[Bdr8629]

I'm at my wit's end with this car. I have had it in and out of the dealership several times, but they have so far been unable to recreate my issue. I feel like I'm going insane.

The car is a 2018 model that I bought in 2018 with 10k miles on it. It currently has 73k miles and has had all of the regular recommended maintenance. No other mechanical issues up to this point.

The problem is that sometimes when I go to start it (via push button/keyless), it runs for a few seconds like it's not firing all cylinders (engine stumbling, car feels like it's rocking side to side, no dash lights except for sometimes the battery light flashes). The car will usually start after 5 seconds or so, but sometimes it just dies again. When it won't start by itself, giving it a little gas gets it going. Once the car is running, everything appears to function completely normally. This only happens after it has been sitting for at least a few hours. I have never had it reoccur after starting successfully. The problem first started about 8 months ago but it was very infrequent to begin with. At this point it's happening several times per week (yet the dealership can't get it to happen even after keeping it a full week).

• The battery is new and the alternator is good
• the oil has been changed several times since this began, no effect
• the dealership suggested that it could be vapor lock due to being parked on the driveway at an incline, which I thought was certainly possible, but it has happened recently on level ground, too, so I don't think it's that.
• the check engine light came in at one point for a few days, but then went off again by itself. The dealership said that the code should still be present, but when they checked they said there were no error codes stored at all, which is odd. Wondering if there's an issue with the ecu??

Any suggestions for where to look next would be much appreciated. The dealership has said they're willing to keep trying, but they're out of ideas at this point.



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[VStar650CL]

There are basically only three things on a VQ35DD which can cause that sort of behavior without throwing any codes. Fuel won't generally do it, a failure in the low pressure pump almost always causes codes in the high pressure side of the system. That leaves:
MAF (Mass Airflow): The DD still doesn't have a MAP Sensor to cross-check the MAF, so when the MAF lies, bad stuff happens. However, intermittent MAF misbehavior is pretty rare.
ECM Relay: This can cause the ECM to go into a cycle of sleep and wakeup if the coil gets weak and the alternator voltage is stumbling (as it would anytime the RPM's stumble). Because the ECM is going in and out on a cycle less than the 2 seconds required for U1000 codes, no codes appear. However, that's also pretty rare and expensive to fix, the IPDM needs to be replaced.
Crank and Cam sensors: Exhaust cam sensors rarely cause starting issues, but crank and intake cam sensors cause them all the time. Nissan ECM's are only good at identifying "flatlined" sensors which are completely inoperative. They're lousy at detecting sensors which are working badly, basically lying about the cam or crank position. Since the crank sensor governs spark timing and the cam sensors govern injection timing, a lying sensor almost always causes a no-start, stumbling start, or an engine that stalls without warning. Those guys would be my prime suspects.

 

[Bdr8629]

Great info, thank you!

Is there a practical way of testing the crank and cam sensors, or is best to just replace them and see if the problem goes away?

 

[VStar650CL]

Is there a practical way of testing the crank and cam sensors, or is best to just replace them and see if the problem goes away?

For the most part, no. Detecting a flatlined sensor is simple but detecting a weak one usually takes an oscilloscope. I'd try replacing the cam sensors first one at a time (because they're easy), then the crank sensor if that doesn't fix it. There will two cam sensors in each valve cover, the inboard ones are intakes, outboards are exhausts. The intakes are the ones you're worried about.