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Discussion Starter #1
Hello all,

So I have had this misfire for around 3 months, at first when it appeared I went for spark plugs since they're cheap and it's been more than 2 years since I bought the car without spark plugs service. The misfire was gone, until 2-3 weeks later. This time I replaced all coils and that didn't fix the issue, so 2 weeks ago I took it to a mechanic who did compression test and other tests. He said that the compression on the 3rd cylinder is about 136 and 170 on others. He said its likely piston ring or or there is hole down the engine.

Suggesting an engine swap. He isn't sure and I don't want to just listen to him, so I was thinking going to nissan dealership to do diagnostics before swapping engine.
Could it be bad fuel injector?
Any ideas what could cause low compression on the 3rd cylinder?
 

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Altima FTW
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If the mechanic used a mechanical compression guage to do that test you can throw those numbers right out the window. Need to use a digital oscilloscope with a pressure transducer in cylinder to get a much more acurate and reliable reading. I'd expect there to be a P0303 code if it was low compression on 3. Since its a random misfire I would look at things such as MAF sensor or possible intake leak. Something that's going to affect all cylinders and not just one. Was there engine oil down in the spark plug tube when you changed out the plugs? Does it seem to misfire more at idle or while going down the road or both?
 

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Discussion Starter #3
If the mechanic used a mechanical compression guage to do that test you can throw those numbers right out the window. Need to use a digital oscilloscope with a pressure transducer in cylinder to get a much more acurate and reliable reading. I'd expect there to be a P0303 code if it was low compression on 3. Since its a random misfire I would look at things such as MAF sensor or possible intake leak. Something that's going to affect all cylinders and not just one. Was there engine oil down in the spark plug tube when you changed out the plugs? Does it seem to misfire more at idle or while going down the road or both?
Thanks for your input.

Im sure the dude used mechanical compression gauge.

It's not random tho, it's been consistently the 3rd cylinder. There is no vac leak or rpms fluctuating.

You mentioned MAF issue, which when I unplug the idle does get pretty smooth. But isn't there a specific DTC code for that?

I just hope it's not cylinder ring.
 

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Discussion Starter #4
I also get lots of gas smell in the cabin on some start ups. Im wondering if it's faulty injector... I tried swapping that with the 4th injector. After looking at it it looked too much work, so I gave up of swapping. Also wouldn't there be also a specific code for the injector?
 

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Altima FTW
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Don't always get a MAF code for faulty MAF. If you have access to scan data the MAF should have a grams per second reading of near the engine displacement at idle with all electrical loads turned off. So if you have the 2.5L the MAF should be roughly 2.5 g/s, 3.5L would be roughly 3.5 g/s. The excessive gas smell if probably a result of the misfire occurring. If misfires on 3 more at idle it could also be an issue with #3 valves. Also if fuel trims are high at idle and normal off idle this would point towards an intake leak.
 
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Discussion Starter #6
Hey twister,

So I have a reader which gave me aprox. 6g/s. I have a 2.5 engine. Looks like it's not piston ring but air flow issue? For some reason I can't get the screen shot attached to the message.

Someone did a YouTube tutorial on diagnosing the MAF sensor. They said if it turns off the engine then it's good sensor else it's bad sensor. I unplugged mine and it didn't turn off, almost did, also the idle went very smooth after that.
 

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Altima FTW
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Another thing to check would be excessive exhaust backpressure. Pull out upstream O2 and install backpressure gauge. Look for a reading of less than 2.5 psi at 2000 RPMs.
 

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Altima FTW
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6.19 g/s at idle is excessively high. That's with all electrical loads turn off right? Lighting, hvac, radio things like that all off. Also vehicle needs to be at operating temp.
 

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Discussion Starter #10
Another thing to check would be excessive exhaust backpressure. Pull out upstream O2 and install backpressure gauge. Look for a reading of less than 2.5 psi at 2000 RPMs.
All the tools i have are this reader and couple of wrenches. I don't have backpressure gauge. But could a bad catalytic converter cause this reading of 6.19g/s?
As for the second reply, I think the car wasn't at normal temperature and I might have had radio on. But no vents or air was on. I also waited for the rpms to drop before doing the reading. Initially the car idles high rpms for about 5-10 seconds.
 

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Altima FTW
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High backpressure theoretically would cause lower maf reading if severe enough. If you can try and let the car get to operating temp and recheck maf reading. If still high I'd look into MAF.
 
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High backpressure theoretically would cause lower maf reading if severe enough. If you can try and let the car get to operating temp and recheck maf reading. If still high I'd look into MAF.
Ok cool, I'll do the test tomorrow. And let you know. Thanks.
 

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Discussion Starter #13
Hi twister,
I did the test after relearning idle air volume, just to be sure the reading on MAF is correct. And Throttle body valve position because I cleaned up the Throttle body couple days ago.

The reading with all the accessories off, and car at normal condition i still got higher than 2.5g/s for 2.5 altima. The new reading is 4.5-4.6g/s. Which is better than the 6.19g/s but still off the 2.5g/s. Should I be looking into getting the maf replaced?

The MAF could've been me cleaning it with rubbing alcohol? I also had MAF code P0102, but that could have been from me unplugging it while the car was running. So I erased all the codes. The P0303 came back and I can feel the misfire and gas smell but I haven't got the P0102 again, yet?
 

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Altima FTW
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MAF is definitely more than I would like to see at warm idle. I'm thinking you may have two issues, one with the MAF and the P0303. Next easiest thing would be to swap fuel injectors and see if the code follows the injector. If not there is most likely a mechanical issue with #3, ie valves or piston ring. What is long and short term fuel trim readings at warm idle?

If you decide to replace MAF don't go with some cheap brand.
 

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I now you said you replaced plugs and coils,, but it would also would be wise to check for spark on the #3 ignition coil. You will need a spark tester for that though.
 

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Discussion Starter #17
MAF is definitely more than I would like to see at warm idle. I'm thinking you may have two issues, one with the MAF and the P0303. Next easiest thing would be to swap fuel injectors and see if the code follows the injector. If not there is most likely a mechanical issue with #3, ie valves or piston ring. What is long and short term fuel trim readings at warm idle?

If you decide to replace MAF don't go with some cheap brand.
I tried changing or swapping the fuel injectors but they're pain in the you know what... So I gave up. But I'll try again. No, I know the PCM is picky about MAF authenticity, so im gonna pull original one from another altima at a scrap yard.

As for your second reply, I did change the 3rd spark plug even after the change of all of them. Still misfiring. I'll try the injector and see and also get the MAF from a yard. Let's see.
 

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Altima FTW
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Swap injectors and don't forget to check spark on #3. Spark testers aren't too expensive.
 

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From post #5.....

'If you have access to scan data the MAF should have a grams per second reading of near the engine displacement at idle with all electrical loads turned off. So if you have the 2.5L the MAF should be roughly 2.5 g/s, 3.5L would be roughly 3.5 g/s.'

Where did that come from?, never heard that before. Not saying it's wrong at all, just wanting to hear more, it can be a valuable tool if reliable.

I'm afraid I'm one of the compression gauge guys but I do agree as to the unreliability of the readings and have worked out streams of method to make them much more reliable than the normal guy doing it is. When I give a number you can pretty much bank on it. There is science to the method that the modern world just doesn't use, too hard to have to think about such things.
 

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From post #5.....

'If you have access to scan data the MAF should have a grams per second reading of near the engine displacement at idle with all electrical loads turned off. So if you have the 2.5L the MAF should be roughly 2.5 g/s, 3.5L would be roughly 3.5 g/s.'

Where did that come from?, never heard that before. Not saying it's wrong at all, just wanting to hear more, it can be a valuable tool if reliable.

I'm afraid I'm one of the compression gauge guys but I do agree as to the unreliability of the readings and have worked out streams of method to make them much more reliable than the normal guy doing it is. When I give a number you can pretty much bank on it. There is science to the method that the modern world just doesn't use, too hard to have to think about such things.
The output of the FET power transistor must be considered. This component is located in the car's computer and sends the signal to fire the spark. This is what caused misfire in my car. Not easy to test without ocilloscope and low amp clamp.
 
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