Re: 89 Ranger
Code 34-EGR valve position (EVP) sensor was above 0.65 volts when the ECA commanded the EGR to close. PCM pin #27, wire color is BR-LG.
Maybe the EGR is stuck open.
Code 41-Heated exhaust gas oxygen (HEGO) sensor shows a lean mixture when the self test commanded rich. PCM pin #29, wire color is DG-PL.
Possibly from the EGR
Code 88-Dual plug inhibit control signal had no effect on Ô¨Āring both spark plugs. PCM pin #32, wire color is grey-OR.
Get the EGR fixed first.
Code 18-Ignition diagnostic monitor signal (IDM) was missing with the engine running or the electronic spark timing signal (SPOUT) was grounded. IDM PCM pin #4, wire color is BK-YL, SPOUT PCM pin #36, wire color is YL-LG.
The computer sends out a timing advance correction to the ICM over the SPOUT wire and then looks for the change on the IDM wire.
IDM - Ignition Diagnostic Monitor
SPOUT - Spark Output Signal from EEC/PCM
ICM - Ignition Control Module
You might check the ECT or the IAT sensor for your problem.
Also check the SPOUT/IDM wire going to ground.
ECT - Engine Coolant Temperature sensor and it has two wires on it.
IAT - Intake Air Temp
IDM (feedback) is a Dark Green with a Yellow stripe wire on the distributor connector.
IDM is pin 4 on the 60 pin ECM connector.