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I have a 1990 YJ with the 4 cyl TBI, I am currently having ecm problems, I have had my ecm rebuilt/rebooted twice and each time the same thing occurs. It runs fine for a year or so then the ecm fails and it won't send signal to the injectors to fire when I try to start it, just cranks. If I spay starter fluid while starting it runs fine and restarts if it only sits for a short time. Thought it was the fuel pump but this last time three weeks ago new ecm installed and it ran great for two weeks than the same thing occurs. Problem is this jeep is in Mexico so limited electrical experience here on the island. Has anybody had this problem and are there relays in the ecm that can be replaced?
 

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How sure are you that it is the computer? The most common fault in the PCM is the three capacitors in there. The 80s/90s caps were sub par, so by now most of them have blown. I've repaired a handful of control boards and if one is bad, usually it's quite obvious.

The PCM in YJs is quite limited, mostly just a bunch of relays, capacitors, and resistors. The only "memory" it has is small things like injector pulse and spark.

If you have new/rebuild PCMs going bad, you likely have an electrical short or too much current flow going through the circuit. The resistors in the PCM shed excess voltage and current as heat. If there is too much voltage or current, then there will be too much heat for the heat sinks to handle. This can burn solder or components.

The easiest thing to do is to check for overcharging of the battery. This is controlled by a voltage regulator. See how many volts you read when the Jeep is running. Don't rely on the dash gauge, use a voltmeter or multimeter.
 
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