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My uncle gifted me with a 2000 Frontier and learned that it didn’t pass smog back in January and has the “Service Engine Soon” (SES) light on.

The code being thrown out on ODBII is P0304 (misfire on cylinder 4). So I began to troubleshoot...

1. swapped the spark plug wire from cylinder 4 with cylinder 2 and reset the SES; SES came back on — so the spark plug wire is okay

2. performed a compression test (dry/wet) on each valve and pressure amounts for each value was consistent

3. dumped seafoam in the gas tank to hopefully clean the fuel injectors — SES still came on

4. spark plugs and distributor cap were replaced within the past year and a half; with closer inspection, they two parts look good but will be replace all spark plugs and distributor cap to rule out those out

5. next steps is to see if the SES comes up after replacing the distrib cap and spark plugs; if it does, i plan to replace the fuel injector in cylinder 4

6. if the problem is not the fuel
injector, i’ll replace the fuel injector harness; it may be a bad electrical connector

If anyone has any feedback, suggestions or recommendations on how I’m trying to tackle the misfire on cylinder 4, I’d appreciate it!






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Is it chugging, as in an actual misfire?
Or just a bad code coming up, maybe from a suspect crank position sensor?
If it is chugging, you can move the injector to another cylinder, see if the code moves with it.
 

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Is it chugging, as in an actual misfire?
Or just a bad code coming up, maybe from a suspect crank position sensor?
If it is chugging, you can move the injector to another cylinder, see if the code moves with it.
It’s definitely not chugging. During idle, I can hear and feel a slight sputter (not sure if that makes sense) but there is no noticeable drop in RPMs. And everything runs smooth when I’m driving.

I’ll look into the crank position sensor and swap the injector to see if the code follows.

Thanks for the suggestions!


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I'm nor sure about the 4-cyl. that year, but on the V6 that year, the crank position sensor is only a monitoring sensor and can't cause a misfire or drivability issue. If you look up the crank sensor info in the "EC" section of the factory service manual, it will tell you. Your diagnostic plan sounds pretty good. I would spray some carb cleaner around the area of where the #4 intake runner meets the head to check for a possible intake gasket leak. I would also recommend sticking to a genuine Nissan distributor cap and rotor and NGK spark plugs. You can use a "noid light" to check if the ECM is "firing" the #4 fuel injector. They may have a noid light kit at your local auto parts store loaner program, if they have one.
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This may be a long shot but you may also want to check your MAF sensor signal. I’m under the impression that these sensors sometimes go crazy on these trucks.

I initially had the same symptoms that you have but from a deteriorating MAF on my 98 4WD. The MAF was putting out the correct average signal but the signal was very noisy. All I was getting initially was some rough idle with sputtering noise at the exhaust and occasional misfire code on cylinder 4 initially, then after a month I started getting misfires on other cylinders. I spent a very long time figuring out what it was. If you have some sort of OBD2 reader you may want to check your MAF signal and see if it is steady under steady engine conditions. Signal should be steady. If it is jumping all over the place then you may have a bad MAF sensor.

In my case with the bad MAF sensor combustion was all over the place rich and lean but the truck drove more or less normally. I did not take the truck for smog check under these conditions but I doubt it would have passed.
 

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Discussion Starter #6
Thanks for the help!!! I’m stilling trouble shooting.


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You didn't get any water into the engine compartment, right? I made a mistake (twice) and had to wash off the engine with a hose, got some water into one of the spark plug wells past the spark wire's seals (1998, 4-cylinder), got misfire code. Had to use compressed air to dry that out.
 

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You didn't get any water into the engine compartment, right? I made a mistake (twice) and had to wash off the engine with a hose, got some water into one of the spark plug wells past the spark wire's seals (1998, 4-cylinder), got misfire code. Had to use compressed air to dry that out.
No water in the engine.


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UPDATE: My ‘00 Fronty passed smog!

I replaced the fuel injector on cylinder 4. And that did the trick.

One thing that came up was that the O2 sensor was not ready even after several attempts of completing the drive cycle. I replace the two upstream sensors and viola... O2 sensor was marked on my OBD II.

Thanks for all the replies and help, everyone!


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