I know you guys get a lot of these questions but i think mine varies slightly. Ill get right to it.

Ive got a 2003 with 170k miles with a P0303 pending code (freeze frame data at bottom of post). Under 2k rpm the code goes away. above 2k rpm CEL comes on. about 4k rpm CEL blinks. No other cylinder misfires at all. No general misfire code.

I put in new plugs and put in a new coil because the #3 cylinder had higher resistance than the other coils. No change.

Nissan datascan power balance test showed very little rpm drop when #3 was turned off. I stuck a stethoscope under the intake manifold with the engine on and the fuel injector sounds quieter than the others (yeah a subjective test but im sure its quieter). So i swapped the one injector with a another cylinder and tested everything again. No change.

So then I pulled out the compression tester. I know this tester tends to read lower on the gauge than what actual psi is (I tested this with an air compressor. While the air compressor had 120psi the compression tester only shows 100psi). I warmed the engine up to 188f (coolant) and pulled the plugs to test compression. All cylinders dry test are 132-135psi. Wet test showed all cylinders 142-145psi.

No coolant in oil, no oil in coolant. No smoke from tailpipe. Has test pipes installed so no melted cats anywhere.

So I guess I need help deciding if that 10psi ring blow-by combined with the low overall compression is for sure worn rings, or if my tester is just terrible and i have a short or high resistance to either the fuel injector or the coil. Or if im missing the mark all together and its a valve lash, or variable valve timing issue.

This is my first rodeo with a VQ motor so I came to the experts to help me out. Thanks so much for reading!
(Possibly unrelated but maybe not: intake has a 5/16 spacer and terrible cai installed by PO. I have seen that the throttle or idle air may need to be adjusted with these mods, but I couldn't find a definitive answer on this. I havent been leaning towards this due to the single cylinder misfire. However, if it bears a check let me know.)

Freeze frame data:
Engine load: 23%
Coolant: 150f
Fuel trim B1 short term: 16.4%
Fuel trim B1 long term: 9.38%
Fuel trim B2 short term: 20.31
Fuel trim B2 long term: -7.81
Rpm:787
Mph: 0

 

CEL on and CEL blinking are from different codes, try to find out what the other code is.

 

Why would they be different codes? CEL blinks to stop you from ruining the cats or excessive detonation. It only blinks above a certain threshold.

"One Trip Detection Logic (Three Way Catalyst Damage) On the first trip that a misfire condition occurs that can damage the three way catalyst (TWC) due to overheating, the MIL will blink. When a misfire condition occurs, the ECM monitors the CKP sensor signal every 200 engine revolutions for a change. When the misfire condition decreases to a level that will not damage the TWC, the MIL will turn off. If another misfire condition occurs that can damage the TWC on a second trip, the MIL will blink. When the misfire condition decreases to a level that will not damage the TWC, the MIL will remain on. If another misfire condition occurs that can damage the TWC, the MIL will begin to blink again.
2. Two Trip Detection Logic (Exhaust quality deterioration) For misfire conditions that will not damage the TWC (but will affect vehicle emissions), the MIL will only light when the misfire is detected on a second trip. During this condition, the ECM monitors the CKP sensor signal every 1,000 engine revolutions. A misfire malfunction can be detected on any one cylinder or on multiple cylinders" (prodemand)

 

Because there are a LOT of things can can trip the flashing CEL and it might not actually be the misfire code, and they can self-clear in the time it takes for you to pull the car over and plug in a scanner. I'm not going to just assume that's what is causing the flash.

As for the P0303 I would next test the wiring for the coil harness. Make sure one side has 12v and ohm the other side out to the ECM (needs to be unplugged). Since you have a LOT of miles on the probably stock ignition condenser you should just replace it because it could be just a matter of bad luck that it is not smoothing out the current only on cylinder 3 discharge due to other cylinder problems that might exist.

As for the idle air recalibration for the plenum spacer, if your idle is at 625 and there aren't any other idle issues then it's PROBABLY already been done at some point.

The part that really sticks out to me is that bank1 is +10% fuel but that's also the bank you're having the misfire, typically it's a lot LOWER since it's trying to subtract fuel due to it not burning in the cylinder on a misfire. Then on bank2 you're pulling a lot of fuel out, just seems a little odd. It's making me think there's some kind of fuel delivery/injector issue, possibly with the fuel dampers?

Does it make a weird "thumping" sound inside the cabin at idle, sound more prevalent around the glove box? That would be a sign of the right fuel damper failing which would definitely screw up fuel delivery on the injectors on bank1 because the pulses of fuel from the pump are not smoothing out and therefor you get differing amounts of fuel delivered through the injector even though the pulse width is the same.

 

Oh I see what you meant about the codes. ive driven it with Nissan DataScan on and a passenger looking for codes to come up and nothing besides the p0303 shows up. Granted that software is from like 2005 or something, but its the quickest tool I have for getting codes off this. I have a generic scan tool and a bluetooth one but I dont imagine NDS would be slower at getting codes than them.

I have tested the pigtail of the injector with the ignition on and its getting battery voltage on pin 1. Resistance between pin 2 and 22 on the ecu harness is at 0.2 ohms. (I tested all other injector connectors while I had them apart and everything tested fine.).

I also did between pin 1 and 61 on the coil for #3 to test continuity and it was at 0.2 ohms. Coil pin 3 with ignition on gave me battery voltage. Pin 2 was 0.1 ohms to ground.

I took of the lower intake manifold to look at the intake valves (because I was close and why not) and they were clean and straight at least. while i was down there i tested the knock sensor. I think it was at ~560kohms.

I have not heard any thumping near the glove box. It is currently off so i would imagine i would be able to notice it easily. I look more into the fuel damper issue, and ill get the ignition condenser changed out asap.

Thanks for all your help so far. Your time an knowledge mean a lot!

 

So quick update on the latest. Got a new Nissan ignition condenser and fuel damper from z1 put in. Cleared codes and drove it and same p0303 came back in the same conditions as before so no change there.

Pulled the new freeze frame data:
Engine load: 41.5%
Coolant: 109.4
Fuel trim B1 short term: -1.56%
Fuel trim B1 long term: 9.38%
Fuel trim B2 short term: -5.47%
Fuel trim B2 long term: -5.47%
Rpm:3325
Mph: 29.83

So still high on that banks trim. I tested fuel pressure and at idle its about 47psi. It fluctuates about 1psi while revving up. IIRC it should be "around 51psi". Not sure what the tolerance is on that.

Im running out of things to test but im thinking the next thing is to pull the valve cover off and measure the cam to tappet spacing. Or check for cam lobe wear i guess. Like I said this is my first vq motor so im not sure if the vvt could make this act like it is.

 

So in case anyone stumbles upon this thread in the future I want to show all the rest of the testing I did and how I finally fixed the problem.

I took the valve cover off and measured the cam spacing against the followers with a feeler gauge. All on bank 1 was within spec.

At this point I was really scratching my head. So I pulled the intake manifold off again to test the injector plug again. 12v coming in and 0.1 ohms of resistance to terminal 22 (or 23? cant remember) on the ECU. After asking a buddy to let me borrow his oscilloscope (3 weeks ago, of course he was out of town so i couldnt get to it) I pushed some needles into the back of the injector 3 pigtail and snaked the oscilloscope cables out. I also took the chance to hook up cylinder 1 and 2 soI could see an injector firing on both banks. After putting it all back together and firing it up I could finally see the dead line of refusal to fire. The ECU just wouldnt ground the injector. (yellow is cylinder 1, blue 2, pink 3)

So I hopped on ebay and found a used ecu for 150$ and got it shipped here. I used the Nissan datascan software and this website: http://keytechtools.com/bcmcodes/index.php to get my key pin code from the BCM. Ran the key reprogramming tool and viola, works great now. I did a power balance test and sure enough that cylinder 3 is helping the cause now.

As far as i know ECU single transistor failure is incredibly rare, but now i can say ive seen it happen myself. If you are running into a similar issue with your car do all the necessary steps to rule out anything else before jumping to a failed ECU. But, like in my case, it is possible.

 

Glad you got it all figured out.