'88 GT Driveability Problem


Founding Member
Nov 13, 1999
Palm Beach Gardens, FL
Been having problems getting my '88 back to roadworthy condition. For now, I'll skip over the fuel pressure problems which have been solved. Also ohmed the ACT & ECT, both within specs. Pulled the 2-10 pins apart, 1st time in 23 years, no corrosion but lubed with Corrosion Block

Been reading jrichker help lately. Got the Equus scanner & extension cable, definitely worth the $50 (through Amazon) for the pair.

FYI, my '88 has the Ford Motorsports MAF conversion kit. 55mm MAF was just replaced with C&L 76mm MAF, starts & idled, didn't get chance to go further, because...

Car starts & runs fine idling in the driveway, vacuum is normal 20inHG. Upon mild acceleration it bucks/jerks, hesitates/stumbles. On a cool day, even after idling in the driveway for 30 min, it can take a few miles before the issue occurs. I can even get a couple pounds of boost. Once the bucking starts, vacuum gage bounces all over between 0 & 20. Had this issue for 10-15 years, never could figure it out, didn't happen all the time. Now has been getting much worse & car is undriveable.

KOEO O-codes are 67,81,82,95. C-codes are 29,95. 67 can be ignored because the clutch wasn't depressed. 95 means nothing because the fuel pumps are working properly. 81 & 82 indicated Air Diverter Solenoid & indeed the TAB solenoid was determined to be bad by pulling the TAB, connecting the MityVac hand vacuum & actuating with a 12V source. TAD was fine, EGR Regulator okay, too. TAB wouldn't hold vacuum for more than 5 sec energized or not. Seems to explain why it somewhat works. Installed new TAB from Ford dealer (same price as everybody online), same codes generated. It appears TAD is normally closed, when actuated vacuum is dumped. I believe TAD is normally open, but "mechanicers" never describe in easy to understand "sparky" terms. Lubed all connectors with Corrosion Block.

Have not yet tried KOER codes. I'm confused why the KOEO would generate the same codes after the TAB replacement.

I'm ending the 1st post of this thread with a quote from jrichker:

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.

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.
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