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Cylinder misfire code driving me crazy

gm558

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My '96 Sport 4.0 has 235,000 miles on it. I get a repeated p0303 (cyl 3 misfire) every few days. It has been well taken care of and uses no oil between changes. It has new Autolite Double Platinum plugs, NAPA wires & coil, new injector on #3 cylinder, new cats, O2 sensors, EGR, DPFE, vacum lines, etc. It will run great for a few days with a new plug in #3 and then begin to misfire. When removed the plug porcelin is brownish-yellow but not fouled. There is some oil residue on the threads of the plug. , I do not have a leaking valve cover. A compression test gave 150 psi in that cylinder. Anyone have experience with this?
 



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robertoa1a

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It sounds like you pretty much know your onions here. If your looking for an odd answer then I have heard of rocker arms comming loose. On such high mileage it's not unusual for valve lash to need adjusting on a 4.0.
 






Tbars4

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...The first thing I would do would be to swap one of the other wires and plugs with #3 and see if it still happens....

...Why did you swap the injector on #3 only and what other codes did you get before that swap???...Have you also checked for stored codes???
 






robertoa1a

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I agree with Tbar4. You should double check the obvious before spending money and time.

A bad wire from the factory or burned on the exhaust? The posibility that you made an error in connecting the wires. Theese things happen with the pros on Discovery channel all the time.
 






gm558

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Thanks for the input guys. I did swap the 2 adjacent wires and still got the P0303 code. There are no pending codes in memory and no other codes stored. A dirty injector was my thought process in replacing the #3 only. I cleaned and replaced the o rings in the other 5 while I had it apart. I am leaning toward a mechanical issue such as a valve guide or valve seal. I am hoping someone has had a similar issue.
 






Heavymentill

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Most engines detect misfire by seeing very small variations in crankshaft RPM. Rough roads and loose serpentine belts can cause a false misfire.
Have you checked the coolant system with a pressure tester? You could have a small leak or worse case cracked head.
 






robertoa1a

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The former posters issues are interesting posiblities. I remember one time a guy had a loose rocker. Fuel injectors can go bad too. If you checked the spark and that is okay then check the compression compared to the other cylinders. I know that's not easy but if its good then that leaves fuel/air delivery.
 






gm558

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The cooling system is in excellent condition: new radiator and hoses, new heater core and serp belt. No crank or cam sensor codes have ever come up. There is no tapping noise to indicate a loose rocker. The misfire will stop temporarily with a new spark plug. The higher the quality of plug the longer it lasts. i.e. an Autolite double platinum will last much longer than a standard plug.
 






Heavymentill

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The cooling system is in excellent condition: new radiator and hoses, new heater core and serp belt. No crank or cam sensor codes have ever come up. There is no tapping noise to indicate a loose rocker. The misfire will stop temporarily with a new spark plug. The higher the quality of plug the longer it lasts. i.e. an Autolite double platinum will last much longer than a standard plug.

First off the cooling system condition isn't the question its the head gasket and head its self. Second the crank/cam sensors are doing their job if its detecting a misfire. I was making you aware of possible false misfires. The serpentine belt tensioner would be at fault for a loose belt if the belt is new. I would ditch the fancy expensive plugs and switch to plain old copper plugs if it keeps fouling them. At least till you figure out your situation.
 






gm558

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The engine has passed a pressure test. Cheaper copper core plugs will not last more than a few hundred miles whereas the Platinum go longer. I should emphasize this is not a false misfire and I am only changing out the #3 plug to keep it running good. You can watch (and feel) the misfire on a scanner an it's always #3.
 






Heavymentill

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When you say pressure test, I assume you are referring to a compression test which is a very vague test only capable of determining compression ring, valves and head gasket condition. If you have a cracked head the coolant system pressure tester and a leak down tester will help determine if this is the case. Its probably not air/fuel related as you would be getting a lean/rich condition code. If it is valve guide or seal related your plug will be fouled on one side with carbon. The carbon will be on the side of the plug that was facing the intake valve. Also you would probably be burning oil.

Also have you tried copper plugs? I have had the same copper plugs in my vehicle for almost 3 years. Plus they are less than $3 for 2.
 






gm558

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The pressure test I refer to is one using a hand held pump and a radiator cap adapter. The system will hold 16 lbs. of pressure indefinitely. I have also done a block test using a special dye that changes color when exhaust gases are detected in the coolant. Results were negative. I appreciate everyones input.
 






Heavymentill

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You did a combustion leak test. Do you have access to a non-contact thermometer? If so see if there is a variation in cylinder temprature at each port in the exhaust manifold.
 






robertoa1a

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I think its cool to have 235,000 miles on an EX. Cylender presure is harder to check than it changing the plugs on those. Can it be done though? It tells a big story about the condition of the engine.
 






gm558

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I do. Good idea. I will try to check in the next day or two.
 






salo

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i had a similar issue with my ranger, the 4th sylinder kept missfiring and it turned out to be a dirty maf sensor
 






gm558

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Update. After a long delay I decided to pull the intake and check for a bad gasket. None found. So, since I went this far, I pulled the heads. No bad head gasket but sending the heads out to a machine shop revealed (via Magnaflux) a crack between the valves in #3 cylinder. The crack was repaired, new valve seats, guides, and some machine work and I'll be on the road again soon!
 






robertoa1a

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Carbon build up can cause a cylinder to diesel. I guess It will all be taken care of with the head work.
 






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Thanks Tbars I solved the issue, previous owner changed all of the spark plugs, but #6 cylinder. All new autolite double platinums on all six amd she pures like a kitten.

Thank you for all of you follow up.
 



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Tbars4

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...Be sure to post that info in your thread...;)
 






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