Hello All,

I have a 2003 G35 coupe, 82300 miles. Note: has K&N Air Filter, and a Stillen Z-tube.

I recently trailered the car from the Los Angeles metro area, to Santa Fe, New Mexico, at 7,000 feet. Average mileage for a tank of gas was about 320-340 miles. That's been reduced by about 35% ~ I'm now filling up the tank at about 200 miles!

Needless to say, I'm freaking out a little. I understand that power decreases at higher altitudes, but my mileage has completely dropped off the charts and no one I've talked to seems to have any suggestions.

Mechanic suggested general tune-up and 3-stage fuel cleaner, but not sure that would account for the sudden and drastic decrease??

Grateful for all suggestions,
J

 

have you replaced your spark plugs yet since it was new? plugs at high mileage will have that affect, also... while i was at mammoth mountain for the week, the drive alone on the high altitude freeway i averaged what i got in the city of los angeles. so yeah, at 7000 you're going from average of 23mph freeway/city to 18 easily. you don't need fuel cleaner, g and z aren't known to have much carbon build up as long as you rev the motor hard every now and then.

also, it sounds like you're averaging your mileage from the computer. you should average from actually math of gallons and miles traveled. the ecu isn't really accurate in the g/z. it's not accurate like the lexus

 

Any CELs? Lost a crapton of milage when I put on my TP's without clearing the CEL.

 

Hello,

@FiveWunOh: No CELs, no error codes being thrown - I had a local mechanic do a check on the car to see if he could figure out the problem.

@power2rice: Spark plugs/tune-up are my next stop. I'm the 3rd owner, so don't have all the previous maintenance at hand, but they are probably due. As for the mileage...actually, I'm calculating the old-fashioned way.

I don't think there's carbon build-up either, especially since I always use premium gas, but I did drop some $$ on a product called CATACLEAN that supposedly cleans the catalytic converter, oxygen sensors, valves and injectors. TBD if it helps.

Two tidbits of information I'd like to add to the mix:

1) When I was trailering the car, I had to go thru a hellacious snowstorm in the Flagstaff, AZ area. The car was COVERED in snow, slush and dirt from the endless line of truckers on I-40 slinging slush over the car as they whizzed by me in my 16' Budget truck with trailer in tow. Wondering if that could have clogged or dirtied some sensitive parts?

2) What are your thoughts on the Mass Airflow Sensor, O2 Sensor, and ECU (retune) as possible culprits?

I have found this forum to be incredibly helpful, and appreciate all the information I've received!
Thanks all.

 

are you getting 200 miles a tank all city? i think something you should check also is egr valve. when they get stuck partial open the car runs sluggish and fuel economy is horrible. would be unusual for the g/z cars to have that problem so soon. it's more common on high mileage cars reaching 200k.

mass air you can clean with the mass air cleaner from CRC. o2 sensor usually last about 80 to 120k for most cars.

you had an ecu retune? if you did..... and not sure why you did for only intake, but the retune adds more fuel to compensate for added air flow. 7000 feet above, air is thin, so yeah you would need a retune again assuming you have osiris or some other standalone / piggy back unit.

snow storm has no effect on a car not running and sitting on a trailer.

premium (91 octane) gas is what causes carbon build up. 87 is most volatile, burns the easiest, 89 harder to burn, 91 even harder. so when fuel can't be burned completely, you get carbon build up. so when you put 91 in a car designed with low compression and meant for 87..... you get so much excessive carbon build up after years of using it.. 91 is marketed as Premium, but its in no way a better gas, it's rated for higher compression engine use, and because it causes so much carbon deposit, it's required to have at least 3 times as much cleaning additives to assist in reducing carbon build up. fyi, g/z engines are high compression, so 91 is needed to achieve the best mpg.

the z forum is better in my opinion, z owners are more enthusiastic about DIY, so you get millions of users and many already having experienced and resolved problems.

i'm chiming in because i'm forced too.... g forum has restrictions to do things that z forum and g37 do not.

 

power2rice,

Yes, getting 200 miles/tank all-city where the same driving in LA (only worse, b-c of stop and go) would still get me to 300 miles/tank.

I will look into the Mass Air cleaner.

I did not do an ECU retune (as mentioned, I'm the 3rd owner) however I read somewhere that it could be necessary. The last owner added an aftermarket exhaust and some other goodies, so it's possible it could have been done at that time (if my understanding of ECU is correct). If this is necessary, is the dealer the only one equipped to do it??

Thank you for the lesson on gas. Now I know! Still surprised though, b/c my CA emissions were passed with flying colors...

Whatever your motivation for response, it's appreciated.

 

i didn't notice the mention of exhaust till now. i lost mpg with cat back exhaust because of lowered back pressure. averaged 250 on a full tank versus 300 with oem exhaust. i'm 5at so exhaust is not beneficial. 6mt drivers won't really lose mpg. at such high altitude the dealer can do a 2 degree timing advance for you. dyno tuning shops usually have equipment to advance the timing for you too. it may help, not certainty but you'll definitely feel some added power.