Fuel economy goes from crappy to intolerable... ideas?

"Chairman Kaga" (mike-mckinnon) 05/05/2017 at 10:19 • Filed to: None

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I have a 2007 Infiniti M35 with 97k miles. My average fuel economy according to the car’s computer and Gas Cubby is 17.6 MPG. That’s not great, but whatever. Since January it has begun to decrease with every fill-up (approximately every 8-10 days), to the point that for the last two it was 15.8 and today 15.4 respectively. The graph is steady decrease over those 13 weeks. The one anomaly was a trip to Dallas. Due to Texas highway speeds paired with car’s 5-speed and short gearing though, I only managed 18.8 on the trip.

There’s nothing obviously wrong. The tires are new and stay at 35 psi. I changed the oil with Mobil1 5W20 in March (no effect). Air filter is new-ish (clean, at least). Plugs are OK. Engine comes up to temp so it’s not a stuck thermostat.

It’s almost time for some major service so maybe it’ll work out then. Coolant, trans fluid, brake fluid, plugs, probably O2 sensors. I also don’t have the tools to do a compression test, although it doesn’t seem to be burning oil. I might have that done at a shop.

But has anyone experience something similar, or have any ideas? It’s ridiculous that I’m filling up the care (with premium no less) every 250-260 miles.

RamblinRover Luxury-Yacht > Chairman Kaga 05/05/2017 at 10:23

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Most of the things I can think of (things that would cause it to dick with your timing and fuel ratio) are things that would also put on the check engine light, so I’m at a loss.

Jcarr > Chairman Kaga 05/05/2017 at 10:23

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Dirty/faulty injector(s)? I know nothing about the VQ, just a thought.

camarov6rs > Chairman Kaga 05/05/2017 at 10:24

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Switching to summer vs winter gas?....I got nothing

Jcarr > camarov6rs 05/05/2017 at 10:25

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That should improve fuel economy, though.

Tripper > Chairman Kaga 05/05/2017 at 10:29

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Plugs? A coworker had a similar symptom on his 4 runner(slowly decreasing to levels of what the fuck? MPG). He had one bad injector and had never replaced the plugs (120k miles).

Lumpy44, Proprietor Of Fine Gif > Chairman Kaga 05/05/2017 at 10:31

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Plugs would be my guess or a faulty sensor. Both, you would think, would give a CEL but not always.

Rico > Chairman Kaga 05/05/2017 at 10:33

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When did you last change the plugs? Have you been filling up at the same gas station or different ones?

Chairman Kaga > Tripper 05/05/2017 at 10:34

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I have plugs on my replacement list for 100k, but I might go ahead and drop in some new NGKs, and go for the hotter ones too. I’m also going to pull the O2 sensors. I think there are only 2 in this car, one in each exhaust manifold.

Chairman Kaga > Lumpy44, Proprietor Of Fine Gif 05/05/2017 at 10:36

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A dirty O2 sensor could cause it to not run stoich , and one way or the other fuel economy would suffer. But as long as they’re producing signal , I don’t know if it would register as a fault.Worth pulling them. Ugh. Crappy job on this car. Almost easier to drop the manifolds than try to remove them in situ!

Chairman Kaga > Jcarr 05/05/2017 at 10:36

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Or bad fuel filter. Worth a check, too.

Chairman Kaga > Rico 05/05/2017 at 10:40

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I checked the plugs when I got the car about a year ago. They were toasty looking (in the correct way) and not sooty, gapped correctly, so I dropped em back in. It’s such a PITA job that I honestly haven’t done it again recently. To be honest I really just noticed the drop in fuel economy a couple of weeks ago and was hoping it was just a thing that would self-correct. Now I’m going to have to do work, dammit.

And I usually stop at the same Shell on the way to work. Same pump, even. I’ll switch it up next time. Could be watered down gas or an unusually high concentration of ethanol. This close to the coast we usually don’t have to deal with ethanol-related problems as the gas is usually mostly petroleum based.

TheRealBicycleBuck > Chairman Kaga 05/05/2017 at 10:43

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Perhaps they have switched to the summer formula? I see a 1-2 mpg drop when the local stations make the switch.

camarov6rs > Jcarr 05/05/2017 at 10:44

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I only said it because it was improbable lol

Ash78, voting early and often > Chairman Kaga 05/05/2017 at 10:44

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Yep, this is my #1 guess. If you want to tackle the last low-hanging fruit item, get some CRC MAF cleaner and spray the MAF and intake air temp sensor, if so equipped. My car is a horrible job to do O2 sensors (you basically have to drop the exhaust and/or use a 3' extension from above), but the wife’s Odyssey makes them easier than changing the oil. Funny how different cars are. But yes, I’ve seen horrible mpgs even with an O2 sensor operating “within spec” and no CEL.

Lumpy44, Proprietor Of Fine Gif > Chairman Kaga 05/05/2017 at 10:46

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Ya, sucky either way!

TheRealBicycleBuck > Chairman Kaga 05/05/2017 at 10:51

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I’d look at the gas first. Summer formulations can change things.

Also check the thermostat and perhaps even try a higher-octane fuel. The engine in my truck is prone to pinging with the standard thermostat and regular gas. This can be corrected with higher octane fuel or by putting in a cooler thermostat. The cooler thermostat works because the computer adds more fuel to try to raise the temperature, eliminating the lean condition, stopping the pings.

If your thermostat is bad and the engine isn’t getting up to temperature, the computer may be using more fuel in an attempt to get the temperature up. It may also be using more fuel to eliminate pinging if it’s equipped with anti-knock sensors. If higher octane fuel increases the fuel mileage, that could be the source of the problem.

Rico > Chairman Kaga 05/05/2017 at 10:52

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I think you should replace the plugs anyway. I think you should switch up the gas station for the next couple of fill ups too.

Chairman Kaga > TheRealBicycleBuck 05/05/2017 at 10:54

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Isn’t it winter formula that usually drops fuel economy though?

TheRealBicycleBuck > Chairman Kaga 05/05/2017 at 10:57

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Maybe you’re right. I could be remembering it backwards.

shop-teacher > Chairman Kaga 05/05/2017 at 11:14

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Have you noticed if your RPMs while cruising have increased? I had a car lose a bunch of MPGs when the torque converter stopped locking up at cruising speeds.

Chairman Kaga > shop-teacher 05/05/2017 at 11:18

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Not really, but then again this car is geared so short with only a 5-speed that I tend to turn 2500-3000 RPM at 70 MPH. I wish I’d have gotten a 2009 with the 7-speed (and an extra 30+ HP).

diplodicus > Chairman Kaga 05/05/2017 at 11:22

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I think it is more to do with colder air being denser, therefore requiring more fuel to run correctly. I always see an increase in mpg when it warms up. Nothing drastic just a 1-2.

ToyotaFamily > Chairman Kaga 05/05/2017 at 11:58

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I had a G35, same engine and trans and experienced the same issue. I don’t know if it’s a quick bandaid fix or a permanent fix, but I did a reset for something related to the trans and engine and it fixed my MPG for a while.

Chairman Kaga > ToyotaFamily 05/05/2017 at 12:28

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Ugggggh. I have a WiFi engine scanner and I don’t see any ability to do anything but clear CELs. Don’t think I can manually manipulate settings. or was it an actual physical reset, like a switch or sensor?

I bought this car as a temporary daily driver until we were in a better position to get a new car for me, like maybe next Christmas. The fuel economy is WAY worse than I was promised though. The PO was averaging 21 MPG, which was only a couple MPG less than my CRV. Of course, premium gas... But I’m now spending almost $200 a month on fuel for this thing, and that number seems to be going higher every week!

brianbrannon > Chairman Kaga 05/05/2017 at 12:31

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A compression test will tell you nothing regarding burning oil nor will a leak down test. The tool for burning oil diagnosis is the dip stick.

Are you tracking by dividing the miles traveled on the odometer by the amount of gas added to a previously full tank? The mileage computers aren’t very accurate.

The temp gauge is also inaccurate. Read off the coolant temp with a scan tool to verify the thermostat is working properly

Chairman Kaga > brianbrannon 05/05/2017 at 12:39

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I log every fill-up in Gas Cubby, which weirdly matches the car’s computer to a tenth.

I replaced the EGR valve a while back. It’s a common issue with the VQ engine. Gets clogged too easily, results in burning oil vapor. MAYBE that sooted up the downstream O2 sensors?

ToyotaFamily > Chairman Kaga 05/05/2017 at 12:49

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It’s funny because I bought mine as a temporary DD too, and the gas mileage just tanked suddenly. And check the infiniti forums, it’s a process you’ve got to do by turning the key thing along with a couple other things.

Chairman Kaga > ToyotaFamily 05/05/2017 at 13:14

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Like this?

http://forums.nicoclub.com/post6657921.html

Justino6969 > Chairman Kaga 05/05/2017 at 14:24

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Plugs/wires, upstream O2 sensor, and MAF all come to mind. I don’t think it would hurt to run some injector cleaner and seafoam through it, either.

ToyotaFamily > Chairman Kaga 05/05/2017 at 14:35

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Yup, that’s it. I did it and it went up to about 18-19 mpg.

brianbrannon > Chairman Kaga 05/06/2017 at 00:53

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A clogged egr shouldn’t be able to cause oil burning. Nor contaminate o2 sensors. Way more likely to be coolant temperature or mass air flow meter issue