That code has to do with the circuit between the distributor and the ECM. If the dist has been changed and you are having the same problem , there is likely a short in the wiring. please see attached diagnostic tree. I hope you're good with electical, It's a little complicated.
another possibility is the e23 code is independant of your problem, which could be maybe a vacuum leak (which i don't think would throw a code, i may be wrong)?
Here are the notes that go with the diagnostics:
The EST distributor produces one 5 volt Distributor Reference Pulse each time that a cylinder reaches 10°BTDC (base timing). The reference pulses are sent from the distributor to the PCM/ECM on circuit 430 (the distributor reference signal). The PCM/ECM adds spark timing information to the reference pulses received from the HEI and sends out 5 volt Electronic Spark Timing pulses (EST) on circuit 423. The PCM/ECM can choose to control spark timing. The HEI bypass circuit 424 is turned on by the PCM/ECM (approximately 5 volts) when the PCM/ECM wishes to control timing. When the PCM/ECM wants the HEI module to control timing, the HEI bypass circuit is turned off and the HEI module grounds circuit 423 (EST). The PCM/ECM sends EST pulses to the HEI; the HEI module shorts the EST pulses to ground when the bypass signal from the PCM/ECM to the module is low (less than .5 volts).
TEST CONDITION #1 CRANK :
1. Bypass line low (0 volts).
2. References pulses being sent by HEI module to the PCM/ECM.
TEST CONDITION #2 RUN :
1. Bypass line high (5 volts).
2. References pulses being received by the PCM/ECM.
FAILURE CONDITION #1 :
EST pulses are detected on CKT 423 in bypass.
FAILURE CONDITION #2 :
No EST pulses are detected on CKT 423.
ACTION TAKEN :
1. PCM/ECM turns on the "SERVICE ENGINE SOON" telltale.
2. The PCM/ECM controlled spark is disabled for ignition cycle.
NOTES ON FAULT TREE :
1. Four jumper wires must be obtained for use in the diagnostic procedure. The jumpers should be about 12 inches long with a male and female weatherpack connector on either end. With the distributor connector disconnected, use the jumpers to reconnect the terminals per instructions provided on the code E23 chart.
2. With only distributor reference and distributor reference ground jumpered, engine will run at backup spark. Checking for proper ground connection between PCM/ECM and the engine and the PCM/ECM and HEI.
3. Checking for PCM/ECM providing an EST output on circuit 423.
4. Checking for HEI module able to ground the EST signal with an open bypass circuit.
NOTE ON INTERMITTENTS:
Possible causes of code E23 are:
1. Reference pulses are being received by the PCM/ECM and the bypass line is low, but the PCM/ECM detects pulses on the EST line. This could be caused by the bypass line shorted to voltage at the distributor.
2. Reference pulses are being received by the PCM/ECM and the bypass line is high, and the PCM/ECM does not detect a signal on the EST line. This could be caused by an open or shorted to ground on the EST line or the bypass line open or a shorted to ground.
NOTES ON FAULT TREE :
5. Checking for HEI module able to recognize a voltage on the bypass circuit and to stop grounding EST (PCM/ECM controlling timing).
6. Checking for bypass signal to the module. If bypass is being sent by the PCM/ECM to the HEI and if the module is interpreting bypass voltage correctly, then the module will switch "OFF" the ground to the EST.
7. If the chart leads to "EST circuit is OK" then a fault may exist in the 4 way weatherpack. Check for proper connector mating and for pins backing out of the weatherpack. If no trouble is found with the connector, reconnect the four way connector, clear codes and retest.