2019 QX50 CVT: Sudden drop in MPG + change in engine noise -- no codes

Question

I'm seeing a sudden, significant (>40%) drop in MPG from my 2019 QX50 (FWD, CVT). I do a fairly regular long drive from SF to Sacramento, and highway MPG for these trips has always been ~31-33MPG; city MPG is more variable, but usually in the low 20s. Now, for the same SF-Sacramento drive, I'm only getting ~22-24MPG; and city MPG is in the 10s.

Events that might have caused it

This all seemed to happen after either/both of these events during our last road trip (they happened on the same day so it may be either, or maybe neither and just coincidental):

1. We were cruising on Neutral for about a mile at ~70MPH (which the manual said we're not supposed to do)
2. We were driving on a fairly bumpy, unpaved road for a few miles.

Other Symptoms

In addition to the drop in MPG, I also noticed these symptoms:

• The engine sound is noticeably louder when the vehicle first starts, and when the vehicle is in reverse. It's not abnormally loud, but definitely louder than normal, and the pitch is low.
• At over 80MPH, the engine is slightly louder than normal--almost like it's running on a lower gear than it should. However, I can put it in the highest (simulated) 8th gear (transmission is CVT but has paddle shifters) and behavior is the same.
• When driving at normal speeds, engine sound is mostly normal.
• There was a slight burnt smell from outside the vehicle (can't smell it inside) for a couple of days after the events above, but the smell is now gone.

Dealer says nothing is wrong

A few days after the events above, I took the car to a remote dealer (while on our road trip). The dealer said they ran diagnostics and did a test drive, but found nothing wrong. Computer returned no codes. He said the weird behavior may be due to a change in elevation, and everything should be back to normal when I get home.

I was hoping that may be the case, but of course now I've been home for a few weeks, and I'm still seeing the same low MPG, and louder-than-usual engine noise.

Questions

1. Can the drop in MPG be related to the change in engine noise?
2. Can I have damaged the transmission while cruising on neutral, causing the symptoms I described?
3. Can something have popped loose during the bumpy ride, causing the symptoms I described?
4. Because the increase in engine noise is rather subtle, it's easy for them to overlook, but I'm pretty sure it's real. Any suggestions on what I can look for myself? And if I take it to my local dealer, what specific things can I ask them to check this time?

More info

• I cross-checked my MPG numbers with both the onboard computer and my own calculations at the pump, so I'm very confident in them.
• The car is ~2.5 years old with 18000 miles.
• I checked the engine air filter, and it looks OK. Also checked tire pressures and they're in range.

Answer 1

Whether or not the cvt was damaged when coasting in neutral against owner's manual recommendations specifically advising against it is unknown. I have a theory why it's not recommended but do not have enough info or familiarity with CVTs to make an informed opinion. Refer to your owner's manual for new vehicle warranty and if it's still warrantied then schedule a diagnostic service with your Infiniti dealer. Continuously variable xmissions have their own unique quirks and problems. A turbocharged 4 cyl engine can have its own issues. A feature of CVTs is the lack of gear shifts. The tachometer shows increasing rpm while the speedometer increases. Conventional automatics upshift into the next gear with rpm dropping on each shift before rpm increases. You may be hearing higher rpm sounds that may suggest cvt problems.