Starting a new post to capture the year-long process of tracking down the intermittent engine stumble. I've been unable to clean, repair, maintain or troubleshoot the stumble. Check out the process in the Seafoam Stumble thread.

Update 9-14-2017: Car is now showing signs of more frequent bad idle stumble and poor acceleration stumble. It doesn't matter if its in gear or not. It doesn't matter what the position of the throttle is or not. It still stumbles. One outing I had the window open and a very loud and distinct backfire, spitting, kicking, no acceleration just violent protest from the motor. A brief CEl came on and went away. Had to pull over, shut down, restart, back to only minor misfire. Put on the analyzer and no stored code So, hopefully it will completely fail soon. I'm suspecting a coil pack.

Update 9-27-17: I placed the OBD analyzer on the port. The LTFT was showing -10 on both banks so it appeared to be running rich. I also started monitoring the ECU voltage. The ECU voltage appeared to be very low; around 13.2V using live data the modulation was around 500mV. When starting the car the ECU voltage was at 11.7 and dropped to 9.2 and then up to 13.5 for about five seconds. Then the modulation kicked in and the ECU voltage dropped to 13.2 with the modulation of around 500mV. I drove with the analyzer on the port. After about five minutes the ECU voltage dropped to 12.2 (note: this should have been the indication of the bad ground that was found later )and the car started stumbling. It jumped to 12.8 and bounced in between the two with the modulation. I placed the analyzer on a friends G37 - his ECU voltage was solidly between 14.0 and 14.4V. I'm going to replace the alternator.

Replaced the alternator. What a PITA. Removed the radiator fans and then the alternator out through the bottom. Battery charge voltage back up to 13.5V. However.... The ECU voltage is still reading between 12.2 and 12.8V. The stumble is very consistent. No CEL or pending codes. (Bad ground won't set a code) Disconnected the battery so it may take a while to relearn the settings. It doesn't appear to have been the alternator but I will see if the battery is just low from always having a low charging source.

OMG: Battery at 14.0V. The battery appears to turn the car over well now. BUT.... the engine started running rough, coughing, spitting, no acceleration, and then died. When I started it back up it is clattering. It sounds really loud. This is not good. Will check the oil pressure at idle. Will try to isolate the clatter but is sounds more like valve train than bottom end. Will check the compression and the plugs for indicators. sigh.

Well... the motor had a catastrophic failure !! It sounds like a rod bearing. I am replacing the motor with a salvage version with 40K miles. How this went from a stumble to a catastrophic failure is beyond my understanding. I'm going to assume that the mechanical failure was not associated with the bad ECU reference voltage. I have a picture of the really loose secondary timing chain that was loose due to the failure of the secondary tensioner on the left cam sprocket.

Update: 10-27 This motor is a bear to replace. I left the tranny in. The top two bell housing bolts are very tough. I used a combination of breaker bar, ratchet and/or combination wrench with a box end wrench leveraged onto the open end. PITA. Then I broke an exhaust bolt at the CAT. Two more steps backward - took out the back CAT bolts then removed the CAT. Heated out the studs and drilled out the broken bolt. PITA. Moving forward now to attach the accessories and see how the newer motor runs. Hope the stumble is gone with the old motor. Stand by....

Update 2-18-18: New/used motor is running great after 5K miles. Changed oil and it looks good. Still haven't fixed the CAT and the CEL. Will do that soon since I wanted to make sure the motor was going to work. I still haven't torn down the old failed motor but I do have a video of the bad secondary right side timing chain tensioner. I'm convinced that the engine start valve train clatter should have been a warning of a failing tensioner. I also replaced the left front half shaft since I messed up the outer boot. I can't figure out a vibration that happens at 55mph. I suspect the front half shaft since it is a remanufactured unit. Don't know how to diagnose the vibration.

Update 3-20-18: Motor working great. Starter spins great. Just got the new CATs in the mail. Installed the aftermarket CATs in about two hours. The top flange bolts for the manifolds were tough to get at. Several extensions needed and one adapter to actually point the socket back toward me and onto the bolt. CATs working great and no codes anymore. 10K miles on the replacement motor.

No more signs of timing chain noise or engine stumble from the bad ground. All voltages normal at 14.4V. I think most of the issues have been tackled. :0

Still have to figure out the vibration at 45-55mph.

Uploaded timing chain video to Youtube:

 

Engine is back up and running. It has been a long process working on the floor without a car lift. Two codes came up when it started. I overlooked two connectors - the bank one O2 and the crankshaft position sensor. The CPS was a real bear to get at. I ended up jacking the back of the tranny to create a small space near the firewall. It was a totally blind connection. Interesting that the car turned over for five seconds and then started. Even with the CPS not connected the long crank time eventually started the engine. The car starts and runs so I now need to reinstall the bumper and headlamps and charge the A/C.

I couldn't resist tearing the timing chain inspection covers off the old engine. The main chain and tensioner was good. The right VVT chain and tensioner was good. The tensioner resisted compression and returned to tight. The left tensioner was really loose. The chain was loose. I suspect the left VVT tensioner failed and jumped a tooth. The knocking I heard was probably a bent valve. There was only 85K miles on the motor.

 

Wow man what bad luck. Glad all is well now.

 

Update 11-15-17: Took the G37 into the dealership. Shout out to Lupient Infiniti service department in Mpls. They found the likely cause of the stumble problem that has been becoming more frequent over the last year. A ground wire that appeared to be fine was corroded and intermittent. They reflashed the ECM updates and dug into the problem resolving it fairly quickly. They added some service items in addition to make sure that the new motor had a good home in the G37X. I'm expecting the new motor and car to last. The starter now spins the motor over really fast, the car starts instantly and the ECU voltage is between 14.2 to 14.5V. Apparently, the cats have also failed and need to be replaced. That could be from the bad sensor readings due to bad ECU reference voltages.

Hope this trail helps someone get to a faster, less painful resolution in the future. I also hope that a TSB would get generated to cover the apparent frequency of problems that this root cause creates.

Don't tackle this motor replacement without the help a a great brother right there to help. Thanks dude !!!!!

FWIW....

 

This sounds identical to the nightmare I experienced last week re: the catastrophic failure while driving. I thought mine might have been an alternator or a/c compressor lock up bc there was a strong odor of hot/singed rubber. I had it towed to Infinti, they loosened the belt and the pulleys spun freely. Got the car started w/ jump pack and after 5 mins of driving it, mech said he thinks car has major lower and upper end engine damage. It's a 2009 G37xS and motor had 130k on it--way too early. I've only owned the car 6 mos and was meticulous about maintenance. I guess the PO used and abused it. Oh and this was about a week after I dropped $770 on a water pump replacement...terrible luck with this car. I put 190k on a G35 w/o it breaking a sweat, damn shame.

Their recommendation was to replace the motor, $11k new and $7k used(including labor). I've been shopping some indy's and they're a few thousand lower. I don't have the time to pull off something like an engine replacement even with help of friends. So, I am now faced w/ a decision to sell as-is and try for as much $ as possible or dump $5k into a car that's probably worth $8k. I guess a third option would be to part it out but that would be such a pita. Only had my 19" coupe wheels and Injen CAI on for a few months before this happened and didn't even get a chance to put on my K-sport coilovers.

 

OUCH LPg37x !! I rebuilt seven 01-04 Chrysler Sebring 2.7L cars that were scrap due to the motor sludging failures. My kids and friends needed reliable college cars and that on fit the bill after a new motor but it cost someone else a lot of money. I used a newer '06-'10 long block to avoid the engineering disaster of the bad timing tensioner, head drain ports, and water pump failures in '01-'04. I never thought I would be right back in the same place of needing to replace a failed motor in an Infiniti car I thought to be bullet proof. The timing chain VVT engineering wonder makes me wonder what the designers are thinking :O I would encourage you to work with an independent shop and find a used motor with much lower mileage than what you have. If the shop can take the motor out the bottom of the car using a lift then job is actually manageable. I was able to do the job out the front by removing the front end but it kicked my butt. The hard part is that there will likely be no guarantee. It's a real crap shoot and the outcome is very hard to predict. A mechanic may be interested in buying the car too. They could likely fix it for the price of a salvage motor of about $1K - and about 20 hours minimum labor.

I would be really curious to see if your ECU voltage reading from the OBD CAN bus is between 14.2 and 14.4.

So why are these engines failing catastrophically???? I take care of my cars !! It's not the owners. Just like the Sebring 2.7L there must be something in the design that is making them fail prematurely. I got this 2009 G37X because my friends 2005 G35X went 250K miles. I would start to be very suspicious of this drivetrain if there becomes a positive correlation to a bad grounding system, ECU voltage and eventual mechanical failure of the VVT. When the timing chain gets loose and you can hear it at start up then it's time to pay attention.

 

Yea, that is what makes me hesitate to get it fixed--no guaranteed outcome. It's either pay for labor, source the motor yourself or pay their mark-up on engine to get warranty on labor. If I source engine and something is wrong with it, I pay for labor 3x.

The Infiniti dealer didn't even scan it; said it was so bad it would've been like giving an x-ray to a dead person lol. I am most likely going to part the wheels, CAI and coilovers then throw the rest of car as is on CL to salvage any little bit of money I can.
This is her...she's a beaut w/ no prom dress:

 

PM Sent...

 

Thanks for the empathy. At this point and as curious as I am, it is not worth finding the exact cause bc it will either cost me more money or time.

 

Posted the video of the timing chain failure....