really bad feul mileage?

red-fox · Jun 7, 2011

So I recently bought a 1992 lx 5.0 and I have been getting really terrible gas mileage. It had 3:73's and removed cats other wise its stock. I ran a bunch of injector cleaner through it and it pretty much doubled my feul mileage but I'm still only getting about 10 maybe 15mpg highway on a good day (canadian gallons) Other guys with bone stockers around here are getting more around 25-30 mpg on better days. Do 3:73's really make it that much worse on gas? I was thinking about changing my o2 sensors incase their is somthing wrong with them. Is there anything else I should check?

2000xp8 · Jun 7, 2011

Well, 25 to 30 is shooting for the stars.
I'd say a high percentage of guys here get 20mpg or less, there seems to be a few oddities where guys suggest they are getting 25-30 but those are exceptions, not the norm.

I'd start with the basic tune up items, fresh plugs gapped correctly, K&N fliter in the box, good fuel fliter. Check the timing.

red-fox · Jun 7, 2011

I set gap on my plugs, I put in a new cap and rotor. Timing is good. I'll get a new air and feul filter and see how it goes. Thanks.

65ShelbyClone · Jun 7, 2011

Have you checked for KOEO/KOER codes? Tried a cylinder balance test? 3.73s should not cut your economy by half.

I was able to consistently average 21mpg in a stock '89 GT 5-speed in mixed driving with a best of 24mpg. With the longblock that's in my '68 right now, mileage dropped one mpg in that '89 GT.

What are "Canadian gallons?"

JustA5.0 · Jun 8, 2011

65ShelbyClone said:
What are "Canadian gallons?"

Lol I'm Canadian, and went 'huh?' to that. Red-Fox do you mean your MPG calculations are based on a 3.78L U.S. gallon?

I religiously keep track of fuel econ in all our cars. Everything emission/tune-up related is 10K or newer on my car (fresh full engine gasket job too), all smog stuff intact except no cats atm. With 4.10s I get 16MPG all day long. Good news is hammering on the car only nets me a 1MPG decrease as opposed to granny driving, have calculated both driving styles on full tanks.

Since you're filling in litres this will apply to convert in MPG
Take KMs driven on tank * 0.621 = Miles driven
Litres to fill tank * 0.264 = Gallons to fill
Then divide Miles by Gallons of course, to get MPG

If she's still under 15MPG, get new O2 Sensors, especially if unsure of age. But if you're getting around 18MPG I'd live with it personally.

You need the 4.6L before seeing 25+MPG consistently and that'd be mostly highway driving. My uncles 3:08 bonestock speed density can hit 25-27 on a pure highway run.

JustA5.0 · Jun 8, 2011

2000xp8 said:
there seems to be a few oddities where guys suggest they are getting 25-30 but those are exceptions, not the norm.

And quite possibly calculations errors, or checking not filled-up tanks a few times.

jrichker · Jun 8, 2011

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 3-Jun-2011. Removed the link to BATAuto.com and troublecodes.net instructions on codes and how to dump them. Post the codes you get and I will post 5.0 Mustang specific code definitions and fixes.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Equus - Digital Ford Code Reader (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.
Or for a nicer scanner see http://www.midwayautosupply.com/p-7208-equus-digital-ford-code-reader-3145.aspx– It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.

red-fox · Jun 8, 2011

I havent checked for any codes becuase my check engine light hasn't came on. I said canada gallon becuase a gallon is diffrent from metric to imperial, metric being 4.5L and imperial 3.7L. My dad has a diagnosis tool that I could use to see if I can come up with any codes, but usually if theres no light theres no code. The o2 sensors on the car are most likely a few years old, I'll change them anyway even if they are not the main problem.

2000xp8 · Jun 8, 2011

red-fox said:
, but usually if theres no light theres no code..

That's definitely not true for a foxbody mustang.

red-fox · Jun 8, 2011

^ Hm ok, I'll scan it tonight or tomorow and see what happens then

65ShelbyClone · Jun 8, 2011

I thinks it's ironic that, for all the Hell we get from Canadians for not adopting the metric system, you guys still use imperial gallons.

Anyway, codes can be stored in the ECU without causing a CEL; it's quite common, actually. Since your car is equipped with a CEL, all you need is a jumper wire to get the codes like jrichker has shown.

What makes you think the O2 sensors are not very old?

red-fox · Jun 8, 2011

Sorry if you misunderstood but I didn't mean that they are not very old. they most likely are fairly old if not the originals, so I'm going to change them out soon as I get around to it.

65ShelbyClone · Jun 8, 2011

If they have Ford ovals and part numbers on them, they are almost definitely original. I have one out an '89 GT somewhere around here.

red-fox · Jun 8, 2011

I put the scanner tool on it just now and nothing came up. I'll order new o2 sensors any way just for a start, hope it helps.

65ShelbyClone · Jun 8, 2011

Was that cold and with the engine off or at operating temperature with the engine running?

red-fox · Jun 8, 2011

operating temp engine off but key was on

JustA5.0 · Jun 9, 2011

red-fox said:
I said canada gallon becuase a gallon is diffrent from metric to imperial, metric being 4.5L and imperial 3.7L.

Reverse that...
Imperial Gallon (UK) = 4.546 Litres (Space taken by 10lbs of water, so yes technically a metric calculation)
US Liquid Gallon = 3.785 Litres

I take it you are doing your calculations based on a gallon being 4.5L then? Something is definitely wrong with your car or your calculating, if you are only seeing 10-15MPG readings. The MPG number should be higher if dividing into 4.5L, in my above example 16(US) MPG would be 20(UK) MPG.

Please try my calculations above, and let us know what MPG result you get. Most everyone's MPG claims on here will be based on a US Gallon, as most live in the U.S.

jrichker · Jun 9, 2011

red-fox said:
I put the scanner tool on it just now and nothing came up. I'll order new o2 sensors any way just for a start, hope it helps.

No code 11 (two flashes when the computer dumps the codes)? The 11 code is computer passed its internal self test.

If you don't get an 11, you have computer or wiring problems. They could cause all the problems you are describing.

Please check and repost.

red-fox · Jun 9, 2011

I will fill it up tomorow morning on my way to school and kepp track of my mpg through the weekend and try and get a more accurate reading. So the code 11 will not show on the computer scanning tool? only by the light flashing twice?

JustA5.0 · Jun 10, 2011

No jrichker explained it above.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems.

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