Is 20MPG too much to ask?

[elwesso]

Ive been battling this ever since I got the new Q..... I dont understand it either... (BTW, dont take this wrong way Fred or anyone towards fred, I am not trying to place blame or anything).....

Anyway... I took a road trip today, and I calculated my exact highway miles.. I filled up, got right on the highway, and then got off about 55 miles later, and it came out exactly to 18 MPG (exact numbers i cant recall) but I calculated it right at the gas station... Didnt drive hard, and was cruising about 70 MPH.. I also get low 12s in town, but thats just alittle lower than my odl Q got....

I checked my timing this weekend, and it was off a little..... Advanced it to EXACTLY 15*BTDC... Noticed a little difference in running.....

Said to have had new o2 sensors... Alos, i recall saying that with JWT that it leans the mix a little, so that could only INCREASE mileage....

THe only thing Ive done to the car is the fuel pump...

On my old Q, I saw a high of about 23 MPG... I was expecting to get that or better on this, or at least no worse than 20..... . Alos, i recall some of Freds posts saying that he was getting 22ish..... Whatever the case is, I *know* he doesnt drive any easier than I do!

The only thought I have on the matter is that somehow the pump is running at 43 (wot) psi all the time.....

[louiegz]

I wish I can get 18mpg, let alone 20. I'm doing some new calculations on my drive to Ohio this Thanksgiving. I hope I do better than my usual 16mpg. I'll see.

[pito11213]

I think we need to face it. The Q is a gas guzzler. I hate when I put 15 dollars in the tank and from E and barely gives me a quarter tank. Then when I am on the road and I hit the accelerator I get pissed at myself because I know my gas is just burning away.

In my defense I probably need knock and oxygen sensors.

[qship96]

wes,i get 23 on all highway driving,16 suburban driving.you get less for at least 2 reasons i can think of-1. the axle ratio in your car,2.much larger tires and heavier wheels.

[elwesso]

I turn 2000 RPM at 60 MPH and currently am running stock wheels and tires (all seasons for snow use)

[qship96]

if you are turning 2000 at 60mph,you must have the original axle ratio in your car?I thought you had the 4.08?

[robmcole]

What are the years of the Q that have the more "fuel friendly" axle ratio?

I am looking into getting a Q45 to begin a Infiniti collection next summer (also it is a pain to drive 4 people on a trip in a J30)

[qship96]

90-96q all had the same axle ratio from the factory,some owners have swapped a lower ratio 4.08 in to improve acceleration,at the cost of an estimated 15% reduction in highway fuel mileage.

[elwesso]

It seems as if I have hte 4.08, from an acceleration stand point, so that could be the speedo sensor maybe..... When im going 60, i should be doing 2500 RPM not 2000, correct? It feels like im moving at the speed it says, if not FASTER..

If I dont have a 4.08, that would make me VERY

However, the 4.08 would decrease mileage by about 3 mpg at cruise... hbut that doesnt explain why Fred got better mileage.....

[qship96]

go out to an interstate and find a "mile marker" sign,go an indicated 60mph and time between markers{one mile} should take 60 seconds if speedometer is correct.

[elwesso]

Next time I am under the car, I will check and see if I have the 4.08

For every turn of the wheel, the driveshaft will rotate 4 (.08) times.....

I'll jack up the rear of the car tomorrow and we'll see what we can come up with.....

[Jesda]

Spoiled!

[AlabamaDan]

I'm only getting about 14 mpg. What am I doing wrong? You want 20? I want your 18!

[Q451990]

I think 20 is reasonable... I'd check on this fuel pressure issue. Sounds like the controller is grounded or ECU is sending the wrong info to the controller. If the pressure is higher than expected - I would think the injectors would not know to compensate and you'd get a slightly richer "squirt" of fuel in each pulse. The O2 sensors should detect it and lean things out - but they may only be able to do so much. Q45Tech will know more - but I'd say it's absolutely essential to get the fuel pressure correct. Is this the car that was set up for Nitro? If so there may be something in the software on your ECU that tells it to run rich to keep the cylinders cooler. Perhaps you could swap your old ECU (if you haven't sold it) and see if the pressure is correct.

Heath

[Q45tech]

New O2 and ecu can compensate for 20% richer than normal [that would be a 53-55 psi fuel pressure with no change in duty cycle but ecu changes the duty cycle].

[rydwhite]

Sucks to be you Q guys. I've been getting near 30 on the highway and 23 in town with the G35 coupe.

Sorry, I had to boast a little. But I do put 94 in the tank which gets pretty damn expensive.

[Falkdesigns]

I got a solid 20 mpg on my trip to and back from Vegas, which includes a climb from sea level to about 4000 feet, going an average of 80-82 (some times like 78, sometimes like 88 and the occasional pass at 95, but mostly cruise control at 80-82). And that's with my 18's, and air conditioning. I get about 16 in town. That made me happy Yours sounds a little low for going only 70. Mine at 80 is at 2750 rpm's.

[elwesso]

I think 20 is reasonable... I'd check on this fuel pressure issue. Sounds like the controller is grounded or ECU is sending the wrong info to the controller. If the pressure is higher than expected - I would think the injectors would not know to compensate and you'd get a slightly richer "squirt" of fuel in each pulse. The O2 sensors should detect it and lean things out - but they may only be able to do so much. Q45Tech will know more - but I'd say it's absolutely essential to get the fuel pressure correct. Is this the car that was set up for Nitro? If so there may be something in the software on your ECU that tells it to run rich to keep the cylinders cooler. Perhaps you could swap your old ECU (if you haven't sold it) and see if the pressure is correct.

Heath

I do too... I got 21 MPG on my way to and from Lexington in the old Q and that was prior to o2 sensors.....

I may throw in the old ECU and see what that does..... This is very weird, and I was kinda hoping Fred would have some insight, because as this seems it something that Ive done, as he was boasting these MPG numbers..

When I did the fuel pump, there was some other wiring going on.. They cut a couple of wires from the fuel pump, and spliced in.....

What I really need to know is which way the toggle switch for the fuel tanks goes into... I **thought** when i bought the car it was pressed foreward, but when I press it foreward now the aux pump is on, and when pressed back its not......

Im about to go out and see if I have the 4.08

[elwesso]

Just went out and the driveshaft turned 3.5 times for one revolution of the rear wheels..... Means I dont have the 4.08

[GreenQ45a]

"Sucks to be you Q guys. I've been getting near 30 on the highway and 23 in town with the G35 coupe." -rydwhite

How much is your G a month?? How many years of payments do you have left on your G?

I'll get a used GX35 when the time is right.(don't think I don't like them)But for the price of a new G you could get a 2002-3 Q45 @25MPG and 340HP Lots more room and every bell and whistle.

But thats just how I would spend 35,000+

Don't feel bad for us all of Q45 owners the Q45a's Active Suspension System is second to none!

[PalmerWMD]

Just went out and the driveshaft turned 3.5 times for one revolution of the rear wheels..... Means I dont have the 4.08

Rex told me that thats what in there, I never checked and always firgured its there.that would maybe explain my stock quarter mile times?

Dennis once mentioned the rpm's I should see at at a certain speed with the 4.08 when I doubted the 4.08, and I did see that number

I will take the car back if you feel you didnt get what you paid for.

Fred

[pito11213]

Wes will not do that I am sure!!

It is still overall a nice car whether or not it has the 4.08.

[elwesso]

I did eyeball it yesterday, and I will have someone else verify..... But isnt turning 2000 RPM at 60 MPH what a stock Q does... Shouldnt it turn at about 2500 at 60 MPH..... It just seems that it turns the same RPMs all the time as it did in my old Q, and it shouldnt... Maybe im wrong somewhere...

In all honesty, if it decreased mileage that mcuh, and I have all this low end power JUST from the higher stall converter, im a happy camper!!! However, I do want to figure this out... Is there anyway to tell if the fuel pump is running at full speed all the time???

Fred, I tihnk you just want the car back

[pito11213]

Wes you are right my Q does 2000 rpm at 60 mph.

[Q45tech]

3.538 diff gear with 215/65 or 235/60/15 tires ~~ 333 rpm per 10 mph = 1980-2020 rpms @60 mph.4.083/3.538= 1.154 thus 1980 x 1.154= 2,285...........2,331 rpms nominal =2.308 rpms with 4.083@60mph.

Need to know what adaptive learn [100% +- 20%] is at a steady 60 mph and with injector opening time and fuel pressure and rpm we can calulate the error [whether the reduction is from driving faster than 55-60 mph or too much fuel being added].

Obviously if the adaptive learn is not 100% on both banks the AF ratio average is not 14.7AF..............even 105% yields 5% more fuel usage ....22 mpg/1.05=20.95 mpg..........110%=20 mpg

[sijoko]

There are a couple of things I would look at:

-Make sure that your TPS voltage is within range. If it is too high, the trans will not go into lockup as quickly as it should. Since you have a high stall converter, this would eat up gas.

-I am running my timing at 17-18 degrees BTDC. The weather is colder and I use 93 octane all the time. This will give you some mileage.

-Make sure that your fuel pressure is not excessively high. If I am right, it should be ~34 psi @ idle and ~44psi @ WOT. Since your car has some modifications, this could be an issue.

-Check the basics such as sensors, plugs, air filter, oil, trans. fluid and diff. fluid. Oil viscosity is also a factor as the temps. drop.

I get about 18 on the highway with the 4.08 gears. My cruising speeds are usually 70-75 mph.

Also, the quality of the gas in your locality can come into the picture.

-sijoko

[elwesso]

3.538 diff gear with 215/65 or 235/60/15 tires ~~ 333 rpm per 10 mph = 1980-2020 rpms @60 mph.4.083/3.538= 1.154 thus 1980 x 1.154= 2,285...........2,331 rpms nominal =2.308 rpms with 4.083@60mph.

Need to know what adaptive learn [100% +- 20%] is at a steady 60 mph and with injector opening time and fuel pressure and rpm we can calulate the error [whether the reduction is from driving faster than 55-60 mph or too much fuel being added].

Obviously if the adaptive learn is not 100% on both banks the AF ratio average is not 14.7AF..............even 105% yields 5% more fuel usage ....22 mpg/1.05=20.95 mpg..........110%=20 mpg

OK... let me analyze.....

First of all, I talked to Rob over the phone, and he said that if the speedo sensor was not changed, then it would be reading stock RPM and stock.. So even though its reading 2000 RPM, i would be going 2300 RPM... So I need ot verify I hvae the 4.08, and then we swap the speedo sensor.

So, assuming my fuel pressure is OK (still have yet to determine what could cause this, as its a new fuel pump!)

Dennis says that the difference in MPG at 60 MPH should be nominal, or at least what Im gathering.... He also says that the adapative learning (o2 sensors) would only produce 20% in either direction... o2 sensors arent very old (30k tops, still within working order). So whatever this is, it seems as if its beyond the computers control...

so the underlying question remains what would cause the fuel pump to run at a higher speed than normal . I will post pictures of how they had this fuel pump wired up before.... I dont know what would make it go, since the fuel pump is plugged right into the FPCU, as it should be..... The ECU, as far as I know, doesnt control it...

Thanks for the advice, sijoko.....

heres a couple other thoughts.....

the 94s have been known for the precat meltings.. If its been running rich, plus the fuel from the NOS, maybe its slowly eating those away.....

[DAEDALUS]

You don't have the stock pressure regulator? The pump puts out fuel but the pressure is regulated.

Bad cats would not have an effect on mileage since there's no feedback to the ECU on their performance.

[elwesso]

You don't have the stock pressure regulator? The pump puts out fuel but the pressure is regulated.

Bad cats would not have an effect on mileage since there's no feedback to the ECU on their performance.

The FPR is regulated by engine vacuum... So, (correct if false) the FPR regulates based on engine vacuum... The base fuel pressure must be normal, as unless i am misunderstood, the FPR will regulate the same wether the pump is pumping 35 PSI or 40 PSI... Thats at least how I understand it.. I remember heraing that the FPRs dont fail...

With the cats, the thing I was saying is how they melt... Remember AZskygod (sp?) and his terrible running 94... Im wondering, due to my rich running condition, this may a precursor to such a failure.....

[qship96]

speedo. gear does not affect tachometer reading-only speed reading.