68 Stang Efi Conversion Issues

Key question is "did it run right in the donor vehicle?" and if it did, then you just need to find the lose wires under your hood and figure out what they plugged into in the '91.

Assuming everything should run right and is in good condition, did you swap over the MAP sensor? I think in 91 these were still speed density, so you should have what looks like a small square black box with a large vacuum line and 3(?) wires somewhere towards the rear of the motor (or on the donor firewall). It is easy to miss, kind of looks like a vacuum distribution tree at first glance.

It's a start anyways. You have a 91 motor, it wouldn't hurt to grab the manual for one and start going through the diagnostic procedure. . .not fun, good luck

make sure you swapped over everything the 91 had, including the mass airflow sensor. check for vacuum leaks, as well. you can try pulling codes from the computer to see what systems are being affected and run the required tests.

Treat it like a 91 Mustang. Check for codes, troubleshoot the wiring like the more modern car it almost is and post your tech questions under the Fox tech section. The Codes and the wiring diagram jrichker posts should get you going.

Did you read "scan for codes"? It is VERY important. Just unplugging and swapping parts will not tell you much on an EFI car, but will cost you money needlessly and frustrate you endlessly. This is really in you best interest.

I do not know how you were able to drive it with the TPS disconnected. The computer should have thought you were not opening the throttle, but had extra air coming in. I wonder if it is in bypass/limp home mode. Anyway, the fox section checklists from jrichker will tell you how to do it right without frying anything.

There is a great one on idle and surging issues, please use them .

@jrichker please help this EFI newbie. He needs to go to the fox section on how to check for codes. Can you repost for him?

I have one with an lcd display, had one with just a blinking led and switches that left with another car, but a paperclip work too. Go to the Fox section and search for the surging idle checklist.

Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

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.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
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 clutch (if present) is pressed to the floor, 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.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10




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 www.midwayautosupply.com/Equus-Digital-Ford-Code-Reader/dp/B000EW0KHW Equus - Digital Ford Code Reader (3145It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $22-$36.