Bad gas mileage

[86bandit]

Looking for some help on this EFI stuff. My car is an 87 LX trunk with a 91 drivetrain, T-5 and 3.55 gears. Normally, I was getting about 24 mpg in combination driving but after changing to GT-40 heads and a Typhoon intake along with a 65mm TB and a 73mm C&L meter mileage dropped to at best 20mpg. So far I've done a complete tune-up, Motorsport wires, swapped back to stock MAF, replaced ECU with spare A9, reset TPS, checked timing, new K&N cone and checked fuel pressure with no improvement . I'm thinking maybe the fuel filter needs replacing (done about 10K ago but could be) or perhaps the ECT is going bad. Any helpful hints on how to test ECT? Or any suggestions on things I may have overlooked? 'Preciate it

 

[MadMatt5oh]

Thats quite a drop but judging from your mods not unusual from what Ive been reading.

 

[86bandit]

Sorry can't buy a 16% or so drop in economy from just those changes. My old 85 with a carb still got 20 mpg with approximately the same mods. Fuel injection should do way better than that IMO. Guess I'll change the filter tomorrow and try to locate the ECT and change it on principle.

 

[bgjohnson]

do you have an adjustable fuel pressure regulator?

 

[86bandit]

Nope, but the fuel pressure looks good at idle and while driving. Tried that for a couple of days just to see what it was doing too.

 

[jrichker]

Dump the codes and see what the computer says is wrong…

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.

See http://www.troublecodes.net/Ford/
OR
See http://www.dalidesign.com/hbook/eectest.html for more descriptive help
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

 

[Black Sun 5.0]

You think that's bad? I oerformed similar mods on my '91 and went from 27mpg highway to 14mpg, wherever or however I drive. Still pointlessly trying to find that cause of the problem.

 

[Nitrous Tommy]

You think that's bad? I oerformed similar mods on my '91 and went from 27mpg highway to 14mpg, wherever or however I drive. Still pointlessly trying to find that cause of the problem.

It still good to me I get 10mpg

 

[siege]

You think that's bad? I oerformed similar mods on my '91 and went from 27mpg highway to 14mpg, wherever or however I drive. Still pointlessly trying to find that cause of the problem.

try letting the car sit for 4 weeks then changing valvesprings and headgaskets and going from 18 city to 10 city...still workin on it