Is your fuel-injected Fox Body misfiring when hot? Try relocating the TFI Module.

During the heat of this summer, I have had some troubles keeping my 5.0 running smoothly. It would stumble and miss whenever the engine got up to temperature, occasionally stalling. I want to share my experience and solution with others who may encounter this problem.

Fuel Injection was new to our Fox body cars when it came around in 1986. And as with any new technology, it had some problems. All in all, Ford did a good job in introducing this new tech to the market. However, arguably the most problematic component in the engine bay of any fuel injected fox body car is the TFI module.

TFI stands for Thick Film Ignition. This is the module which directly controls the ignition coil and thus the spark generation and timing of the engine. This unit is found on 2.3L, 3.8L, 5.0L and more. It affects many Ford cars beyond the Mustang, from 1983 through till 1995. A complete listing and other useful information can be found here.

As these modules are what directly energize the coil, they do generate heat. When introduced, they were mounted directly to the distributor, using it as a heat sink. These modules had three bladed terminals along the top of the casing. These plug into the pickup coil inside the distributor. The main 6 pin connector sends the pickup coil signal (PIP) to the main computer, and the computer sends back the spark timing signal (SPOUT).

The most common symptom of a failing TFI is missing and stumbling when hot. Once warm, the car will start to miss more and more, and may eventually die altogether. But, once it cools back down again, it will run just fine, like there was never any problem. In my case, my 5.0 would miss when idling, but never stalled. I might have to give it some gas to start a hot motor, though. You can diagnose this yourself. If your car is stumbling, open the hood and feel the TFI module quickly. If you can’t hold your hand on it for about 10 seconds or so, it is probably too hot.

Well, after a lawsuit in the early 90’s, Ford finally realized that using the distributor on the hot engine as a heat sink was not really the best idea. They then mounted the TFI in a remote location on its own heat sink. These modules did not have the three bladed terminals, and the PIP signal is now an input on the main connector. Differences between these modules is shown by the illustration below.

So, for those of us with a Distributor mounted TFI, we have to struggle with a stumbling motor that might leave us stranded. However, we can retrofit our cars with a heat sink and some soldering skills. Ford had a special tool for removing the TFI, as the 5.5 mm screw is hard to get to. A skinny deep socket can also work. I actually carried my dizzy into Autozone to find and test a socket on the screws before buying it, as I did not already have one that would fit.

The first step is to find a heat sink from a later Ford car. These can be found in some vans and V6 Cougars. I got mine from a 1993 Ford Aerostar (never thought I would find a useful part for my car from one of these!)

As our TFI has three bladed terminal connections, it will require removing a few of the heat sink fins to properly mount our modules. I was lucky enough to use a mill to machine mine down, but this can be done with a handsaw, pliers, and patience to carefully break them off. Be sure to use thermal paste between the TFI and the heat sink to ensure good heat transfer.

Finding a place to mount this heat sink on your car is very important. I originally moved mine to the passenger shock tower. I quickly found this to be a big mistake, as my TFI would then heat soak off of my aftermarket exhaust headers! I then decided upon a place in front of the AC condenser behind one of the driver side parking lights. This allows air to get to the heat sink from the grille, but not rain water.

Next is the fun part, the wiring. I would highly recommend having some soldering skills and proper tools and supplies. It will require a soldering iron, solder, shrink tube and/or electrical tape, crimp terminals, crimp tools, 18 gauge wire, wire strippers, some RF shielding tape or three-conductor shielded cable, a multimeter, and be willing to open up your existing wiring harness without fear. It goes without saying disconnect your battery before starting.

To connect the TFI bladed terminals back to the distributor will require some crimp blade terminals; three female for the TFI and three male for the pickup coil. These can be found at a hardware store. Be sure to get the correct size terminals that are the same size as the TFI blades. Make three lengths of wire to run from the TFI back to the dizzy, crimping the appropriate terminals on each end. Be very sure to connect the terminals up correctly. These three wires should have RF shielding tape to protect the signal.

Next is cutting and lengthening the existing harness for the main six pin connector. As you open it up, you’ll notice that the PIP, SPOUT, and ignition ground signal are RF shielded and the other three conductors are not. Be sure that as you extend these wires to maintain shielding on them. This ensures minimal electrical noise and that a good signal is sent to and from the main ECU.

After making these connections, tidy up your wiring harness, reconnect the battery, and start her back up. You should find that the TFI will run much cooler. Mine went from somewhere in the neighborhood of 180 degrees Fahrenheit down to nearly ambient temperature! Even in the 100 degree summer heat, my car has not missed or stumbled once since moving the module. It was more work than I had anticipated and took me a whole weekend by myself, but it was well worth the added reliability.

Thanks for sticking with me this far. I hope this information can be useful to someone else experiencing similar issues.