For some time now I've been chasing a random stumble. I eventually tracked it down to a misfire / ignition. I could see in the data logs that the RPM would drop, then catch up again. Shortly after that the wideband would show a lean condition. It turned out to be the TFI. I replaced it with a DY1077 motorcraft TFI (they no longer say motorcraft on them, but it is a ford part). Theres a bit of a story behind it though.
Some time ago the stock TFI failed. Car died, towed home wouldn't start. I had an old fox distributor with a working TFI, so I stuffed that in the fender, plugged in the TFI and it started right up. I removed the TFI from the stock distributor, modified it to fit and installed it with some dielectric grease. For a few years, the car ran just fine, so I didn't expect there to be an issue. I can only assume that the ignition demands of the new motor made the stumble present itself. Thats when I did some research. Long story short, the 86-93 Fox TFI are Push Start, and dwell is controlled by the TFI module alone. 94/95 are TFI-ICCD (increased computer controlled dwell) and the ECU will control a few additional parameters related to dwell (coil charging). When a Fox TFI is used in a 94/94, the ECU defaults to PIP to control dwell, which is very short. here is a great write up!
http://www.myo-p.com/Ford-EEC/EEC%20Help%20files/Files/TFI_grey_or_black.html
now the car runs awesome with the correct TFI installed. I'm also playing around with the spark tables. I found that adding an additional 2 degrees really didn't add to the performance / seat of the pants feel. But it wasn't pinging either. So when I needed fuel I used regular rather than premium, and it still runs awesome. I do have the fuel type set to 10% ethanol, so the calibration accounts for that.
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