Engine - Power Loss, Hesitation, Shudder/Backfire

Bulletin No. T 83-25
File In Group 6D
Number 15
Date Jan. '83

ENGINE PERFORMANCE DIAGNOSIS HEI IGNITION SYSTEM

Power loss under load, hesitation, shudder and/or medium throttle intake manifold backfire on HEI equipped cars may be caused by an irregularity in the secondary side of the ignition system (cracked spark plug insulator, plug wire leakage, etc.). These symptoms are often mistakenly diagnosed as a DFI or carburetion related condition.

The following procedures may be used in diagnosing this condition:

1. With the engine running, visually inspect the spark plug mounting area of the engine block for a blue arc as the spark shorts to ground. This is more visible in a darkened area.

2. With the engine running, apply a water mist using a spray bottle to the plugs, wires, and cap. This procedure may reveal an electrical arc and/or cause the engine to quit.

3. Attach the lead of a standard electrical test light to the tach terminal of the distributor.

With the engine running, touch the probe of the test light to the positive terminal of the battery. If the test light maintains a steady brightness level with only minor variations in intensity, the secondary side of the ignition is functioning properly. If the test light does vary in intensity with noticeable difference in brightness, the secondary side of the ignition should be examined further for evidence of the following conditions:

a. Cracked plug
b. Plug wire integrity
c. Distributor cap integrity
d. Rotor integrity

FIGURE 1 - CRACKED SPARK PLUG PATTERN:

FIGURE 2 - NORMAL PATTERN:

A diagnostic scope displaying the primary pattern may also be used in diagnosing this condition. The firing line will be irregular and higher, and the oscillations from the primary field collapsing will be distorted when a secondary circuit problem exists (refer to Figure 1 & 2).
When attaching the diagnostic scope, the ISC motor should be disconnected and the air conditioning control should be in the "OFF" position to insure constant engine speed.