2015 Infiniti Q70 Misfires on All Cylinders - Causes & Fixes

Working off what you gave me, the leading suspect is a global issue (fuel, air, power, or timing), not eight separate bad parts. On a 2015 Q70, a fuel delivery problem or intake/air metering issue is more likely than eight coils or eight plugs all failing at once. Second place is low fuel pressure from a weak pump or clogged filter/strainer; fuel pressure data will separate that from an air/MAF problem.

Most likely culprits

• Low fuel pressure / fuel delivery issue – Weak pump, clogged strainer, or bad fuel pressure regulator can lean out all cylinders and set misfires across the board.
• Air metering / major unmetered air issue – Bad MAF sensor, big intake leak after the MAF, or stuck-open PCV can throw fuel trims off and cause all-cylinder misfires.
• Power/ground or timing issue – Poor engine ground, low system voltage, or jumped timing chain can cause misfires on all cylinders, especially if it runs rough all the time.

How to narrow it down

1. Scan it properly (not just “no codes”)

   • Use a decent scan tool on the engine module.
   • Check for:
     • P0300 (random misfire) plus any P0301–P0308
     • Fuel trim codes, MAF codes, cam/crank correlation codes
   • If you can, look at:
     • Short-term and long-term fuel trims at idle and 2500 rpm
     • Misfire counters per cylinder
   • What it means:
     • High positive trims (+15% or more) = likely lean (fuel or air leak).
     • High negative trims = likely rich (leaking injector, bad MAF, etc.).

2. Basic under-hood checks

   • With engine idling, listen and feel for:
     • Loud hissing around intake manifold, throttle body, brake booster hose, PCV hoses.
   • Spray brake cleaner or carb cleaner lightly around suspected leak areas (intake gasket area, big vacuum hoses).
   • What it means:
     • Idle speed or sound changes when you spray a spot = vacuum leak there.

3. Fuel pressure test

   • On the 3.7 VQ, you’ll need a fuel pressure gauge on the rail (or read fuel pressure PID if your scan tool supports it).
   • Check pressure at:
     • Key-on engine off
     • Idle
     • Snap throttle / under load if possible
   • Compare to spec for your exact engine (3.7 vs 5.6 – tell me which you have and I’ll tighten this up).
   • What it means:
     • Low or dropping pressure under load = weak pump / clogged filter / regulator issue.
     • Normal pressure but misfires = look more at air/ignition/timing.

4. Quick ignition check (don’t shotgun coils yet)

   • Pull a couple of coil connectors while it’s running.
   • If killing a cylinder doesn’t change the idle much, that cylinder is already weak.
   • Pull a plug or two and inspect:
     • Wet with fuel = not firing (spark or compression issue).
     • White and clean = lean.
   • If you have a spark tester, verify strong blue spark on at least a couple of cylinders.
   • What it means:
     • Weak/no spark on multiple cylinders = power/ground to coils or ECM issue, not just bad coils.

5. Battery/charging and grounds

   • Check battery voltage engine off (~12.6V) and running (~13.5–14.5V).
   • Inspect and tug on engine grounds (block to body, battery to body). Clean and tighten if corroded.
   • What it means:
     • Low voltage or bad grounds can make all coils and injectors act up and cause global misfires.

6. MAF sensor sanity check

   • With scan tool, look at MAF g/s at hot idle and 2500 rpm.
   • Unplug the MAF and see if it runs any better on default values (it will set a code, that’s fine for testing).
   • What it means:
     • If it runs noticeably smoother with MAF unplugged, the MAF is suspect or there’s an air leak skewing it.

7. Mechanical timing/compression (if it’s really bad)

   • If it cranks uneven, has no power, or backfires, do a compression test or at least a relative compression test with a scope or scan tool.
   • If you have cam/crank correlation codes or obvious timing noise, you may be looking at a timing chain issue.
   • What it means:
     • Low compression across the board or cam/crank out of sync = mechanical/timing problem, not just fuel/air.

Possible parts (if needed)

• Fuel pump / fuel pump module – If fuel pressure is low or drops under load.
• Fuel filter / in-tank strainer – If pressure is borderline and pump is noisy or old.
• MAF sensor – If trims are way off and it runs better with MAF unplugged.
• Ignition coil set – Only if testing shows weak spark on multiple cylinders and power/grounds check out.
• Intake manifold gasket / vacuum hoses – If you find a confirmed vacuum leak with spray test.

A few questions

1. Engine and mileage? (3.7 V6 or 5.6 V8, and how many miles?)
2. Any codes besides misfire codes? (MAF, O2, fuel trim, cam/crank, etc.)
3. When does it misfire most? (cold start, hot idle, only under load, all the time?)

Give me those details and any scan data you can pull (fuel trims, fuel pressure, misfire counters), and I can narrow this down a lot tighter.

You’ve already done more than most shops. With what you gave me, this is engine drivability and the pattern screams bank‑specific lean plus a local issue on cyl 1.

Key points from your info:

• New plugs and coils
• MAF looks right
• Bank 1 LTFT ~ +25% (I’m assuming you meant +25%, not 125% – if it really is +125% we’re in “massive lean” territory)
• Compression good
• Battery good
• Oil buildup on cyl 1 plug

So we’ve got:

• Global stuff (MAF, fuel pressure, compression, power) basically OK
• Bank 1 only being corrected lean
• Cyl 1 showing oil contamination

Most likely culprits:

1. Vacuum / unmetered air leak on bank 1 (intake gasket, PCV/hoses, brake booster feed, etc.)
2. Fuel delivery issue on bank 1 / cyl 1 (weak/plugged injector, wiring)
3. Oil in plug tube from valve cover leak causing intermittent misfire on cyl 1

O2/AFR sensor on bank 1 could be skewing trims, but I’d prove the basics first.

1. Confirm it’s truly bank‑specific lean

On a scan tool, engine hot:

• Look at:

  • STFT B1 / LTFT B1
  • STFT B2 / LTFT B2

• Tell me:

  • What are B1 and B2 trims at:

    • Idle
    • 2500 rpm in park

What it means:

• B1 trims high, B2 normal at idle, both closer at 2500
  → Classic vacuum leak on bank 1.
• B1 trims high at idle AND 2500, B2 normal
  → More likely fuel delivery or O2 sensor issue on bank 1.

2. Hunt a bank 1 air leak (cheap and quick)

Engine idling:

• Spray brake cleaner/carb cleaner or use propane around:

  • Bank

Next step: find the bank 1 air leak.

With it idling, use brake cleaner/propane and carefully spray around bank 1 intake manifold gasket, PCV/valve cover hoses, and any vacuum lines on that side while watching fuel trims and RPM on the scan tool.

If trims drop or RPM changes when you hit a spot, you’ve found your unmetered air leak. Fix that before chasing anything else.

Check fuel delivery on bank 1, starting with cyl 1.

Next steps, in order:

1. Swap injectors 1 and 2

   • Clear codes and fuel trims.
   • Drive or run it and see if the misfire and plug/oil/carbon fouling move to cyl 2.
   • If the problem follows the injector → bad injector.

2. If it doesn’t move:

   • Do an injector balance test or at least a noid light / scope check on cyl 1 injector connector to confirm it’s being pulsed correctly.
   • If pulse is good and balance is off on bank 1 → likely restricted injectors on that bank or a bank‑1 O2/AFR sensor skewing trims.