At my wits end with P0303 cylinder 3 misfire code

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Manny

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I am not mechanically inclined so I rely on my mechanic quite a bit.
I bought a 2002 Jeep TJ Sport 2 years ago for a summer car as ive always wanted one and figured since my boys were grown, mom deserved it. It has been nothing but a headache and a money pit.
The CEL comes on all the time.
The code is P0303. So I know its the cylinder 3 misfire (googled that)
We changed the spark plugs because they were long overdue. improved the idle and ride but the code still comes up.
Now I know we havent investigated much but I really dont want to keep throwing money at this thing. Ive replaced everything to the tune of 6000 dollars (Canadian but still!_)

I googled what else it could be and seems like alot of expensive maybes.
So here it is.
It sputters a little when idling. It doesn't ride rough
I held a clean paper towel at the end of the exhaust and it had some slightly tinged black liquid.

So looking for some best guesses
 
What kind of spark plugs did you use and what did you gap them to?

Did you test the coil packs?

Have you ruled out a bad head gasket or cracked cylinder head?
 
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All great questions! I will ask my mechanic when he calls me back.
What I did notice, and I dont know if this is related but the hood and the engine are really hot. I drove it, only about 15km total about an hour ago and its still really hot. It is 28 degrees but it really seems excessive
 
Manny said:
All great questions! I will ask my mechanic when he calls me back.
What I did notice, and I dont know if this is related but the hood and the engine are really hot. I drove it, only about 15km total about an hour ago and its still really hot. It is 28 degrees but it really seems excessive
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That’s typical for a TJ. Vehicles that get poor fuel economy in general (such as TJs) tend to put out large amounts of heat. All that energy has to go somewhere.

If the temperature gauge is showing signs of overheating, or steam is escaping, that could be an issue. But heat is normal.
 
Another question, on the top of my air intake box, there is pooled oily water in the creases around the edges?

jeep.jpeg
 
Steel City 06 said:
That’s typical for a TJ. Vehicles that get poor fuel economy in general (such as TJs) tend to put out large amounts of heat. All that energy has to go somewhere.

If the temperature gauge is showing signs of overheating, or steam is escaping, that could be an issue. But heat is normal.
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Thanks I just had the rad and thermostat replaced yesterday because the rad was cracked and the heat gauge was climbing
 
Just went through the P0303 code on a buddy’s LJ. It would sometimes start bad, ran rough then smoothed out and finally persisted to run poorly. We thought injector, heat soak, connector etc. but his turned out to be a chafed wire:
I
1590695952759.jpeg

Something else to take a look for.
 
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A #3 cylinder misfire is commonly caused by heat soak at the #3 fuel injector which causes the fuel inside it to vaporize. The factory issued a Technical Service Bulletin to fix it which is nothing more than wrapping the #3 injector with insulation.

This is the insulation specified by Jeep for that TSB... https://www.amazon.com/gp/product/B007O3QHDK/?tag=wranglerorg-20

My '04 TJ had the same #3 cylinder misfire and after installing the insulation 7-8 years ago the problem went away.

This is what it looks like installed...

Fuel Injecdtor Insulation .jpg
 
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Jerry Bransford said:
A #3 cylinder misfire is commonly caused by heat soak at the #3 fuel injector which causes the fuel inside it to vaporize. The factory issued a Technical Service Bulletin to fix it which is nothing more than wrapping the #3 injector with insulation.

This is the insulation specified by Jeep for that TSB... https://www.amazon.com/gp/product/B007O3QHDK/?tag=wranglerorg-20

My '04 TJ had the same #3 cylinder misfire and after installing the insulation 7-8 years ago the problem went away.

This is what it looks like installed...

View attachment 165220
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🤔 I’m guessing nobody read post number 5.
 
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I've been throwing parts at my 2005 4.0 for a few years now. Today I replaced all 6 injectors and plugs (though I knew it didn't really need it). No luck. I've replaced damn near every sensor (all quality parts - no cheap China crap), the coil, the PCM, drove with the hood off as an experiment, added the fuel rail and injector heat shields last year and the jackets today. I've checked all the wiring for chafe, compression check is good. Replaced the radiator and water pump two years ago. Flushed it last week. I bought it new in '05 had zero trouble until 2015. It's driving my crazy.
 
What turned out to be the cause of my random misfires was a bad pre-catalytjc converter. Mrblaine replaced all of my cats and reinstalled the existing O2 sensors and no more misfires at all.
 
Jerry Bransford said:
What turned out to be the cause of my random misfires was a bad pre-catalytjc converter. Mrblaine replaced all of my cats and reinstalled the existing O2 sensors and no more misfires at all.
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I was thinking that may be one of my next moves. Since you mentioned it - I just ordered 4 new O2 sensors. I'll try them first. Thanks.
 
Jerry Bransford said:
A #3 cylinder misfire is commonly caused by heat soak at the #3 fuel injector which causes the fuel inside it to vaporize. The factory issued a Technical Service Bulletin to fix it which is nothing more than wrapping the #3 injector with insulation.

This is the insulation specified by Jeep for that TSB... https://www.amazon.com/gp/product/B007O3QHDK/?tag=wranglerorg-20

My '04 TJ had the same #3 cylinder misfire and after installing the insulation 7-8 years ago the problem went away.

This is what it looks like installed...

View attachment 165220
Click to expand...
Jerry, I did order this from the link above but not sure what I received the correct part. It is too small to go around the injector. I had enough legth to cut in half and wrap it over the exposed area and tie wrap in place. Is this normal? It looks like that's what you did in the picture.
 
Just to relate my misfire woes to the group. I'll preface by saying I had a Cylinder 1 misfire, not a Cylinder three. I went through the normal trouble shooting. Replaced plugs, swapped injectors, replaced plugs again (after a check and found them fouled). Nothing seems to get rid of it permanently. Other than throwing the code, the engine ran fine. Good power, never had any bucking or heard any pinging (like pre-ignition)


I also had a bad fuel pump...in that the check valve was bad and it would leak down over time. You all know the symptoms of that...gotta cycle the key a few times to pressurize the system. I lived with that for a while, until it was too much. I finally replaced the pump with a new Bosch unit. My misfire has not come back in over two years! I don't know why the two things were related...but they certainly were. In my mind, I should have had random misfires, if the fuel pump was causing a problem. Anyway, YMMV, but it just might be something else to check, ESPECIALLY if you need to crank the engine a lot before it starts.
 
Just replaced complete exhaust system and O2 sensors. Still with the misfire. Can't find any chaffed wires. Replaced the plugs and coil pack. Checked compression (good). Replaced all injectors and installed the heat soak insulators. Replaced the exhaust manifold heat shield. I guess I'll pull the head next.
 
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