Low MPG Q45

[peaceofmindinhim]

Hey everyone,

Just a quick question: lately on my 1995 Infiniti Q45, I've been average about 12-14 mpg combined highway/city. Is this normal for the car being as old as it is? I fill with Premium fuel (98% of the time) but this past fill, I've put in regular because I was thinking of adding one of the fuel injector cleaning bottles you find at the auto parts supply stores (I can't think of the specific brand off the top of my head), and I believe the instructions are to add to a full tank of regular.

What can I do to increase MPG? Also, as of late it seems any time I go up any type of incline/hill, my fuel gauge noticeable goes down a tiny bit, and doesn't return back to its original spot. This happened today after just filling the tank. It'll move about 1 mm/2mm. Am I really blowing through gas THAT QUICKLY? We're not talking San Francisco type hills, but your average normal going up hill type roads (if that makes any sense, I apologize if it sounds as if I am rambling).

Don't have a lot of money right now, and I realize many people will flame me for looking for "cheap" repairs. I love my car, and I do realize it is a luxury car, and deserves the proper maintenance etc, but being unemployed at the moment, food/shelter is more a priority. I hope the readers can empathize with that!

Any suggestions are welcome. I should mention I had the plugs replaced about 2 years ago. I'm not having any backfiring, or idling problems.

Thanks!

Eric

[MinisterofDOOM]

Definitely not normal. Even driving aggressively you should see high teens at worst as far as combined fuel economy. Should be able to cross the 20mpg mark on the highway.

Make sure your air filter is clean. If you haven't replaced it in a while, do that. Dirty air filter can cause the car to run rich, which can hurt fuel economy. That alone shouldn't result in 12mpg average, though.

[goody90q45]

.....lately on my 1995 Infiniti Q45, I've been average about 12-14 mpg combined highway/city. Is this normal for the car being as old as it is?.....

My Q runs well and I get very close to 15mpg with 100% city driving and 21-22 on road trips (going to Santa Cruz today) so I would say it's not normal to get 12-14 combined mpg. What is normal is for the O2 sensors to wear out after 15 years and cut in to your pocketbook with low mpg almost as bad as an unemployed teenage child. I've had both.

It sounds like you may have a faulty fuel pump sending unit if the gauge doesn't return to the same position when you're back on level ground. A common problem for the Q. At 12mpg the tank will be empty quick but you shouldn't see any difference unless the hill you're climbing is to the restaurant you work at on top of Pike's Peak.

Is the CEL lit up? Have you checked ECU codes? Ohmed injectors and KS? Ohm tested the O2 sensors? If you have a voltmeter these are all simple, no cost tests you can do yourself. Read up on the how-to's in the Tech Help section at Q45.org.

Do you have plenty of power going up those hills or are you bogging down? You didn't make it sound like it was a fuel pump issue. Removing and cleaning the TB, IAC valve and EGR valve will give the engine a noticeable improvement in power for the price of a couple of cans of brake parts cleaner and 3 gaskets (stealership parts), about $25. Finally, cleaning up the MAF, CAS, sub CAS, EGR solenoids and TB connectors, tightening the pins and coating with dielectric grease will help. These are all similar connectors that are prone to corrosion and loose connections if they've been removed a few times (....MAF).

If the CEL is lit I'd start by pulling ECU codes and ohm testing the O2 sensors.

Tell us where you live. There may be someone close with a Consult program that can make the diagnosis very easy.

[peaceofmindinhim]

My understanding is that the onboard controller would adjust the fuel mixture to make up for any clogged air filter.

I'm averaging between 240-320 miles per tank.

[peaceofmindinhim]

My apologies for my last post - didn't see goody90q45's response! Thank you very much, although I will admit I am rather lost with all of your abbreviations. I'm not much a techie at all, so I wouldn't know how to do any of the test with the voltimeter that you're talking about.

I live in Claremont, California, to answer your question. No lights are lit up either. Fuel pump is relatively new - 4 years old.

In addition, I have plenty of power going up the hills, yeah, I mean, everything seems normal - I'm not noticing a lack of power by any means.

Performance wise, I seem to notice the most rev/kick once I hit 30 mph. I'm slow(er) from 0-30, and then at 30 it feels like a rocket ignited. I realize that's performance issues, and maybe not necessarily related to MPG. But I thought I'd share. Better too much info, than not enough.

Eric

[goody90q45]

Unless you have a leaky injector (don't think so) It soulds like it's time to replace the O2 sensors. Testing them is simple with no tools other than a voltmeter required. A digital voltmeter is $10-$15 at Radio Shack if you want your own.

The last page of the EC (emissions) section of the FSM shows that the ohms resistance of the O2 sensors should be 2-1000. The connectors are mounted close to the firewall at plenum near the throttle cables. You can follow a small wire up from each exhaust manifold to find them. Unplug the connectors and measure across the two pins to get the reading. Any friend that owns a voltmeter will be able to do this for you once the connectors are located.

http://www.nicoclub.com/FSM/Q45/1995_Q45/1995_Q45/

The Q is a rocket even running poorly but above 4000 rpm the O2 sensors don't function under hard acceleration anyways. They let the fuel flow. It could explain what you're feeling above 30mph.

[Q45tech]

Original EPA sticker on 90-96 Q was 16/22 mpg assuming operators drove EXACTLY LIKEEPA TEST PROCEEDURE.

NO AC not to exceed 33 mph in rural city or 58 mph in rural highway.

Consumming E10 will further reduce MPG from 0.5>1.5 MPG is different situations.

Everything depends on RPM and speed of acceleration and of course warm up and idling.

[qship96]

Even with E10 and a/c on and driving 80-85mph I can get 21-22 mpg on pure highway long trip in summer....in town only driving can be brutal as I have seen as low as 14 mpg in winter, and generally 15-16 in summer.

[Q45tech]

After all the displacement times rpm is all that counts since AF is held steady except in acceleration.
The HP consumption of AC compressor, alternator, PS pump varies very little except above 2500 rpm.

It is educational to view the injector open time 1.2 > 11 milliseconds under different speed and load conditions. Don't forget that 34 > 44 psi rail fuel pressure increases the fuel flow by 15% [idle/cruise vs acceleration]........that richens AF from 14.7 to ~ 12:1 A/F.