My car's MPG

[ajk23az]

First off, I drive a 2005 Nissan 350Z.

I took my car into an auto place a few months ago to get my radiator flushed because I havent had it done since I bought the car with @ 32,000 miles, had it flushed at 50,000.

The day before I had it flushed I changed my oil as well.

Ever since I took it to the auto place I have only been getting about 17.5-18 MPG. Previously I was getting around 21 MPG.

A friend said that it could be the summer heat that was causing this but I've kept track of my MPG for 2 years every time I filled up and in previous summers, its never dipped below 20 MPG.

I've changed my oil once again since then but it had no effects.

Anyone have any ideas what could have dropped my MPG that much?

I mean 3 MPG per tank, have around a 17 gallon tank, I'm losing 51 miles per tank. If gas on average is 3.30 for premium, I'm losing $15 every time I fill up!!

 

[Signals]

Perhaps there is an issue with the gas gauge? (I am assuming this is what your using to track your mileage) or maybe even an problem with the floater in the gas tank.

I would think that there are a variety of mechanical things that are determining factors in your cars mileage and any major repair or change in one component can affect other components performance.

 

[Vtwin]

Have you recently changed to ethanol blended gas from non-ethanol?

 

[casmith07]

Check your tire pressure too. Could also be that they went to a blend oil instead of full synthetic 5w30. I noticed a dip in my car's performance when the dealer put Pennzoil (Pennzoil!) in my Infiniti instead of Mobil 1 5w30, and our cars have the same motor.

 

[ajk23az]

Perhaps there is an issue with the gas gauge? (I am assuming this is what your using to track your mileage) or maybe even an problem with the floater in the gas tank.

I would think that there are a variety of mechanical things that are determining factors in your cars mileage and any major repair or change in one component can affect other components performance.

I don't believe so because I also used to look at how many miles per tank I got and it was right around 300, 320 if I REALLLLY took it to the end of the tank, and now I am only getting about 260ish.

Have you recently changed to ethanol blended gas from non-ethanol?

No, I just use one type of gas. Shell. And it is always premium.

Check your tire pressure too. Could also be that they went to a blend oil instead of full synthetic 5w30. I noticed a dip in my car's performance when the dealer put Pennzoil (Pennzoil!) in my Infiniti instead of Mobil 1 5w30, and our cars have the same motor.

I change my own oil and I use fully synthetic Mobil 1 5w-30 with a K&N oil filter. Tire pressure is a-ok as well.

 

[Yeagermeister]

Do you change the air filter also? If it hasn't been changed in a while it could be clogged up with dirt.

 

[TwoCentPlain]

You drive a Nissan 350Z and you are worried about gas mileage, and particularly 3 lost MPG?

 

[Hostile]

Talked to a mechanic friend for you. He said if you changed your air filter to a K & N air filter (not oil filter) you will increase power and reduce mileage due to the way they are manufactured to restrict air flow. He said if you got a K & N air filter, go to another brand and you should see mileage come back up but power go down a little.

He recommends as well that after a flush and oil change that you tune the car up with new plugs, etc.

 

[Vtwin]

I think your mechanic friend has it backwards Hoss. K&N replacement filters are less restrictive then stock or aftermarket paper filters. Just replacing the filter won't gain you much performance or fuel mileage improvement as the engine can only pull as much air through as it can push out. The K&N filters really shine when you modify the intake and exhaust systems to allow more air/fuel through. I keep diligent records on my truck and the mileage barely moved after putting a K&N air filter in. I've done the intake, filter and exhaust on all my bikes and there you can feel really the difference.


To the OP. You might not have noticed if the gas stations in your area made the switch to 10% ethanol. Around here you could still get non-ethanol gas until just a few months ago. My fuel mileage dropped 2+ mpg when I was forced to run 10% ethanol gas. I wouldn't have noticed the gas station made the switch if I wasn't paying attention. They did not advertise it as they got a lot of business from folks who would go out of there way to avoid the ethanol. They are only required to put a sticker on the pump letting customers know the blend.

 

[Hoofbite]

Any other symptoms?

Rough idle?

Could be a vacuum leak.

 

[CowboyMike]

Might be something as simple as your plugs. After you got it flushed and at 50,000 miles you might consider just getting them replaced. Usually it's done at the 100k tune up but it doesn't hurt to do it at 50, especially since you seem to take great care in upkeeping your vehicle.

 

[67CowboysFan]

I'll toss my two cents in since I specialized in electronic engine controls for a couple of decades before I got sick of working in car dealerships and moved onto something else.

A couple of things that can go wrong that won't turn a check engine light on or hurt performance enough to notice.

K&N filters are oiled I think. If so, they can build up enough of an oil film on the mass airflow sensor to throw the calibration of it off enough to call for a little more fuel. Over time it will build up enough to turn on the light.

I'm not familiar enough with the Z but if it has a single ignition coil instead of multiples, it could have high resistance. 9 ohms across the terminals is the limit. More than that it should be replaced.

I'd double check the air pressure specs on the tire (not the door sticker). I have seen hundreds of fuel mileage problems related to tire under inflation.

Since you have had recent work, open the hood and look for obvious things that might not have been put back together properly. Any air leak in the induction system (hoses) can cause problems. You will have to look close and make sure everything is 100% in place and secure.

Make sure the pcv is full seated where it belongs. Oil dipstick too.

Those are the first things I would check. If that does provide any clues. Find out from the local refineries if the blend of gas has changed recently. Back in 90, my Mustang GT went from 28 to 24 mpg overnight. I finally talked to a guy that work at a refinery and he told me the fuel blend had changed.

And finally, jack up each wheel and make sure it turns easily. A wheel bearing about to fail will cause a drag as will a brake caliper sticking.

 

[joseephuss]

Wash and wax it. All that dirt build up creates more drag.

 

[ajk23az]

Do you change the air filter also? If it hasn't been changed in a while it could be clogged up with dirt.

I'm at 55,000 and haven't changed it yet. Today I took it to a Nissan Dealership and had him check the noc? sensors which were normal, and the air filter which he said looks like brand new yet.

You drive a Nissan 350Z and you are worried about gas mileage, and particularly 3 lost MPG?

Haha touche. But since I'm still in college at the ripe old age of 23, I like to save my money where I can. =) After I graduate with my CIS degree, then hopefully I won't have to worry about a 3 MPG loss. :laugh2:

Talked to a mechanic friend for you. He said if you changed your air filter to a K & N air filter (not oil filter) you will increase power and reduce mileage due to the way they are manufactured to restrict air flow. He said if you got a K & N air filter, go to another brand and you should see mileage come back up but power go down a little.

He recommends as well that after a flush and oil change that you tune the car up with new plugs, etc.

Like I said above, the Nissan dealership said that I wouldn't even have to look at the plugs until about 90k miles. But I think changing them and the air filter would be a start. I can't just sit status quo. Thanks for asking him!

Any other symptoms?

Rough idle?

Could be a vacuum leak.

No other symptoms that I have been able to tell. Everything sounds fine, no pinging, screeching or anything of the sorts. Power is still there as well.

 

[67CowboysFan]

Wash and wax it. All that dirt build up creates more drag.

Not to change the subject but did you see the mythbusters show where the put golf ball like dimples in a car and it got better gas mileage?

 

[joseephuss]

Not to change the subject but did you see the mythbusters show where the put golf ball like dimples in a car and it got better gas mileage?

No, I didn't catch that one, but I believe it. There was a company(don't know if they still exist) that sells full car body form fitting stickers that have micro-dimples in them.