Despite best efforts, problem lingers
I have a 1994 Lincoln Town Car that had been intermittently stalling at low speeds and failing to start. After waiting 15 minutes or so, the car would eventually start. I have replaced the battery, spark plugs, coil pack and fuel pump. The last mechanic, who said the issue was the coil pack, said my fuel pump pressure was reading low, but I had it changed just two months ago. The car runs pretty well these days but did go through a brief period of intermittent roughness after the coil pack was changed. I amat a loss and almost out of money. Any ideas? — Desperate
I think you may have had two problems occurring and might still have one. Let’s look at your ignition system first and then possible fuel issues.
Your Lincoln’s 4.6-liter V8 engine employs a transitional distributor-less ignition system known as a waste spark system. There are four ignition coils, each firing two spark plugs using spark plug wires. The four coils are contained within two coil packs. If renewing one of the coil packs improved performance, it’s likely one of the coils within it was leaking spark, causing either an intermittent or continuous problem with two cylinders. A faulty spark plug wire can also cause problems with two cylinders as the loop from the coil through the two spark plugs and back may be interrupted.
Coil and wire performance can be easily checked using an OTC 6589 or similar spark tester (about $20, a super tool!). My students call it the spark plug from hell because it requires about three times the energy to jump as a normal sparkplug, really proving out ignition performance. Temporarily installed at the end of each spark plug wire (or coil-on-plug coil on newer engines), it verifies robust spark-making and spark delivery capabilities. Wrapping the spark plug wire boot or coil-on-plug (COP) coil in aluminum foil, touching engine metal, provides a convenient leakage path, proving or flunking the boot or coil’s insulation. This finds a lot of sketchy COP coils!
Your fuel system may have issues contributing to hard starting, rough running or power loss. Possible faults besides the pump include a faulty electrical circuit to the pump, a restricted fuel filter, or faulty pressure regulator. Verifying reliable power to the pump is more easily done at the inertia fuel-shut-off switch (pink and black wire) near the left trunk hinge. A restricted in-line filter could starve the engine of fuel, particularly at higher speed or load. It’s also possible a faulty fuel pressure regulator is not holding the specified pressure (the pump merely moves fuel, while the regulator makes pressure happen). Purchasing an inexpensive OTC, Actron or Equus fuel pressure gauge (about $40 at Amazon) and making its simple hook-up should point out any pump, pressures or flow problems, if there are any. These gauges come with fittings that easily attach to most Ford and GM systems.
Be sure to follow the safety instructions that come with the gauge, particularly when disconnecting it. Fuel pressure varies between about 35 pounds at idle (high vacuum, low engine load) to 45 pounds (higher engine load, low vacuum). If it falls below these values at any time, there’s a problem.
It’s also important to check the car for diagnostic trouble codes and follow any indicated code charts and procedures. A dirty or faulty mass air flow sensor could also be a suspect.