Why does my gas mileage suck?

Lynx331 · Jun 3, 2005

Hey guys. I did some work to my 87 recently... and now my gas mileage completely blows. I am getting roughly 10 mpg, if that. 8 gallons got me 88 miles. So heres what i did to my car. I converted to mass air using a A9P Computer. I put on a good set of heads, a lumpy cam, new lifters, valvesprings, roller rockers, pushrods, Headers, Off road X pipe. The car ran a 13.50 at the track, but its also fuel starved like crazy since i am running a stock mass air meter, injectors, fuel pump, and stock regulator. I switched out the A9P for a A9L, and it seems like my gas mileage has gotten a bit worse. Im not even lead footing the car really cuz i dont like taking it past 3 grand because i know its leaning out. I changed the o2 sensors.. still the same thing. Why am i getting such bad gas mileage with all stock fuel parts?? What do you guys think?? Thanks for your help, i appreciate it.

jb89coupe · Jun 3, 2005

Lynx331 said:
Hey guys. I did some work to my 87 recently... and now my gas mileage completely blows. I am getting roughly 10 mpg, if that. 8 gallons got me 88 miles. So heres what i did to my car. I converted to mass air using a A9P Computer. I put on a good set of heads, a lumpy cam, new lifters, valvesprings, roller rockers, pushrods, Headers, Off road X pipe. The car ran a 13.50 at the track, but its also fuel starved like crazy since i am running a stock mass air meter, injectors, fuel pump, and stock regulator. I switched out the A9P for a A9L, and it seems like my gas mileage has gotten a bit worse. Im not even lead footing the car really cuz i dont like taking it past 3 grand because i know its leaning out. I changed the o2 sensors.. still the same thing. Why am i getting such bad gas mileage with all stock fuel parts?? What do you guys think?? Thanks for your help, i appreciate it.

Why do you think you're leaning out? With that kind of mileage I'd think you're running rich if anything, unless you're leaking fuel somewhere.

What heads and what cam are you running?
Your name would imply you have a 331, but I don't see it in your sig.

With my powerheads, ported gt40 intake, and 1.72 rockers (stock cam) I still get 20+mpg around town, and that's stomping on it every now and then. You should expect to lose some mileage with a bigger cam, but not that much!

Do you have the right speedo gear? This will throw your odometer way out of whack..

nickg42oz · Jun 3, 2005

Pull codes. I'm having the same problem now and I pulled codes and most my sensors that controll air/fuel ratios are giving me errors.

Insane3D · Jun 3, 2005

O2's are probably due for a change at the very least, and will have the biggest effect on mileage. Also a good idea to change out the ACT and ECT sensors, and clean off the wires in the MAF with some q-tips and rubbing alcohol. Reset the computer and you will be good to go.

The O2's are about $39ea, and the ACT and ECT are about $15 each. You can get both at your local Auto Zone, or on their site.

Good luck!

Lynx331 · Jun 3, 2005

The heads are cnc ported Gt-40's. I just recently changed my o2 sensors because a code came up saying the o2s werent switching. Now the only codes i have are the fuel pump montior that i didnt hook up when i did the maf conversion, and that my smog pump isnt working ( cuz its not hooked up ) other than that i dont have any codes.. thats why im at a loss oif what to do here... I kno its a modified v-8 mustang and its not going to get great gas mileage.. but there are guys making 12-15 with many more mods than me... I just really dont know what to do. My injectors do have 130,000 on them and 2 of them are missing the caps on the ends.. so a nice set of 24 lb'ers probably wouldnt hurt... i dunno.. im just so confused...

Insane3D · Jun 3, 2005

Couple of things that come to mind...

Is your car still speed density? I think that MAF stangs are generally more efficient. Injectors could be a possibility as well. You really are supposed to have those caps on. The new 24's I got had a different injectors design, so that might not be a bad idea...even a set of new 19's.

The 3:73's are probably another reason...

It's not likely one thing, but a bunch of things together. The ACT and ECT sensors play a big role in the adjustment of the fuel/air ratio, and your's are 18 years old unless you've changed them before.

Good luck!

Michael Yount · Jun 3, 2005

He said he converted to mass air and that he changed the O2's.

Pull the codes and see if anything else is up. How much stop and go driving is in your driving cycle? I can see 8 to 10 mpg in heavy stop and go for a 5.0 that's had a "lumpy" cam put it it. That usually means that your engine's ability to make good torque efficiently on the bottom end (idle to 2500 rpm) has gone out the window. You may end up wishing/wanting to put the stock cam back in. Many perform with MUCH quicker times than yours with the stock cam -- and mileage/drivability is usually significantly better. Sounds like new injectors/maf are in order anyhow.

Insane3D · Jun 3, 2005

Ooops..sorry..I should have read a little better.

It's a long shot, but are you sure the O2 harness is grounded?

Lynx331 · Jun 3, 2005

Im running AFM n-21, Theres a guy running the same cam with a s-trim and 110 octane and getting 20 mpg... I dont get it.. Actually i still have pretty decent low end... and i love that lumpy idle. What are the ACT and ECT sensors?

jb89coupe · Jun 3, 2005

act and ect are both on the lower intake. If they are that far out of range the computer will throw codes.. but they're not going to give you 8mpg.

You have a 302, right.. not a 331?

Stock fuel pressure regulator? What is your pressure reading?

You do have the correct speedo gear(s), right?

Michael Yount · Jun 3, 2005

lynx - if you have a good tune, and keep your foot out of it - the supercharger can actually make things MORE efficient. And, boost does wonders for low end.....

Bobert · Jun 3, 2005

Michael Yount said:
lynx - if you have a good tune, and keep your foot out of it - the supercharger can actually make things MORE efficient. And, boost does wonders for low end.....

Say huh? You're gonna have a bigger fuel system on a blown car, and that's always hurt mpg from what I've seen. Boost and low end aren't happening with a s-trim. And how is the motor constantly turning a blower going to make anything more efficient?

Michael Yount · Jun 3, 2005

On a properly designed system - at low rpm/small throttle openings (remember - keeping your foot out of it) the VE increases can be greater than the extra power consumed driving the supercharger (doesn't take much power to drive it from idle to 2500 rpm) -- that's how it can make things more efficient. And even the mosdest boost a centrifugal makes down low can significantly improve torque down there.

"If you have a good tune...." - if you've got your maf/ecu transfer functions matched properly, those bigger injectors will only inject what's called for at small throttle openings. But very few have a real good match...you've got to work at it with a good chip burner/tweecer/etc.

I'll stand by the post - even with a centrifugal, you can make decent mileage. Even better with a well tuned positive displacement blower. The key is a good tune AND driving VERY gently - something not often done with these.

The challenge is that most folks set up the supercharged cars with big boost and tune them for hard charging in the 4000-6000 rpm range without much regard for efficiency on the street.

Lynx331 · Jun 4, 2005

Yes my car is a 302. It still have the stock fuel pressure regulator. And last time i checked i believe it was at about 40 psi. Those injectors are really making me wonder tho... I dont know if those plastic ends make a difference or not.. but two of them are missing them. I cant really think of anything else it could be, because the computer comes up with no codes. The car smells pig rich at idle.. Im hoping a new Maf Sensor and some 24lb injectors will clear this up. What do you think? Any other suggestions?

Lynx331 · Jun 4, 2005

At idle.. when i jst start it up.. the gas pedal feels like a carburated car thats running very rich. I give it some gas it hesitates for a couple seconds then revs up.. it seem like as the car warms up it gets better. What do u think guys? New injectors and a mass air meter?

Bobert · Jun 4, 2005

Michael Yount said:
On a properly designed system - at low rpm/small throttle openings (remember - keeping your foot out of it) the VE increases can be greater than the extra power consumed driving the supercharger (doesn't take much power to drive it from idle to 2500 rpm) -- that's how it can make things more efficient. And even the mosdest boost a centrifugal makes down low can significantly improve torque down there.

"If you have a good tune...." - if you've got your maf/ecu transfer functions matched properly, those bigger injectors will only inject what's called for at small throttle openings. But very few have a real good match...you've got to work at it with a good chip burner/tweecer/etc.

I'll stand by the post - even with a centrifugal, you can make decent mileage. Even better with a well tuned positive displacement blower. The key is a good tune AND driving VERY gently - something not often done with these.

The challenge is that most folks set up the supercharged cars with big boost and tune them for hard charging in the 4000-6000 rpm range without much regard for efficiency on the street.

Well when you say "more efficient", I'm thinking of comparing mpg of a stock motor vs a blown motor with a bigger fuel system. I know of no blown or turbo cars that get at least as good gas mileage as a stock car. Now none of the cars I know of have had anything but dragstrip and backroads tuning, no dynos with widebands and custom chips. One friend with a s-trim is about to have a dyno session and chip burn, but I'll be suprised if it gets his mpg back up to stock.

As far as the VE increase, that only happens when you get boost, correct? Never owned a s-trim car, but I don't think they get much boost from idle to 2500 to make the burn more efficient so mpg will go up. Now I can believe guys get 20+ while cruising on the interstate with blown cars....just don't see them getting better than stock mpg.

ninety15.0 · Jun 4, 2005

Me and my buddy just installed a Kenne bell on his 94 gt a few weeks ago....and we also use the tweecer software to tune the car and after a couple weeks of messing with different tunes and scalars....his blown stang with 255 lph fuel pump and 42 lb injectors gets like 4 or 5 mpg better on the higway....with 3.73's. So i would have to agree with yount on this one...a properly tuned street driven 5.0 with a blower on it is much more efficient in low load street driving. Its a diferent story at WOT....

KnowPlay · Jun 4, 2005

ECT = engine coolant temperature sensor
Change it. Not with an Autozone or other cheap piece, but get a Motorcraft if possible. Next, borrow a scanner with as to check datastream info. More than likely you would see the O2 sensors both showing rich. Most ppl would then change them...this is a mistake. They are reading rich because your is running rich. I deal with this kinda diagnostic nightmare everyday.

Check ect voltage, tps voltage and insure that all sensors are getting their 5 volt reference. This can be done with a scan tool or a Ford breakout box. Only so many things control air/fuel. Here's some things that can cause rich/lean conditions.

Rich:
Non-functional ect showing COLD(default on most is -40F). ECM adds fuel to increase rpm/richness to heat the car.
Bad O2 heater. Not available on your car.
TPS being out-of-range.
IAT showing cold air constantly. Intake Air Temperature sensor.
Dirty Mass Air Meter
Bad ECM
Many more things, but these are the most common.

Lean:
Vaccum leak. And I say again VACUUM LEAK. Kinda common.
Dirty MAF
TPS out-of-range
O2 out-of-range
ECT showing hot when car is cold.
Dirty or clogged injectors.
LOW FUEL PRESSURE!
IAT showing hot air constantly.
Bad ECM.
Many others.
The caps on the injectors are dust covers. Will not affect anything. If it gets better as the car warms up, sure sounds like a bad ect. The hesitation at idle is a result of the car being rich.
I would pin out/check the main sensors named here. Insure all voltages are within proper range. Check fuel pressure. Check fuel pressure bleed off after killing the engine. A blocked fuel return line can play havoc on an EFI system. Bad fuel mileage is usually the result of an extreme rich condition. Check the TPS,ECT and fuel bleed off first. Hope this helps.

TheUser · Jun 4, 2005

KnowPlay said:
Lean:
Vaccum leak. And I say again VACUUM LEAK. Kinda common.

Doesn't a vacuum leak create a rich condition? The metered air is escaping, while the fuel is not, therefore there is enough fuel for the metered air, but since some of the air is escaping, there is not enough air for the fuel = rich

Lynx331 · Jun 4, 2005

Just a quick question.. if my Engine Coolant sensor wasnt working right... would my temperature gauge still work?

Why does my gas mileage suck?

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