Agustin Jimenez
Technical Editor
October 2, 2015

We’ve all been there before. You’re idling in bumper-to-bumper traffic and all of a sudden your beloved 5.0L Fox-body Mustang starts to pop and sputter a bit. While the issue might clear itself up and go unnoticed in the short term, it can lead to bigger trouble over time. Usually the Check Engine light coming on should be a clear indicator of a problem that needs to be addressed, so it is best to deal with the issue soon rather than hold off until it’s too late.

For instance, you might have a faulty oxygen sensor that’s telling your ECU that there is a lean condition because of a false reading it took that isn’t really there. Of course, this will typically trigger a rich air/fuel mixture that could wreak havoc on your catalytic converters if ignored long enough. This means you’ll be pulling some of your modification funds to replace those expensive precious metal pollution filters.

So what’s a Fox-body owner to do when the Check Engine light comes on or stays on while driving around? Check for trouble codes of course. Sure, you could spend the money on a good Ford-specific EEC-IV diagnostic code reader, but why do that when you could just as easily test for trouble codes manually? Yes, you heard that right. The 1986-1993 Ford Mustang’s EEC-IV has the capability to manually test for codes. Not just that, but you can also check the Continuous Memory codes that are stored in the system. These codes can give you an idea about other systems that might be having problems that you might otherwise not catch; you can even perform a cylinder balance test that will pinpoint a misfire or a cylinder that might have lower compression compared to the others.

What you’ll need

A paperclip will work fine to make a bypass jumper connection at the test port, but we prefer to use a simple electrical wire about 3 inches in length with crimped-on spade terminals on both ends for a solid connection. If you don’t have access to a repair manual, do a quick online search for a list of trouble codes for the 5.0L EFI Fox Mustang. Beyond that, all you need is a piece of paper and a pen to write down the number of times the check engine light flashes and a few minutes of your time. Make sure you bring the car up to operating temperature before checking for codes, as this will increase accuracy.

Where to make the bypass connection on the test port

The EEC-IV (also known as OBD-I) test port is located just behind the driver-side strut tower (1994-1995 will have it behind the passenger-side strut tower) and is mounted against the inner fender. The test port will have a cover that holds it and a smaller square port. Free them from the cover and you should see a large test port that resembles a house and a smaller square plug that has one wire leading to it. Take your bypass jumper connection (or paperclip) and connect one end to the smaller square terminal. This terminal is known as the Self Test Input. Take the other end of your bypass connection and hook it to the upper right corner of the larger test port. If you look at it as a house, the correct port would be the upper window near the roof on the right. This terminal is known as the Signal Return. Once you have the bypass connection hooked to both the Signal Input and the Signal Return, you can proceed to the actual test.

How to manually read the codes

Open the ignition switch to the On position without starting the car. This will initialize the Key On, Engine Off (KOEO) test. Watch the Check Engine light closely because it will begin to flash. The first flashes will be very quick, but don’t bother counting them, as they aren’t actual codes. Once the light starts to flash slower, pay attention. These are the hard trouble codes that need to be addressed first. Count each flash until it stops flashing. If there is a brief pause followed by more flashes, combine the two flashes to make a two-digit code. For instance, our car started off with five flashes followed by eight. This means we had trouble code 58, which is a problem with the vane air temperature sensor having an input that is greater than the self-test maximum. Keep in mind that trouble codes will be separated by a longer pauses. We only had one code in our case.

The codes will repeat twice and will immediately be followed by a slightly longer pause, and then a separator pulse will flash. Once this happens, the system will begin to check the Continuous Memory codes that are stored on the computer. Here’s where you get to see what other codes might be lingering or momentarily tripping throughout your routine drives. You would check these codes much the same way as the hard codes mentioned above. Once the Continuous Memory codes have cycled four times, you can turn the ignition key off for roughly 10 seconds.

After a moment you can fire up the engine, at which point the idle will increase a bit followed by the Check Engine light flashing four times in a row. This indicates how many cylinders you have (each flash counts for two cylinders). Once it has flashed four times (on a V-8 car), the Key On, Engine Running (KOER) test will commence. Check the codes just the same as you did for the Continuous Memory codes. Write them all down and look them up in either a repair manual or your Internet cheat sheet.

Cylinder balance test

Once your KOER test is complete (after all the stored trouble codes cycle four times) you can initiate a cylinder balance test. Activate the test by quickly tapping the throttle. The idle will rise and fall a bit, but don’t worry because it’s just the good old EEC-IV doing its job. The codes will appear in a different format from all the previous tests because the flashes now hold a value of 10. Each flash is worth 10, so if the Check Engine light flashes seven times, this would mean code 70, which means that cylinder No. 7 has failed the test. This could be due to an ignition problem or a compression leak. While the code won’t pinpoint the actual cause, it will get you in the ballpark and show you where to look. One to eight flashes correspond to their respective cylinder, while nine flashes indicates code 90, which means that all the cylinders passed the test.

Clearing the codes

Generally we would stay away from clearing the codes unless you’ve already fixed all the trouble codes you initially had. You can do this by disconnecting the battery for about five minutes. This will trigger code 15; this means that the Keep Alive Memory (KAM) test failed. This is done so you don’t cheat the sniffer test by clearing trouble codes whenever you want. Every time the codes are cleared, the ECU will have to relearn its driving pattern by completing a full drive cycle where every potential driving situation is simulated, and mileage can vary from 30 miles to 1,000 miles in some vehicles. To clear code 15, you will need to perform a KOEO self-test. Once you turn the ignition switch on and the codes start to flash, you’ll have to disconnect the Self Test Input (STI) jumper wire. Once you’ve done that you can turn the ignition off and then run another KOEO self-test to make sure that code 15 is no longer present in the Continuous Memory codes.