?
Thanks so much for any info.

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post #2 of 10 Old 10-16-2021, 01:57 AM
Ralph77
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Not the firing order.
So 2nd and 3rd plug from the front.
Any idea when the plugs were last changed?
So I believe that most times misfires are symptoms of an underlying problem.
But you start with the simplest thing first. Pull the plugs and give them a look.
Couple years ago going to work I thought I could feel an intermittent misfire.
Wasn't sure thinking maybe I was just imagining it.
Same on the way home. Figured get in there and see if anything jumped out at me at some point.
Next morning start the Jeep and it feels like it is running on 5 cylinders and throws a code.
P0304. So again simplest thing first. I pulled the plug. Looked fine.
Swapped in a new one anyway.
Problem solved.
My point is while I feel that misfire codes are symptoms of an underlying problem sometimes a misfire is a just a misfire.
And I am sure I am going to catch some flack for saying this but cheapo Champions. 438's. LOL.
And I have a reason for thinking this.
Jeep was bought as a kinda running not driveable project.
I had read about the misfire and heatsoak issues with the '00's. I didn't seem to have this problem. And not to date either.
When I finally got around to pulling the plugs you should have seen them. I really could not believe that the Jeep ran as good as she did.
The plugs in there were cheapo Champions. So my logic was that is what she had in there and that is what she is getting new ones of.
Oh and you might want to consider the Viper Coil Mod.
I have no problems with the coil bar and the Viper Coil Mod does not improve performance.
What I feel it does do though is make it easier to diagnose some things If you have too.
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post #3 of 10 Old 10-16-2021, 06:51 AM
CJ7-Tim
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Consider the spark plugs or the fuel injectors as the most likely suspects. When my 2000 has minor misfires, some fuel injector cleaner is usually all that is necessary.

Inspect the spark plugs. Look for excessive wear and fouling. Make sure they are fresh spark plugs, either plain old Champion copper, a basic NGK, or that they are or AutoLite APP-985. If you are unsure of their age/mileage, install some fresh plugs.

Make sure the fuel injectors are clean. Use a good quality fuel injector cleaner in the next two gas tank fill ups. I use Techron. Inspect the injectors for cracks, and inspect the injector wires for damaged wire insulation. Very carefully remove the injector connectors and clean them with some plastic safe electrical contact cleaner.

Some people have changed to another style of coil for the 2000-01 Jeep 4.0L, but I see no reason to do, mine runs great same as it was built at the Jeep factory.

ďWe the people are the rightful masters of both Congress and the Courts,
not to overthrow the Constitution but to overthrow the men who would pervert the ConstitutionĒ Abraham Lincoln, 1859.
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post #4 of 10 Old 10-16-2021, 11:30 AM Thread Starter
xj2000cherokee
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Thanks so much for the great info. Hopefully Iíll have time this weekend to start investigating. The misfires started happening after I filled the tank up with gas yesterday. I drove about 50 miles after getting gas. When I restarted the car, the engine was noticeably off. I drove for a little bit and thatís when the check engine light came on. Iím wondering if I possibly filled up with bad gas. Iíll check the plugs soon. Iím also thinking it could potentially be a blown head gasket since it is occurring on two cylinders that are right next to each other. Iíll do a compression test as well.
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post #5 of 10 Old 10-16-2021, 11:39 AM
Ralph77
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Quote:
Originally Posted by xj2000cherokee View Post
Thanks so much for the great info. Hopefully Iíll have time this weekend to start investigating. The misfires started happening after I filled the tank up with gas yesterday. I drove about 50 miles after getting gas. When I restarted the car, the engine was noticeably off. I drove for a little bit and thatís when the check engine light came on. Iím wondering if I possibly filled up with bad gas. Iíll check the plugs soon. Iím also thinking it could potentially be a blown head gasket since it is occurring on two cylinders that are right next to each other. Iíll do a compression test as well.
I didn't want to bum you out so I purposely did not go right to the bad place.
Probably time you educate yourself a bit about the 0331 head in your Jeep. Google it.
I would go with a cracked head over a head gasket.
But I am pretty sure the crack happens between #3 and #4 so there is that.
And it is kind of a coincidence that this did happen after a fill up so....
Just don't get all nervous just yet. LOL.
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post #6 of 10 Old 10-16-2021, 01:03 PM
CJ7-Tim
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Start with the basics, logical and simple. Bad gas, plugs and injectors.

Be aware that a cracked 0331 head or a failed head gasket is possible, but many people report no symptoms in the rare instances the 0331 head does fail. A coolant test kit from the local auto parts store should detect any combustion byproducts if you notice any milky looking oil, unexplained loss of coolant, possible overheating or goopy stuff on the oil fill cap.

ďWe the people are the rightful masters of both Congress and the Courts,
not to overthrow the Constitution but to overthrow the men who would pervert the ConstitutionĒ Abraham Lincoln, 1859.
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post #7 of 10 Old 10-17-2021, 11:54 AM Thread Starter
xj2000cherokee
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Ok Guys. Thanks for the great info. I am aware about the 0331 cylinder head and that was something I was thinking about. I thought Iíd be able to investigate things today, but Iíll have to wait until later in the week. Iíll start by looking at the plugs. While they are out Iíll do a compression test. If I get low compression on those two cylinders, Iíll remove the valve cover (since it would have to come off eventually) and examine the cylinder head between cylinders 3 and 4. If it does turn out to be bad gas, though, what do people do in that case? I still have almost a full tank in there. Is it recommended to drain it all out, or do you put additives in there until it is used up? I have used Techron in the past.
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post #8 of 10 Old 10-17-2021, 11:57 AM
Ralph77
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My first thought would be water in the gas.
I would just dump about 15 ounces of 91% isopropyl alcohol in there and see what happens.
Same stuff that some dry gas companies put in theirs.
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post #9 of 10 Old 10-24-2021, 02:48 PM Thread Starter
xj2000cherokee
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Ok. Got around to checking the Jeep out this weekend. Spark plugs look fine. Compression test was as follows: #1-156 #2-170 #3-160 #4- 160 #5-175 #6-176
So Iím a little puzzled by the compression numbers. The Haynes manual listed the compression for the inline six to be between 120-150 psi (+/- 30 psi difference). My compression numbers were above that. Is that normal for this engine? I do not know too much about the engineís history.
At this point, I am just going to put in new spark plugs since theyíre cheap enough and I already have the old ones out. Iíll either add Techron or 91% isopropyl as suggested and see what happens.
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post #10 of 10 Old 10-24-2021, 05:46 PM
CJ7-Tim
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Factory specs for a compression tests are between 120PSI-150PSI, with a variable difference of 30PSI between cylinders.

Different compression testing tools & different testing processes ( wet or dry ) can yield significantly varying values on the same motor. The best indicator of a healthy motor is consistency.

If all the cylinder values are above 100 psi and similar to each other that's a good indicator of healthy compression.

ďWe the people are the rightful masters of both Congress and the Courts,
not to overthrow the Constitution but to overthrow the men who would pervert the ConstitutionĒ Abraham Lincoln, 1859.
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