Is 20MPG too much to ask?

[elwesso]

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

What I said above is what Rob told me.. he has real world experience in this manner, and I wont doubt him......

he said on the phone today "if you have the 4.08 and the stock speedo sensor, it will read, RPMS and MPH, just like a stock Q45"

[Q45tech]

If you change the speedo gear to the one that come with the 4.083 then the speedometer will read nomally [within the calibration range accuracy of the speedometer. Otherwise it will read 15.4% off............just as using the wrong loaded radius tires will create an error.

Duh! otherwise why would they bother to make a dozen different replaceable gears for the speed sensor.

More modern designs [G35] sample the front/rear tires as part of the ABS/TCS system and create a speedometer signal.

Always research what people tell you [posts] and never accept things as facts, even my comments. I'm wrong on average 2-3% of the time because I skip research rigor sometimes!

[sijoko]

The tach will always read true regardless if the right speedo gear is installed.

-sijoko

[DAEDALUS]

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.

So check the fuel pressure.

Quote »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.....[/quote]It's not like you're dumping raw fuel all over the cats. (Besides, running rich lowers cylinder temps.) 18mpg is not a bad number, I rarely see that on my Q and I've never seen 20, even with a spotless intake and many new sensors.

Various gears aside, I think the bottom line is you're looking to get a solid 20mpg on a modified vehicle that was EPA-rated at 17/22 when brand new, with testing methods that are under scrutiny for possibly being too optimistic. The fact is, 10% mileage difference is well within the "noise" of all the variables that can affect mileage. Driving habits, weight, temperature, fuel quality, condition of various subsystems (intake, fuel, cylinder, etc), various sensors, etc. You can check everything to make sure they're all "in spec" and functional, but I guess what I'm saying is don't be surprised if they all are.

[Jesda]

Is the BAD Q45 a California vehicle like the late Crimson Q? Does that matter?

-Jesda

[elwesso]

I looked last night, and there is some funky wiring going from the FPCU to the fuel pump harness..... I'll replace the harness with an OEM one from my old Q, or ill buy a new one, or whatever..

I have a feeling this is my problem, or I hope...

The reason I think I should get 20+ is because my old Q did it, and this car is in greater condition, assuming I dont ahve the 4.08, there isnt much excuse....

[DAEDALUS]

You're thinking that somehow the pump is working too hard, thereby heating up the fuel more and causing more evaporation in the returned fuel?

[elwesso]

if the fuel pump is grounded somehow (full 12 v signal) then its running at full speed... Correct?

If the FPCU isnt controlling the pump, and the car still runs "fine" this seems like a possibility...

[rsiwicki]

What I said above is what Rob told me.. he has real world experience in this manner, and I wont doubt him......

he said on the phone today "if you have the 4.08 and the stock speedo sensor, it will read, RPMS and MPH, just like a stock Q45"

Wes....sorry but I think that there was some confusion as this is not easy to follow with all the stock vs. non-stock scenarios we were talking about

Anyway....what I was saying is that if you have a 4.08 rearend and the stock speedo gear, then you will not be able to tell if you have a 4.08 rearend by looking at the relationship between the rpms and mph as it will appear if everything is stock (2,000rpms @ 60mph but your car should actually say 2,000rpms at 52mph if you had the correct speedo gear installed). This is becuase your mph will be off but not your rpm guauge so just by looking at the gauges with the above scenario your car will appear to be running a stock 3.54 rearend. 1qsker and I called another NICO member to verify this when the both of us were trying to figure out whether or not I had the 4.08 installed....it turned out I did have the 4.08 installed but you can't tell by looking at the guages unless you switch out the speedo gear as well.

[DAEDALUS]

if the fuel pump is grounded somehow (full 12 v signal) then its running at full speed... Correct?

If the FPCU isnt controlling the pump, and the car still runs "fine" this seems like a possibility...

I'm just not sure how you're drawing a connection between fuel pump operation and the car getting poor gas mileage. You could run the pump at 24V and the pressure regulator, if working properly and with enough capacity, would ensure the injectors get as much gas as they're supposed to. The rest is returned to the tank. Granted, perhaps the regulator doesn't have the capacity to bypass 12V worth of fuel at idle. But the right approach is to check the fuel pressure, or at least take a voltage reading across the fuel pump terminals at idle. Otherwise you'll be replacing parts one gas tank at a time.

[elwesso]

OK... I did some digging before dinner tonight, and I found a wire that was spliced in between the FPCU and fuel pump... It was an orange one with blue stripe.. Couldnt find a wiring diagram in the FSM (albiet didnt try too hard).. So I spliced the wire back together...

So right now the fuel pump should be under complete control..... Now im fairly certain it cant be a fuel related issue, so we'll see if gas mileage continues......

EDIT: now as far as I can tell the aux fuel pump and fuel cell is useless... So i really do have a lot of "junk in my trunk"

[elwesso]

Id like to bump this.....

For the purpose of determining if I have 4.08, i did some calculations and at 2500 RPM I should be going about 66MPH...

However, when I use the .694 gear ratio as the OD, does that take into account the torque converter locking.. I seem to drop about 200 RPM or so when it locks up?

[qship96]

yes-stock rear ratio,locked up equals 2500rpm@80mph

[Q45tech]

333.3 rpm [stock tires and stock 3.538 diff gear in 4th gear [0.694]] per 10 mph = 2,000 at 60 mph and 2664 rpm at 80 mph.

MULTIPLY ABOVE by 1.154 if you have a 4.083 in a 90-96Q.

You can't use 26.0" as the tires because the weight squishes them down to 25.25" [24.75-25.25] depending on brand [sidewall stiffness and inflation pressure] plus tires grow at speed so this varies even more at 80 mph depending on temperature!!!!!!!!!!!!!!!!!!!!!!!! Rolling loaded [by car weight] radius is what determines rpm!

Important to know measure sidewall stiffness of the tires you buy.51 psi are horrible ballons compared to 44 psi, very few stiff 35 psi still available..........number of sidewall plys give you a clue. Remember Michelins are 50% stronger per ply so single is like a 1.5 ply.

[elwesso]

Thank you dennis..

I do think that before the TC locks up, in say 3rd gear, that its whereit should be with 4.08...

Really I dont care.. I just want 20ish MPG.. Gotta do BG44k and maybe a couple other things...

I may throw in a different MAF from my old Q, as I was reading some posts of dennis' that there was a thought to be good maf that would give weird higher voltage... Might explain my somewhat lumpy idle....

[louiegz]

Yes, I think it is too much to ask for 20MPG. I've been driving this POS loaner for two days. A Mercury Sable with the Duratec 3.0L V6. Car sucks but it does have one nice feature. A computer that not only has a trip odo and an MPG gauge, but also tells you your driving time. Lots of trafic around here for the holidays. I've driven about 40 miles in about 4 hours. My MPG is 11.4. Now I know why I get such bad milege. It's not my car, but my driving conditions.

[texasoil]

A couple of things to look at. Air filter not clean? Plugged CAT convertors will significantly increase back pressure and reduce mileage. Also, a 'nervous foot' will significantly lower fuel economy. Use the cruise control or brace your foot tohold it rock steady. My stock 94a routinely delivers 19-21 at a steady 75, A/C on , hot humid LA/So Tex summer days. I do notice a difference with different gasoline brands. Chevron and Shell work a little better for me.

[elwesso]

I have no cats

I bet the high stall converter has a negative impact on city driving since it revvs a little easier.. hard to describe, but i like it..

But the bottom line is... im not gonna not drive the car because of this, so i should either try and figure it out, and or live with it.....

[qship96]

FYI-just returned from a 400 mile highway trip -baltimore to bethany beach-23.1MPG averaged 70-80MPH with a few blasts to 105MPH-96q original oxygen sensors!

[91QAZ]

23.1! Way to go. I just drove LA to Phoenix in a stock 91Q at 80 - 90 mph (with a few blasts to 105). Used the cruise when I could. I got 19.2 mpg...

[Q45tech]

In 15 years of driving 271,000 miles my Q has averaged about 19.77 mpg in the blend of city and interstate........I keep accurate records for taxes.

I'll bet >65% of those years miles were at Interstate speeds 60-75 mph.

Now each day [in 100 miles~~600 week] I average 16 mpg [60 miles at 60 mph average] and 40 miles idling and stuck in traffic.............. $12 per day for gasoline@ 1.90/gallon..........about $1.40 more per day than regular.

My Q pings on midgrade even at 50F.

Then the can of 44K every 6-7 weeks prior to oil change.

Easily $20 per day x 6 or $480 per month in simple gasoline and M&R.

Getting 19 mpg average much less 20 requires just the right blend of driving.............I am pleased at 16 mpg plus considering.

Everyday some emergency crops up where I must use the braking, handling or superior acceleration power of my Q......ATL Traffic and drivers.

I feel for those Policeman driving Crown Vics or Impalas.