2004 FX45 - Poor City MPG Questions

A forum for lovers of Infiniti's hot-rod crossover, the FX! In 2014, all FX models will be named Infiniti QX70, in line with Infiniti's new naming structure.
Posts: 76
Joined: Mon Nov 15, 2021 1:48 am
Car: 2004 Infiniti FX45

2004 FX45 - Poor City MPG Questions

Postby imnprsd » Sun Sep 11, 2022 8:52 pm

My 2004 Infiniti FX45 gets normal Highway-MPG (20-22), but horrible Inner-City-MPG (6-10 MPG).

...And I have replaced the MAF sensor and today replaced the spark plugs, but my City-MPG is still very low. So I'm wondering what other FX45 owner's see when they query their "MPG" after driving 1 mile in stop-and-go traffic?

Note: By "query" I mean you reset your fuel economy button and travel 1-3 miles... and read your MPG.

Note: How you drive also affects your results, because in theory, if you are not moving in your vehicle then your MPG would equal zerro.

That said, you really have to test your MPG under different driving condions, and here is how I have tabulated my test results after driving 1-3 miles in three different categories:

A) Stop & Go Inter-city driving, where 50% of the time I am stoped at the light and 50% of the time I'm driving at 25MPH ==> and I get 6-8MPG. ...And this poor number bothers me the most, but maybe it's normal? What does your FX45 get?

B) Stop & Go over 3 miles when I'm only stop 20% of the time and travel 35MPH ==> and I get 10-13MPG.

C) City driving where I cruise on through "green lights" over 3+ miles at 25-35MPH ==> and I get 13-18MPG.

D) On expressways where I can maintain 35MPG for 5+ miles ==> 18-20 MPG

E) Flat highways doing 60MPG ==> 20-22 MPG

It would be a great helf to me if you can verify you get these same results in your FX45 or M45 or Q50/60?

Spark Plug Comments: After replacing my MAF my engine was more responsive, but my MPG did not change. And I can say the same for replacing my stock spark plugs (NGK # LFR5A-11) (NGK6376) with Denso Iridium SK16HR11. Both plugs are the same heat range, but it has been said the Iridium plugs burn a little cooler and the spark is a little stronger. So I did change my plugs to Iridium, and my old spark plugs definitely were worn, but my City-MPG did not improve, even though my engine has a few more "ponies," which is nice.

Further, We all know the FX45 has a sensitive gas pedal, so if you find your g-forces are waning, then you might replace your MAF first and if you have over 100K miles you should go ahead and replace your spark plugs too.


I recommend you tighen your valve cover bolts, but not over tighten. I have 115,000 miles on my FX45 and my valve cover gaskets were leaking in the back on the driver's side, and in the front on the passenger side. Then I put ~1/8 to 1/4 turn on eacy bolt and the leak stopped!!!! Note: Be sure to do a cross-torque-pattern starting in the center of your valve cover and work your way out.

* Also, you will need a combination of 3/8" and 1/4" sockets with different length extensions. and here's a tip: Put a small piece of duct tape in the 5/8" deep socket to hold the spark plug in place. And sometimes you can use a piece of black electrical tape to hold the 10 mm bolts in the socket so you don't drop them. And you should start with the back spark plugs, because if you can get these out then all the other plugs will come out easyly.

* The passenger side spark plugs are the easiest to R&R. And when you get to the driver's side, you first need to remove the entire MAF sensor tube, when remove the whole air tube to the throttle-body as one assembly. To do this you just need to remove one 10mm nut that holds the secondary air box, and then you break loose the 3/4" suction hose on the backside, and loosen the 2-large hose clamps. ...Then you can filp the "whole mess" out of the way and you can see your spark plug coils underneath. ...Also, don't over tighten the ingnition coils 10mm bols.

* And I think anytime you change spark plugs you should tighten your valve cover bolts (all at room temperature).

* This is also a good time to clean your throttle body, but don't be surprised if your engine is hard to start aferwards since you sprayed carburator cleaner of MAF cleaner into the throttle body.

Good luck. The whole thing is a 2 hour job. Maybe 2.5 if you go slow. And it's not difficult.

Posts: 76
Joined: Mon Nov 15, 2021 1:48 am
Car: 2004 Infiniti FX45

Re: 2004 FX45 - Poor City MPG Questions

Postby imnprsd » Sat Sep 24, 2022 10:22 pm

Copied to from another thread on the same topic related to replacing O2 sensors and Crankshaft Position Sensor:

Re: 2004 FX45 (V8) O2 Sensor Discussion
Postby imnprsd » Sun Sep 25, 2022 2:05 pm

TO CONCLUDE: I openned this thread based on the goal of increasing MPG by replacing the Upstream 02 Sensors with the hope that would improve my city-MPG, and my journey has tought me several things:

1) This VK45DE (V8) in my 2004 FX45 is a pig when it comes to city MPG, albiet what's a 8MPG in this Infiniti I enjoy driving vs 12MPG-city I used to get in a V6 Honda. I mean who cares? s*** MPG is just that.

2) I found replacing the upstream O2 sensors was not as easy as I thought so I paid a shop to do it. $400 later, because they upcharged me, my city MPG is still in the toilet at 8MPG in heavy city driving, but my highway MPG is acceptable at 20-22MPG.

Would I recommend you replace your O2 sensors if you are getting 20-22MPG on the highway? ...No!

3) Would I recommend you replace your crank position sensor in you were not experiencing any no-start conditions? ...No!

Conclusions: It's all about weight vs. HP and you can't fight that. The only upside is that I may not end-up stranded on the side of the road anytime soon, because I elected to do some preventive maintenance (PM) by replacing my CKPS tend to fail somewhere between 120K-200K miles. And as for the 02 sensors getting replaced, that was a crap-shoot from the beginning and I lost.

So now my wallet is $600 lighter. ;) or :( ...You decide?

On my 2004 FX45 the CKPS was on the passenger side where the transmission mates with the engine block and not on the driver's side as implied in the video above. And I decided to us the OEM-Hitachi Part because Japanese cars tend to be finicky about sensors and coil packs. And the O2 sesnsor were NGK.

May thanks to VStar650CL weighing-in. I'm satisfied now.

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