Back in the days of Nirvana and Clinton, Ford's trademark Explorer wasn't quite the beast that it would become in later years. Originally, the Explorer landed somewhere between the Ranger and F-150 in terms of weight class and came only with a 4.0-liter V-6. But, while fairly large for a V-6, the 4.0-liter didn't rely solely on its displacement for power; its computerized control system and coil pack ignition helped to make the most of every drop of fuel going in. Though this system has historically proven pretty reliable -- at least compared to older distributor-driven systems -- time and wear still take their toll. Sooner or later, you'll run into some sort of coil failure symptoms.
Coil Pack Basics
On a traditional engine, all of the cylinders get their power from a single ignition coil; a distributor mechanism routes power from that coil to the individual spark plugs. The coil pack on the Explorer V-6 is actually three coils bundled together; the front coils power the plugs on cylinders one and five, the middle powers six and two and the rear coil powers cylinders four and three. When the computer triggers the coil, it sends power to both of the cylinders. But since only one cylinder at a time actually needs the energy, this arrangement is known as a "wasted spark ignition."
Misfire
The primary result of any coil malfunction is misfire, or failure to cause a spark and ignite the air/fuel mixture in the cylinders. If a single cylinder misfires, you've got a V-5 instead of a V-6. But this won't happen with the Explorer's 4.0-liter, because each coil triggers two cylinders. So, while a single-cylinder misfire is bad, you're guaranteed to drop from a V-6 to a V-4 when you have a coil failure with this particular arrangement.
Misfire and Conditions
Hot wires don't conduct electricity as well as cold wires; and because hot wires trap electricity, they get even hotter with use, causing a cascade effect of failure in the coil. Because of this, the coil may work well at first, and gradually lose power and cause misfire over the course of a few seconds or minutes (depending upon the severity of failure). Misfires are particularly problematic here not only because you lose two cylinders instead of one, but also because V-6 engines -- because of their odd cylinder count per side -- are inherently prone to vibration. Although losing two cylinders will drop you down to an even cylinder count per side, it'll still throw off everything the factory did to balance the engine in the first place.
Symptoms and Codes
The first thing you'll notice is a severe drop in horsepower under acceleration, along with a noticeable drop in power. As the coil heats up, it'll fail to ignite the mixture even under less aggressive conditions like idle and part-throttle. When the cylinder misfires, raw fuel will pour out of the engine and exhaust pipe, resulting in a noticeable stench of raw fuel from the exhaust. This will, in turn, trigger a check-engine light when the oxygen sensor detects raw fuel in the exhaust. But the 1994 Explorer doesn't use the Onboard Diagnostics, Series II protocol that all modern cars do. So, you'll either have to take it to someone with a specialized Ford OBD1 code reader, or pull the codes yourself using the proscribed procedure. It's a bit involved, but the information's available online; enter the keywords "1994 explorer dtc codes" into your browser, and peruse the results. "DTC" stands for "diagnostic trouble codes," which is the technical term for the numerical codes that the computer uses to communicate panic.
