Bone stock 90 GT - 10mpg??

If you haven't, pull codes for peace of mind. Otherwise, it sounds quite reasonable to me. If you guys ever get out for a weekend trip or some highway driving, mileage should go way up.

See the mods in my sig, but my car averages 18 MPG with mixed driving. Sumpin' 'aint right! Either she's got a lead foot and not telling you about it, or someone's draining your tank.

i have had good luck with seafoam. try putting a can of that in with a full tank. im thinking along the lines of mike with a bad injector too.

fwiw, i just got 130 miles on half a tank with my 425+ rwhp notch in the city. i've got atleast 5 pulls with my foot in the radiator on that tank too.

An inoperative injector would be noticeable and would not allow a car to pass emissions with flying colors.

HISSIN50 said:

If you haven't, pull codes for peace of mind. Otherwise, it sounds quite reasonable to me. If you guys ever get out for a weekend trip or some highway driving, mileage should go way up.

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i'm with hissin on this one...

Take into account any long stays in drive-thru lines and long redlights, at which time you are essentially getting 0 MPH. Also if the car has the original cats, could they possibly be clogged (in which case you would be down on power as well) ?

I converted my dad's built 89 5.0 to a carb setup and it gets 11 mpg around town.

Cylinder balance test:

Revised 21-Nov-2010 to improve readability of the cylinder balance test.

Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode (see dumping the codes below).

Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady.

The computer will then shut off each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures. Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should.

See the Chilton’s Shop manual for the complete test procedure.

Dumping the computer trouble codes
Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

Here's the link 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.

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.

Here's the link 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.

See Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical





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.

Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder balance test. Low compression on one of these cylinders rules out the injectors as being the most likely cause of the problem. Look at cylinders that fail the cylinder balance test but have good compression. These cylinders either have a bad injector, bad spark plug or spark plug wire. Move the wire and then the spark plug to another cylinder and run the cylinder balance test again. If it follows the moved wire or spark plug, you have found the problem. If the same cylinder fails the test again, the injector is bad. If different cylinders fail the cylinder balance test, you have ignition problems or wiring problems in the 10 pin black & white electrical connectors located by the EGR.

How to do a compression test:
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the throttle wide open, crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 psi. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

See the link to my site for details on how to build your own blow down type compression tester.

run some seafoam through the intake and the gas tank...

I used to live about 3 miles from work and would drive about 6 miles a day. My car would get about 12 miles to the gallon on a week when I went only to work, but on any long trips it would get 18-19 mpg. So that doesn't sound too far off.

Mustang5L5 said:

It doesn't sound that far off....but every once in awhile someone chimes in claiming their 400 hp supercharged 5.0 gets 29 mpg highway.

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i get 27 so far with my 440 rwhp s/c notch sue me.

I go 5-7 miles of city driving each way to work, and average 190-210 miles per tank. I usually put in 14-14.8 gallons at a time.

Any idea how old the O2 sensors are? If you think they are over 6 years old maybe time to put some fresh ones in. Just a thought...

Does the car get to operating temp? Cold engine = rich = poo poo gas mileage. at 5 - 7 miles one way sounds about right.