(2005 coupe 6MT) My car has been running P0300 for about a 2 years off and on (currently at 200k miles). For the last 4 months it's stayed on. Within these years, I have replaced the following:

• coil packs (Hitachi) (Feb 2021)
• spark plugs (NGK Double Platinum) (Feb 2021)
• valve covers (tiny bit of oil in spark well of cylinder 6) (OEM) (Feb 2021)
• Both Fuel Sending Units and fuel pump (OEM) (March 2020)
• All four cam sensors (OEM) (Late 2019)
• All four O2 sensors (Denso) (Late 2019)
• Thoroughly cleaned out throttle body along with the upper and lower plenum. (Feb 2021)

Next replacements:

• MAF (Hitachi)... sometime this week

The only known issue that I have is an exhaust leak at the flange where the OEM muffler connects to the OEM downpipe. Could this cause a random misfire? The car is drivable and I cannot tell it's misfiring, it does a slight sputter at idle.

Here's my mod list (none suspension):

• Z1 high flow cats (Early 2020) (These have an integrated non-fouler J-bend bung for the downstream oxygen sensors)
• Z1 plenum spacer (Early 2020)
• Z1 Fuel Gauge Adapter and Gauge (Early 2020)
• DIY catch can (from the forums) (Feb 2021)
• 350z y-pipe (Early 2020)
• Stillen Z-tube (Early 2020)
• K&N OEM Air Filter replacement (Early 2020... Cleaned Feb 2021)

Does anyone have any suggestions of why the p0300 is still haunting me? The front four fuel injectors sound normal... I can't get my stethoscope to cylinder 6 and 5 without removing the plenum.

 

Try the crank angle sensor that's down low on the trans. Use the Hitachi branded one. For the O2 sensors, I've stuck with the OEM NTK branded ones (for upstream at least) since many OG posts indicated issues with other brands.

Check for vacuum and exhaust leaks. For vacuum leaks, you're checking everything after the MAF; boots & couplers are known to get old and tear, same for the hose that goes from the intake to the back of the valve cover. Can get a PVC hose kit from Z1 for that issue. For exhaust leaks, check around the manifold and cats, basically before/near the primary O2 sensors. After the cats/y-pipe, exhaust leaks won't cause the misfire, just be annoying.

I'd also clean the throttle body throughly (remove it from the car to clean it up, don't move butterfly while plugged in) then do an idle relearn after the car is warmed up.

 

That "sputter" you hear at idle is your misfire.

Have you ever recalibrated the throttle closed position and idle air volume algorithms? You probably had the throttle body unplugged at some point yes (gotta recalibrate it every time you unplug) and you added the plenum spacer (gotta recalibrate idle air volume anytime you clean/replace the throttle body or modify the intake plenum volume).

 

I checked the Manual Transmission repair manual and didn't see a "crank angle sensor" anywhere on the transmission. Just the "back-up lamp switch" and the "neutral position switch." EDIT: I see where it is. I'll get one tomorrow.

I just checked... my upstream O2 sensors are NTK and the down streams are Denso... all replaced at the same time. I remember seeing the same thing about NTK.

I checked for vacuum leaks today using the carb cleaner method with no instantaneous change in the idle. I sprayed around where the plenum mates together, the top bolts, intake couplings, throttle body connection to intake, valve cover and all the PCV hoses. There was no change in the idle. I assume there was no vacuum leaks, unless it's at the headers, which I couldn't get to because of the heat shields. I also leak tested the exhaust with the vacuum blower method and no leaks except the one at the flanges where the muffler meets the downpipe. When I installed the new valve covers, spark plugs, and coils a few days ago, I thoroughly cleaned the plenum, throttle body and intake manifold... along with the K&N air filter.

Yes, I did the idle relearn (pedal dance party) and the MAF reset (where you disconnect MAF, run engine for 2-3 seconds, generate CEL, clear CEL, then plug MAF back in and turn car on). I did this today. I just finished a 30 minute drive on the interstate. No CEL, but when I got home I figured I'd run the OBDII scanner and run another diagnostic and a p0300 is pending. I'm sure when I drive to work tomorrow the CEL will illuminate. One thing to also note is that my cruise control will not work. It'll hold the rpms for a few seconds then release... I'm thinking the random misfire is disengaging it. Only other thing I could think is fuel... I check my fuel pressure gauge... in the run position, it's a little above 40 psi, while idling it's at 51-52. The pressure doesn't drop immediately once the vehicle is off. After 2 mins it dropped from 40 psi to 37. At this point I'm thinking it's the cats or maybe a short in a wire. I am completely stumped.

 

So... I was tinkering around to make sure there were no more vacuum leaks. I sprayed carb cleaner on the plenum... and notice a very dull change in the idle, not like the abrupt ones I see on youtube. I start rubbing my hand over the top of the plenum and my the palm of my hand was sucked into the 4th bolt hole. So I loosened it and torqued to 61 in-lbs (Which I may have torqued to factory spec months ago). Put my hand over it again and it there was still suction on my palm. I take a closer look... and... crack in the plenum at the 4th bolt hole. Apparently I may have over torqued a while ago and cracked the upper. I'm off to LKQ to find another upper plenum. I'm guessing that's why the plenum spacers from Motordyne and others come with 6 individual spacers to make up for the space between the upper and lower.



You can clearly see it here.

From the front it was camouflaged.

 

Looks like you possibly found the problem.

Two things of note though, first don't ever spray carb cleaner into a fuel injected engine, you risk damaging the O2 sensors, throttle body cleaner only.

Second, your fuel pressure at idle should ALWAYS be within 1-2psi of 51 psi. If you're dropping down into the 30's something is wrong, usually sediment clogging up the strainer baggy on the end of the pump.

 

Oh wow, a vacuum leak will definitely cause the misfire problem, glad you finally found the issue. Basically running lean causing the misfire. That's kinda lame the Z1 spacer doesn't come with the six spacers for the plenum posts, just asking for something like this to happen. At least they use washers with a seal built in, but torque specs in their installation instructions are a bit low (44-61 inch pounds), it's basically just "snug". Also, a common thing with the spacers is accidentally getting the oil filler rubber "pinched" in between the plenum when putting on the spacer. I recommend removing the rubber part while installing the spacer so there's no chance of that happening.

Factory service manual shows 51 lbs at idle as the spec for fuel pressure. It doesn't list anything beyond that. So fix the vacuum leak issue before spending time/money trying to replace the fuel pump, since it may not be needed.

 

The fuel pump was replaced last year. It should still be working fine, especially since the fuel pressure gauge seems to give in-spec readings. The oil filler catch rubber thingy did try to get pinched, but I caught it before I torqued the plenum. I wonder if buying those plenum post spacers are worth it... wonder where I could get some.

Ok, quick update. I thoroughly cleaned out and installed the new upper plenum from LKQ careful to not over torque the central 6 bolts (somewhere around 50 in-lbs). After doing so, I connected and reinstalled everything back up. Did another MAF and Idle relearn... cleared CEL. Started the car back up and the idle was fluctuating badly between 300 - 600 rpms. Took it for a spin and it almost felt like it was going to stall every time I came to a stop sign or stoplight. While driving I notice the heat was not strong and I remembered I didn't bleed the coolant after removing the coolant lines from the plenum. By the time I got home the p0300 had lit up. I woke up this morning and properly bled the coolant (took a syringe and filled the upper coolant bleeder port until it was full, then used the funnel kit) and now there's scorching heat coming out the vents at max heat settings. Also after bleeding the system, the idle is much more smooth and stable between 750 to 800 rpms (with little to no fluctuations). However... I haven't reset anything since bleeding the coolant. I will do another idle relearn and report back. It'd be weird if air in the coolant system was the cause.

In case this next reset doesn't work.... I need to take my time and take some advice that @cleric670 gave me and others probably a thousand times over the years. Follow the service manual. This Sunday, I will run through the service manual EC-275 through EC-284 (forDTC P0300 - P0306 MULTIPLE CYLINDER MISFIRE, NO. 1 - 6 CYLINDER MIS- FIRE). I will follow it to a "T", and report my findings sometime next week. If the instructions in the "diagnostic procedures" or the "symptom matrix chart" (for "rough idle") doesn't fix it, I'll take it to Nissan/Infiniti for a proper diagnosis.

 

First thing I would do is an idle air recalibration since you no longer have a massive leak in the plenum.

Make sure to follow the steps including having the transmission up to temp.

 

Ok, I'm done messing with it (should have said this 2 years ago). I'm taking it to Infiniti for a proper diagnostic. I'm a little nervous to take it there because of my light mods. Especially since the high flow cats have "no fouler bungs" integrated into them.

Idle Air Calibration: No Go

• I did the idle air calibration about 7-10 times to make sure I did it correctly and each time I got the same results with the little sputter fluctuating idle at around 500 - 700.

Compression Test: Go

• Cylinder 1: 165 psi
• Cylinder 2: 157.5 psi
• Cylinder 3: 165 psi
• Cylinder 4: 165 psi
• Cylinder 5: 157.5 psi
• Cylinder 6: 165 psi

Idle

• In spec at 650 +/- 50 rpm... but still sputters and fluctuates

Ignition timing advance #1 cylinder

• between 14 and 18 degrees. (I believe this is in spec)

Absolute throttle position at idle

• 0.4%

O2 sensors at idle

• B1, Sensor 1: 0.29 V
• B1, Sensor 2: 0.28 V
• B2, Sensor 1: 0.57 V
• B2, Sensor 2: 0.28 V

New Spark plugs Inspected : Go

• No fouling
• No oil in spark plug cylinder
• Gaps at 0.043 inches

New Coil Packs Inspected: Go

• Dry with no signs swelling, oil leaks, or signs of damage

Fuel Injectors and Fuel System: Go...Maybe

• All sound the same
• Continuity between [F33] injector harness and [F108] ECM harness
• While idling fuel pressure is 51 psi... Fuel pressure holds after engine is off.

- Swapped out K&N filter for new cheap filter (To see if my K&N was the problem)
- Installed Z1 PCV hose kit.
- Cleaned out every terminal in the engine bay with terminal cleaner.
- Cleaned MAF with MAF Cleaner

I didn't check the O2 sensors... i'm just tired. I heading to Infiniti now... I will let you know what they say.

 

I just got back from the Infiniti Service shop. The service associate told me they could not find out why my CEL (P0300) kept generating. He went on to say that my non-factory parts would not let them diagnose the issue properly (Z1 Plenum Spacer, Z1 hi-flow cats, Z1 PCV hoses... and the Z1 fuel gauge port). I asked if I could talk to the mechanic that was running the diagnostic, but that person magically disappeared. I sat in their lobby for 2+ hours. They didn't charge me anything (which leads me to believe there was nothing for them to fix) nor did they tell me anything. There is currently no pending p0300 or confirmed p0300. But I'm guessing they did a CEL reset right before handing me the vehicle.

I guess I have to fiddle around with the fuel lines and the double check the Z1 cats and O2 sensors. Could the evap canisters cause this? I've never touched them. Or maybe the ECM?

I'm determined to post a solution!!!!! There are way too many p0300 post without solutions beyond replacing coil packs.

 

I would remove each coil pack and use a digital multimeter to measure resistance across each combination of pins 1-2, 2-3, 1-3 and see if they're all EXACTLY the same resistance reading. You might have one that's just on the edge of failure.

 

I've checked the resistance on the old OEM coil packs (they had different readings but were within spec) and the new Hitachi coil packs before installation (resistance was identical on each one). I have not measured the resistance after the Hitachi's were installed, but will try later this week, along with checking the cats and O2 sensors.

 

The new Hitachi coil packs still have identical resistance (1.57 kΩ). I tried fixing my exhaust leak but made it worse...debating on getting the Tanabe Y-Pipe Back , or sourcing a local shop to repair it (There's ugly black soot around both tips of the muffler). The flange is rusted badly at the muffler to downpipe flange and sounds like someone is sipping soup when the cars decelerating. I found a local motorsports shop that specializes in Nissan/Infiniti cars Soho Motorsports. I'm taking it to them tomorrow morning to do a diagnostic. I'm starting to get the feeling that it's the ECM... in which I'm guessing might be a nightmare to swap for the 2005 6MT Coupe... being that it's a mixture of 03-04 and 06-07.

 

I'm back from Soho motorsports. Really cool and clean place. They say my P0300 is coming from low oil pressure and that my gallery gaskets needs to be replaced.

I should note that a new OEM water pump and OEM thermostat were installed about a year ago. So they should still be fine... coolant system is in great condition. They quoted me $2,907.68 ($746.59in parts and $1,930.40 in labor and additional shop fees/taxes) to do this repair.

• OEM Timing Kit - $495.00
• VQ35DE/VQ37VHR Oil Filter - $13.49
• 5W-30 Full Synthetic Oil - $35.88
• EPS Oil Gallery Gaskets VQ35DE - $75.00 (found for $45.00 here Nisformance)
• Permatex Ultra Black - $35.92 (How much Ultra Black do they intend to use?????)
• 1 Gallon Antifreeze (Green) - $22.98
• OEM Driver Side Valve Cover Gasket - $22.67
• OEM Passenger Side Valve Cover Gasket - $22.67

I would rather buy a lower mileage VQDE rev-up to swap, than pay that much to replace those items.

So now, I'm off to purchase gallery gaskets and a timing chain kit. If you have any suggestions on the best place to buy these items let me know. I'm weighing options between Z1 Motorsports and CZP. Once these items are installed... I will report back again.

The exhaust leak is making the car loud as ****, almost like there's no muffler. Slowly edging towards the Tanabe y-pipe back, but first I want to see if the gallery gasket will solve my P0300.