Weird problem with my 3.5 coupe

[CoupeVQ35CVT]

So yesterday I swapped my car with my brother's, so he could get my car's inspection and oil change done. It just rolled over to 40,0xx miles last evening.

Today I got the car back from my brother and drove it to work. I noticed unlike before, the "average mpg" display read 23.7 mpg, when it almost never changes from 23.9 mpg. Yes that seems trivial but for 40000 miles I know how the car behaves, and to get that average to drop that much, I'd have to drive the car around hard or slow around the city and really drop the mpg. My brother doesn't drive my car crazy, he knows better, and he's taken it before many times, for an oil change etc., without that drastic of an MPG change.

The next thing then was the "Range," it showed 100 less miles than it should have (153 miles to empty), given the level of gas in the car (I just refilled the night before and so the car was driven 150~ miles before today). I easily get 400 miles out of it, the #s were adding up to less than 320 miles total out of the tank (way less than anytime before).

Then the thing that got me to notice this being a problem was on the highway. Cruise ON, at 68 mph on a level road, the live MPG gauge usually shows (whatever its accuracy is) I get over 27-30 mpg. On this same particular road, today the mpg gauge bar was showing less than ~25, maybe a hair over 21-23 mpg. Only other accessory on was rear defrost. I KNOW from prior experience it never hits that hard, even with the A/C on. But I turned it off (the air was already off). The engine RPM needle, and engine sound as well, would change, up and down just a little, at a steady rate. Then at one point going up a hill, the RPM bogged down a bit unusually. At another point on another stretch of level road, I hit CRUISE ON and noticed almost perfectly every 3 seconds, the MPG bar would move up to 30-35+ for a second then level off at ~25-27. The engine sound was a bit less noticeable but the RPM needle would twitch a bit. I turned CRUISE OFF and held the gas pedal steady, noticed the same kind of thing, engine RPM and MPG bar changing just more than usual, on a level road. Never happens that exact way before.

This is very odd because I have never had the car do this before, ever. Even with A/C and defrost on, in winter etc., I've not noticed such a trend in the car. The engine rpm thing is what bothers me. I don't know if the CVT is screwing around its ratios and causing the RPM to change that way, or what.

Going to take it to the dealership tomorrow if this continues on my drive home today. Other than the above symptoms, the car is fine, drives with power and all that, can't notice any unusual sounds, and the tire pressure is fine.

[Hussain]

the only thing I could think of is a bumpy road or something to cause the MPG to spike up and then drop down.... I'm averaging about 17-18 MPG with it being 100% city driving. on the hwy I can pull off 27-32 MPG.... But I never go on the hwy for more then 2-3 miles...

[CoupeVQ35CVT]

No the thing was just off today. I've driven on these roads for 2 years, and with the car over a year and few months. It just was not acting normal. I know road unevenness can and does cause the MPG bar to change all the time, but this was almost clockwork every 3 seconds. That and the engine RPM thing was very unusual. Never had it do that, as subtle as it was.

[CJizzle]

Change the grade of gas?

Since I drive the exact same car as you lol. (with the difference in color)

I noticed that whenever I pump 93 or 89, the range tells differently.

For example, after a full pump of 89, my car will tell me 359 miles to drive. Where as after a full pump of 93, my car tells me 420 miles to drive. I think it's just the car trying to estimate how many miles the gas can support it although I can usually drive more than 400 miles on 89.

I'm not sure about your other problems though...

I would get it checked out if I were you... but damn 40k miles...

Last time I saw you, you still had like 30k-ish lol...

[zilla19]

The range and MPG are low could be due to long idle time during inspection. Basically the car calculates MPG based on the miles driven divided by drive-time. So if you car is idle for a long time the MPG will decrease, and thus would affect the range to empty estimate.

[QR25DE]

Or your o2 sensor is slowly going...

[Hussain]

Or your o2 sensor is slowly going...

hmmm has anyone else had problems with their o2 sensor? my car started getting much worse gas mileage from last year... when my car was new, If I used cruise control and reset the average MPG thing, it would say 32-38 MPG. now it says 26-28. plus my city MPG sucks really bad. I get 17 MPG without going crazy on the gas.

[QR25DE]

I replaced mine... however, I kinda cooked it with a blow torch LOL (it wouldnt come out, stfu) lol

[CoupeVQ35CVT]

Hey thanks for the suggestions. Yeah I drove it back home yesterday, couldn't tell the symptoms due to constant unsteady speeds in rush hour traffic. Today going to work I had the chance to observe the details again. It seems like it is working OK now.

As for the O2 sensor I'll keep that in mind. I know Nissan is known for their great O2 sensors. Won't be the first time I've experienced it ('01 Quest...).

And yeah at the car meet I had ~38900 or 39000 miles. Two nights ago I hit 40,000. It really sucks, I need another cheap beater to drive to work.