Help! P0300 misfire giving me headache!

[JoshTSquared]

As the title suggests... 

 

I have a rev up 350z with 133k miles on it which had a misfire when we got it. The check engine light flashes from start up and sometimes disappears under load but the car sounds like it's missing a cylinder.  It sounds really rough, although it doesn't stall or splutter. A weird point to note is unplugging coil makes NO difference to the sound or speed of the idle. None. No matter which cylinder. Which seems mental to me. And I've checked that the wires for cylinder 4 and 6 are the correct way around.

 

Cylinder 3 had a particularly black spark plug but changing all coil packs, all spark plugs, and the injector on that cylinder didn't make it go away. We've replaced the PCV valve, tried a working MAF sensor, removed and inspected the flywheel (incase it was a timing thing), changed the fuel pump, none of this worked. 

 

We compression tested it dry and all cylinders were consistent (at the lower end psi-wise but within the limits according to this forum), and the only code it throws is p0300, nothing else.

 

I'm really struggling to understand what's going on here and really need some suggestions for what to try next 

 

[ZMANALEX]

Stop guessing as guesswork can be very expensive.

 

Get a proper professional diagnostic check carried out preferably on Consult or similar to pinpoint the issue.

[JoshTSquared]

Anyone you recommend?

[ZMANALEX]

31 minutes ago, JoshTSquared said:

Anyone you recommend?

Where are you based and how far are you prepared to travel?

[JoshTSquared]

North Manchester. Can do anywhere in the North West

[Kieran O'Quick]

I had that with a collapsed cat. Might not be but worth a check

 

 

[ZMANALEX]

On 09/11/2019 at 21:33, JoshTSquared said:

North Manchester. Can do anywhere in the North West

I have had no personal dealings, but these guys may be able to help.

 

http://www.tdi-north.com/

 

These guys are also well thought of:  https://www.lockwoodandgreenwood.co.uk/service/

[JoshTSquared]

I can send them a message. Does any of what I've described sound like it points to anything in particular though while I wait for a reply?

[ZMANALEX]

My thoughts would only be informed guesswork.

 

if I had the car physical I would plug into diagnostics and run a few checks, which is the same as what TDI North would do.

 

Do not expect TDI North to give you any pointers as to what the fault may be as that is how they make there money, in diagnostics and repairs.

 

TDI North will want you to book the car in with them for a chargeable diagnostic test.

[JoshTSquared]

Bumping this thread because I managed to get my hands on a proper Snap on OBD reader and got these readings while the misfire was occurring. Can anyone help interpret this or does anyone know if this points to anything in particular? One of the O2 sensors on bank 2 is jumping between rich and lean very quickly

[JoshTSquared]

[JoshTSquared]

[fijifixit]

Have you actually scanned the fault code to see what it gives you? If so, share what it is please.

[JoshTSquared]

On 09/11/2019 at 21:56, Kieran O'Quick said:

I had that with a collapsed cat. Might not be but worth a check

 

 

I could try swapping the cats if we can get the nuts off

[JoshTSquared]

2 hours ago, fijifixit said:

Have you actually scanned the fault code to see what it gives you? If so, share what it is please.

P0300 random misfire is the only code it's generating. Which is why it's been so hard to track down

[AW11mike]

I had that code a few months ago. It was a coil pack. Mine was number one.

[JoshTSquared]

20 hours ago, AW11mike said:

I had that code a few months ago. It was a coil pack. Mine was number one.

Changed all the coilpacks and spark plugs, sadly made no difference. Also done MAF, fuel pump, crank position sensor, PCV valve, vac leak test, compression test....