G35 Coupe MPG Going Down.....WHY?

[enflixer]

I keep track of my MPG and track it with the computer in my 2004 G35 Coupe. Over the past few months I have really noticed a dip in my MPG. I was getting at least 300 miles out of one gas tank fill up and sometimes higher, no matter how I drove it and with what ever gas I put in it. Now I am lucky if I am getting 275 a tank full. I have tried different things like using different grades of fuel, you know the 87, 89, 93. There is no difference that is noticeable. I have added stuff like the upper cylinder lubracant to the gas, I think its called tune-up in a bottle and was recommended by a mecanic at the dealership. It seemed to work for a while but it's not helping now. What I am thinking is my car either needs service or maybe the quality of gas has gone to sh$t with the addition of the ethanol. Anyone have any guesses?? This problem is getting expensive!!Thanks

[telcoman]

I keep track of my MPG and track it with the computer in my 2004 G35 Coupe. Over the past few months I have really noticed a dip in my MPG. I was getting at least 300 miles out of one gas tank fill up and sometimes higher, no matter how I drove it and with what ever gas I put in it. Now I am lucky if I am getting 275 a tank full. I have tried different things like using different grades of fuel, you know the 87, 89, 93. There is no difference that is noticeable. I have added stuff like the upper cylinder lubracant to the gas, I think its called tune-up in a bottle and was recommended by a mecanic at the dealership. It seemed to work for a while but it's not helping now. What I am thinking is my car either needs service or maybe the quality of gas has gone to sh$t with the addition of the ethanol. Anyone have any guesses?? This problem is getting expensive!!Thanks

How oftern do you change your air filter and when is the last time you changed it? How often do you check your tire pressue? How much weight do you carry around? Perhaps the wife gained a few pounds? How many miles on your car?

Telcoman

[Floppydiskz]

I keep track of my MPG and track it with the computer in my 2004 G35 Coupe. Over the past few months I have really noticed a dip in my MPG. I was getting at least 300 miles out of one gas tank fill up and sometimes higher, no matter how I drove it and with what ever gas I put in it. Now I am lucky if I am getting 275 a tank full. I have tried different things like using different grades of fuel, you know the 87, 89, 93. There is no difference that is noticeable. I have added stuff like the upper cylinder lubracant to the gas, I think its called tune-up in a bottle and was recommended by a mecanic at the dealership. It seemed to work for a while but it's not helping now. What I am thinking is my car either needs service or maybe the quality of gas has gone to sh$t with the addition of the ethanol. Anyone have any guesses?? This problem is getting expensive!!Thanks

When was your last tune up? Have you ever had a valve adjustment or your fuel injectors cleaned (not just the stuff they sell in the bottle)

[joe603]

When is your last oil/air filter change?

[rn79870]

Do they offer different blends of gasoline at different times of the year in your local? That could account for some of it.

After reading the responses to your question, I'm going out to check my air filters, tire pressures and weigh my wife.

[enflixer]

Well let me think about this. I got my car with 31,000 miles on it and that was 16 months ago and now it has a little over 57,000 miles. I had the 30,000 mile done at 35,000 after I got the car. The tire pressure doesn't move really, plus I just got new tires on it and they check out. My ex-wife has gotten bigger but I dont cart her *** around They dont do different blends of gas here I dont think, I am in Delaware. I have not had the valves adjusted or the fuel injectors cleaned other than that stuff you add to the gas. So I guess it could be the air filter, the valves needing adjustment or just a good old fashion injector cleaning. I will check my air filter when I get it back from the body shop (that's another story in its self). What about the valves and the super injector cleaning? Any feed back on where to get it done and what to look for to see if I need to have it done?Thanks for the feedback so far...!!

[rn79870]

The way they used to clean the injectors (the good way) was to disable the fuel pump, and hook up a aerosol can to the fuel rail. Then they would start and run the car off the contents of the can. It really did clean the injectors well. It was several (15x +/-) times more expensive than the fuel additive method. They may have a new magic way to do it now.The fuel additives were never anywhere near as effective at cleaning the injectors.

[zozoka1212]

You should change the spark plugs also.

zozo

[slam]

Can the valves be adjusted??? Couple months back I ask my local dealer here in Costa Mesa CA to adjust my valves they said it wasn't possible because they are hydraulicly controlled or something like that and is not adjustable then again this is coming from a dealer that said low beams aren't adjustable too.

[C-Kwik]

When was your last tune up? Have you ever had a valve adjustment or your fuel injectors cleaned (not just the stuff they sell in the bottle)

I doubt a valve adjustment would be necessary. IIRC, the G35 uses a shim-on-bucket lifter design. It's notably lightweight and requires the lowest spring rate of any design so it typically has a low wear rate. When I had my 240sx, I had replaced the valve stem seals and the shims showed very little sign of wear at some 100K miles. Used the same lifter design.

As far as the topic, itself, I'd check the air filter, plugs and fuel filter. Perhaps even the O2 sensors.

[03gz]

you should check your spark plugs it could be that.i change mine out an put in uridium an that really save me some gas.an got some extra power.

[Focusedintntions]

Also make sure you use the same grade of gass...you're supposed to be using 91+ octane...if you're running lower grade gas your ecu is changing the way the engine runs to protect it.....use 91+ for about 2 tanks and it should relearn and be back to normal.

[Sentientbydesign]

I like pointing to alignment for problems like this:

Here's a recap of what everybody else has said + my $.02:

IV style injector cleaner. You can buy one yourself or go to a mechanic to get it done. The "IV" bottle isn't cheap, so I'd just pay to get it done.

You can run whatever grade of gas you want, but I firmly believe that you get what you pay for. I'd stay away from BP stations and if you must, go with their high grade stuff. The low grade feels like I'm running a gas/water combo.

Tire pressure/Alignment- Also, did you stick with the same tires or change to a different size? This makes a big difference!

Air Filter

Spark Plugs

If you're feeling like Mr. Fix it, I'd bust out a 10 mm socket and wrench and clean out the plenum and throttle body (including butterfly valve). That seemed to make my G respond a little better.

Since it's still under warranty, you could do some of the recommendations and see if they help. If not after a tank or two of gas, then take it to the dealer and ask for your MPGs back.

They'll probably then and start singing "I want my M P Gs"

I'm happy if I can get around 20 mpg. Anything lower than 17 makes me

[enflixer]

I will try that stuff. I wish I was getting 20 mpg, I was use to getting about 17-17.8. Now I am getting like 15.2, this is why I am getting a little worried about something being wrong. You guys have helped a lot. The tires are the same size by the way. I am due for an alinment so I will get that done. I get one every 3-4 months just to save the tires. I have had problems in the past. Thanks

[Floppydiskz]

I doubt a valve adjustment would be necessary. IIRC, the G35 uses a shim-on-bucket lifter design. It's notably lightweight and requires the lowest spring rate of any design so it typically has a low wear rate. When I had my 240sx, I had replaced the valve stem seals and the shims showed very little sign of wear at some 100K miles. Used the same lifter design.

As far as the topic, itself, I'd check the air filter, plugs and fuel filter. Perhaps even the O2 sensors.

that's good to know. Thanks!

[BrandAidDesignG35]

I tried the lucas fuel lubricant injector cleaner stuff which seems to work well, I've been getting better mpg in the city than I usually get on the HWY... I'm about to change my plugs as well, looking for an increase however I can, I too get pessed when it goes around 15-17mpg...

[BrandAidDesignG35]

also, I lost some level of efficiency when I popped in the fujita intake, I lost around 30-45 mi before the light comes on, be it from new found driving habits, or too lean a mixture I'm not sure, both likely, I'm thinking about maybe reinstalling some key factory intake pieces to restore the cool air to the intake, or at least as much as possible,

I''ll mess around with it soon an get back to this post.

[telcoman]

I will try that stuff. I wish I was getting 20 mpg, I was use to getting about 17-17.8. Now I am getting like 15.2, this is why I am getting a little worried about something being wrong. You guys have helped a lot. The tires are the same size by the way. I am due for an alinment so I will get that done. I get one every 3-4 months just to save the tires. I have had problems in the past. Thanks

Don't assume your tire pressure remains the same. Tire pressure @ 35lbs measured in December with the temperature at 40F will not be the same in February with the temperature at 20F. Obtain a good quality tire pressure gauge & don't rely on the ones at gas stations. In the winter you need to add a few extra lbs to account for temperature drops.How are you calculating your mileage? Some gas stations post 10% ethenol and others post 15%. My mileage was 22.4 MPG when I filled up this morning. It drops in cold weather and improves to between 23 & 25 in warmer weather. I only use regular 87 octane. BP posts 10% ethenol where I fill up.

Telcoman

[Sentientbydesign]

Telcoman:

I'm not trying to argue with you here, just trying to make sense of something. I don't check my tire pressure religiously, but I've never had a drastic drop in pressure over time.

I did have one occasion where I lent my car (not my G) to a friend and it came back with 50 psi on one tire and a "replaced" valve stem cap

Other than that, the pressure was always within 1-2 psi.

[telcoman]

Telcoman:

I'm not trying to argue with you here, just trying to make sense of something. I don't check my tire pressure religiously, but I've never had a drastic drop in pressure over time.

I did have one occasion where I lent my car (not my G) to a friend and it came back with 50 psi on one tire and a "replaced" valve stem cap

Other than that, the pressure was always within 1-2 psi.

Correct me if I'm wrong but I think Riverside,Ca is near LA?If so you do not experience the wide temperature swings that we have here in the east. We can reach 100 F in the summer and zero or below in the winter. There is a drop of a few lbs for each 10 degree change in pressure. Also some tires will lose a few lbs over time the same as women tend to gain a few over time

[Sentientbydesign]

Yeah, we're about half way between LA and San Diego.

What are seasons again???

Good point. Very good point.

I want to start putting helium in my tires so they are lighter