Instantaneous Fuel Economy Always Indicates 0 Miles/Gal Or 99 Miles/Gal

Ensure speedometer operates properly. Verify continuity and absence of short circuits in circuit No. 305 (Light Blue/Black wire) between cluster connector "A" (terminal No. 11) and Data Output Line (DOL) circuit (terminal No. 34) at EEC-IV connector. See Figure . If continuity does not exist or short circuits are present, repair circuit No. 305 as necessary.
If continuity exists and short circuits do not exist, ensure a DOL signal from EEC-IV module exists. With key off, connect breakout box to EEC-IV module or check for signal output at cluster connector "A" terminal No. 11 (Light Blue/Black wire). Start engine. Measure voltage at test pin No. 34 (DOL circuit) at breakout box or at connector "A" terminal No. 11. Voltage should be 10-12 volts.
NOTE: EEC-IV module computes fuel usage signal and transmits it over DOL circuit.
If EEC-IV module signal on DOL circuit is present, replace EIC (fuel computer module). If signal is absent, replace EEC-IV module. Reconnect components, and retest system.