The high energy ignition (HEI) ignition system has been a vast improvement over the old standard cap, rotor and point ignition system used in the past. The HEI design incorporates the vacuum advance mechanism, the ignition coil and a permanent magnet assembly that has a pole lined with teeth and a pickup coil. Only a few models have the external coil, but most today have the coil combined with the cap. Bad HEI ignition distributors give off some warning signs that can be diagnosed by the vehicle owner.
Engine Fails to Start
If the positive hot wire from the battery that feeds the distributor has no voltage to it, the HEI distributor will not function. The starter will crank the engine but there will be no fire coming from any of the spark plug wires or spark plugs. With the ignition key on, grounding the lead of a test light against a metal source and placing the test light probe on the "BAT" side of the distributor pole will determine voltage to the distributor. If the test light does not illuminate, the battery voltage is absent. Check the battery charge.
Plug Wire Engine Miss
Any type of constant engine miss that resembles a dead cylinder, can point to a faulty HEI plug wire that has a corroded connection or too much resistance. Plug wires that measure 15 to 25 inches in length, normally carry 400 ohm resistance at low engine rpm, and 15,000 ohms resistance at high engine rpm. The resistance of each plug wire can be measured by hooking up the leads of an ohm meter to each free end of the plug wire. Plug wires that measure "infinity" have broken connections.
Reduced Fuel Economy
The HEI system controls the spark timing changes for the engine, which regulates emissions, engine performance and fuel economy. Any reduction in fuel economy can indicate the electronic spark timing (EST) in the HEI control system has failed. This system works in conjunction with the vehicle's computer, and will announce a failure via a trouble code light on the dashboard.
An HEI distributor that has a worn pickup coil or corroded magnetic poles will produce a weak spark, with insufficient voltage to ignite the combustion gases. This can be seen by a sporadic miss when the engine pulls under a heavy load or the vehicle has to climb a hill or steep mountain pass. Testing for a weak spark requires pulling the plug from the engine and grounding it against a metal source while the engine runs. A yellow or intermittent spark indicates weak HEI coil output.
Electrical Arc and Shorts
Examining the HEI distributor at night with the hood up can pinpoint any stray electrical arcing from the cap, wire connections, distributor cap retainer and connector terminals. Cracks in the cap will be evident at the top spark plug wire poles, where the pole necks have the least material insulation. Look for blue-white bolts of electricity jumping from pole to pole or from the bottom of the cap to engine ground. Electrical arcing will be accompanied by an audible "clicking" or "snapping" noise.
If the engine requires repeated cranking to start, it could indicate worn or carbon-corroded cap electrodes that have increased in gap size or chipped, losing material. Overall poor engine performance and weak acceleration points to weak spark in the HEI system.
Smog Check Failure
Insufficient spark or coil voltage in the HEI distributor will allow excessive hydrocarbons and carbon monoxide emissions. This results when raw gas has not burnt completely in the combustion chamber and has passed through the catalytic converter. Weak ignition spark will also contribute to catalytic converter failure by soaking the palladium pellets with fuel and destroying the chemical process within the converter.