5-8(max)mpg city/ 22 mpg Highway- please help me.

[cackster]

Hello FX community, I am writing to you all in desperation. My family has owned a 2003 FX45 since it had 20k miles (certified pre-owned). At the moment, it has aprox. 88k miles and we have done absolutely everything to keep the car in good working order. I changed all the crank sensors when asked by the vehicle, always use premium fuel (tried many brands) and synthetic oil, and do regular tune ups and service to the car such as changing tires, alignment, rotation, brakes, plugs etc... HOWEVER, I have a HUGE problem with CITY MPG that I cannot seem to figure out. As it stands, the car is running perfectly fine with no codes. In the city, driving as if my leg had muscle dystrophy while never letting the rpm's exceed 1500-2000 in first or second gear, the max avg mpg I can squeeze out (after resetting the computer at fill up) is about 5-8 mpg city!!!!! However, when I go on the highway and take a 500 mile trip, the car does aprox. 380 miles to the tank which, if I'm not mistaken, is pretty normal for this engine. Guys, I'm not going to pretend that I know jack about cars...I'm just HAVE to help my mom and dad figure out why this is happening. I convinced them to buy an Infiniti back when my own selfish desire of having a flashy car outweighed any practical sense of having a V8 engine in NYC. All I know is I have a huge problem with city mpg, and this is where I stand with the car. I'm begging you guys to help me figure out this mystery. We absolutely love this car, but at this point, we can't afford to drive it in it's current state. Thank you all so much.

Best,
John S. :disapointed:


EDIT:
-Performed an intake vacuum test and everything seems to be normal. 20 psi idle, and when I slowly rev up to higher rpm, the psi increases and when I let off the pedal it goes back to normal.
-Performed a volt meter test on TPS sensor to see if it was bad.
-performed seafoam test on vacuum line yesterday. Got minimal smoke, and performance of engine definitely improved but no change in mpg.
-checked IR temp on both cats to test for restriction- At operating temp got reading of 100degrees F higher on inlet for both. I don't know how accurate this test is, but if I had a restriction in the exhaust or the intake, wouldn't it show up on the vacuum test which showed itself to be normal?

 

[StuLax18]

Try calculating your actual mpg to make sure there isn't an issue withe the computer. Pretty easy, just reset your odo at fill up and then divide the mileage by the number of gallons at next fill up.

Also, Do you spend a lot of time sitting still in traffic? That's going to give you horrible mpg alone. I think I went from 18 avg to 13 avg today just from sitting in stop and go traffic on the way to work.

 

[shadow191]

I don't think anything is actually wrong with your car. If your reset the computer, then immediately drive in the city, it will read very low. It's not accurate, as Stu says, you need to actually do the math based on miles driven and gallons used. Basically the computer looks at the previous period from reset and if you're just idling at red lights, it will skew the results a lot. My FX35 will read as low as 8-9 if I reset then drive in stop and go. But if I let that average out for an entire tank, I'll get like 12mpg in the city which is low but normal. I'd bet that if you actually drove in the city for an entire tank, you'd be closer to 11-12mpg. Which is horrible, but normal for a V8 SUV that weighs 2 tons and has AWD.

Your highway mileage is normal so that would indicate to me there's nothing wrong with the car. Anything that would lower mileage in the city would lower mileage on the highway as well such as bad cats, leaks, bad O2's, etc.

 

[cackster]

I don't think anything is actually wrong with your car. If your reset the computer, then immediately drive in the city, it will read very low. It's not accurate, as Stu says, you need to actually do the math based on miles driven and gallons used. Basically the computer looks at the previous period from reset and if you're just idling at red lights, it will skew the results a lot. My FX35 will read as low as 8-9 if I reset then drive in stop and go. But if I let that average out for an entire tank, I'll get like 12mpg in the city which is low but normal. I'd bet that if you actually drove in the city for an entire tank, you'd be closer to 11-12mpg. Which is horrible, but normal for a V8 SUV that weighs 2 tons and has AWD.

Your highway mileage is normal so that would indicate to me there's nothing wrong with the car. Anything that would lower mileage in the city would lower mileage on the highway as well such as bad cats, leaks, bad O2's, etc.

I think you're absolutely right... wow, to think all of this work and worry was for nothing Either way, it's a bitter sweet deal! Thanks so much bro...