10 MPG with good AFRs?

My decently modified 87 740 is getting close to 10 MPG.

Here are things I?ve eliminated:

- bad o2 sensor (fluctuates good)
- running very rich (AFRs are good. Idle = 14.4 boost = 10s)
- coolant temp sensor (car enters open loop (sudden shift in AFR))
- TPS is set correctly

Possible causes:

- base timing set to 14 degrees BTDC (not 12)
- weak spark (plugs are new and gapped but maybe dist/wires/coil ?)

Thoughts?

Sounds par for the course, V8 swap next?

Are you measuring the 10 mpg correctly, Unless you have a fuel leak or some other weird issue, you should be getting a lot more,. and is that 10 mpg at steady cruise, stop and go with heavy throttle etc? I get anywhere between 12-22 depending on how I drive I don't see your base timing or spark settings causing this, assuming motor runs good

I hate MPG. It should be G/100 Miles for better refs.

But real talk... what were you getting normally? What mods do you have?

CEL on? EGR leaking? Brakes dragging? Trans slipping? ODO not accurate?

Timing won't be the cause. Junk O2, even if switching can be an issue. I won't run Bosch anymore, not accurate. I swapped to NGK, better mileage and performance.

FWIW mine on E85, beating the piss out of it this weekend in the hills still got 13mpg, with 20-30 miles of that in 2nd or 3rd, high rpm, high boost.

Have you checked alignment and tire pressure? Checked to be sure you don't have brakes dragging?

I'm slowly starting to realize there are many more variables than I have considered. Have not checked tire pressure or alignment (is there a way to check/set alignment by hand without paying someone else to do it for you?). The driving was city driving, stop and go with a couple boost pulls here and there. To be totally honest I haven't truly calculated my mpg yet but it was still the first thing I noticed after driving that car. I regularly drive another car that averages around 20 mpg (actually calculated) city driving and I could tell very quickly that I was using up fuel much faster than that. My 10 mpg estimate was simply based off me mapping out how far i drove using google maps and then estimating my fuel usage based off the gauge and tank capacity.

EX: gauge goes down by about 1/8. divide the 15.85 gallon tank by 8 to get rough amount of gas used...

I haven't driven the car in a while so there is some rust on the rotors that may be causing some dragging but would that be enough to drop my mpg by 10 points (assuming around 20 is average)?

O2 sensor is pretty well brand new, only maybe 100 (not 100k lol) km put on it

To my knowledge, 87s don't have a CEL, nor do I have any EGR to my knowledge.


Mods include:

-Ebay header
-Ebay 60 trim turbo
-big ebay intercooler
-browntop injectors
-external wastegate
-cold air intake
-free-flowing exhaust
-AC delete

EX: gauge goes down by about 1/8. divide the 15.85 gallon tank by 8 to get rough amount of gas used...

Click to expand...

Now, I'm not too familiar with volvo's fuel gauge system, so take this with several grains of salt, but I know that on a lot of cars the gauges aren't exactly linear, so the first half of a the gauge might actually indicate 2/3 of the tank, or something along those lines.

Best way to keep track of it would be to reset your trip odometer after a fillup, and then calculate your MPG after the next fillup based on total miles traveled/fuel used.


FWIW, my bone stock NA 87 740 makes about 21-23 mpg with mostly highway miles.

But are you actually getting garbage mileage or did your sender die? That's why I'm asking if you see a CHANGE in normal ops and what that change is telling you.

10MPG
- speedometer is wrong
- fuel leak

I got about that when my MAF was fried and I disconnected it on my 83.

My V8 gets better mileage.

Seriously start car and let it idle. Look from clear drips of fluid. My 240 rusted a line thru under the drivers seat.

Magnanimous said:

Now, I'm not too familiar with volvo's fuel gauge system, so take this with several grains of salt, but I know that on a lot of cars the gauges aren't exactly linear, so the first half of a the gauge might actually indicate 2/3 of the tank, or something along those lines.

Best way to keep track of it would be to reset your trip odometer after a fillup, and then calculate your MPG after the next fillup based on total miles traveled/fuel used.


FWIW, my bone stock NA 87 740 makes about 21-23 mpg with mostly highway miles.

Click to expand...

This is the only way to actually test your fuel mileage. Don't trust any gauge, especially not in 1/8 tank increments.

My tank reads differently in first half vs second. It's not linear, most tanks are not

Fun fact. At least for aviation, the only time any fuel gauge is required to be 100% accurate in it's reading is when it's on empty. Gauges are garbage.

Interesting, never would have thought the gauge to be non-linear. I know for a fact I don't have a fuel leak as my car has been left idling for quite a while with no "clear fluid" to be seen.

It seems as though further testing is necessary in order to confirm actual MPG

FWIW all my measurements were being taken from half a tank and below which could help justify the significant needle movement after little distance traveled if the gauge is indeed non-linear?

Yeah m8... also, keep an eye on how much is going in the tank... it may be your sender or float ball.

That's a good point, will do.

maxitoman007 said:

I haven't driven the car in a while so there is some rust on the rotors that may be causing some dragging but would that be enough to drop my mpg by 10 points (assuming around 20 is average)?

Click to expand...

i doubt that, any service rust will wear away quite quickly after you start driving the car.
Things you could check:
after you drove the car for a few miles step out of the car and walk around it and touch each rim. If 1 wheel feels excessively hot that brake may have a stuck caliper piston or (in case of a rear wheel) a dragging handbrake shoe.
On older cars the same symptom may also be caused by a deteriorated flexhose. They expand inward causing the pressure inside the caliper to not be released when you release the brake pedal.

How is the coolant temperature? If it stays too cold all the time the engine might run on too rich of a mixture all the time. This can be caused by a thermostat being stuck in the open position.
Or by a visco coupling that keeps the cooling fan engaged all the time.
Or by a broken coolant temp sensor that tells the ECU that the coolant is cold all the time causing a cold start mixture to be injected all the time.

Thanks for the reply. I have a wideband so I can monitor AFRs to make sure I?m not running excessively rich, but I have yet to do the ?hot rim? check. I will give that a shot when I get the chance.

Ok so it sounds like from everyones comments you need to actually calc your mpg correctly before even trying to fix anything.

This is true