I'd like to add a little insight to this since I've turned quite a few wrenches to date.
These engines are extremely similar to a BMW M62 4.4 liter V8 - which is an amazing powerplant in its own right, but also suffers from similar issues that I've been reading here. So what we've got is a
coil-over-plug setup, where the "boot" feeds down through a tube that's O-ringed in the M62. By the sound of things, the 3.9 Liter we have works the same way - 4 O-rings and a
valve cover gasket. If the O-rings fail, oil gets in, pools up around the plug, boils, cooks the boot, ruins the
coil.
The BMW
coils are generally Bremi's or Bosch's with a giant heat-sink surrounding it - the T-birds are much smaller - and I suspect prone to failing due to heat-soak. Generally when a
coil like this starts to fail, it'll sometimes arc right against the tube the boot feeds down in to, which generally can cause a misfire. This is pretty easy to spot because you'll see a visible discoloration or burn mark on the
coil near where the boot attaches. If the
coil is failing internally within all the resin insulation, well... that's a different story.
Each
coil is
supposed to act like an "In-Cylinder Sensor" so when a misfire starts to develop, you're
supposed to get a CEL on the dash with a corresponding "Misfire Cylinder X" code. Sometimes the misfires come and go, so the
ECU will continually get battered with a fault code that comes and goes constantly.
In any event, the best thing to do in this case is stop as soon as you notice the misfire. Pull the
coils and
plugs and inspect them (I know, file this under 'duh' but you never know, this might help someone less mechanically familiar). If you have a fouled plug, you'll notice it even if you're not familiar with engines (one of these things will not look like the other... one will be sooty, pretty black, maybe even have a carbon chunk on it). Replace the
coil and the plug immediately.
The pinned thread in the subforum has a great link for a set of
8 coils on
Amazon for less than 50 bucks - just do that. I have only owned my Bird for 6 days and I've got a set on order "just in case" because 50 dollars for 8 is an absolute blessing compared to the 90-140 dollars EACH the BMW ones I used to go through cost.
If you have 1 fouled one, go ahead and replace all 8 with that set so they're all new, replace all 8
plugs, and keep the remaining old
coils as "rainy-day spares" in case you ever get stranded somewhere with a misfire. It's just cheap insurance.
