gap the plugs between 42 and 46
Bad coil pack or spark plugs...
i have a 1988 ford ranger xlt 4 cylinder and my spark size is 42-46 hope this will help
you will need a 5/8 spark plug wrench let the engine cool, then disconnect the negative battery cable remove the spark plug wire from the first plug and remove and replace the plug dont forget to gap the new plugs between 42 and 46 and repeat on the next one be sure to only do 1 at a time as not to mess up the firing order
C3I ELECTRONIC SPARK TIMING (EST) FAILURE is code 42 timing can be off by a lot of factors check vacume leaks plugs wires etc..
42% = .42
According to all-parts.com/diagnostics/mechanics/Ford_Mustang_GT.html, "If this is a 5.0 litre, I have the answer... And this one took a lot of hours to find.... The spark plugs have to be gapped at .52 to .54 on this..... Not the .42 to .46 like all the other Fords."
Chilton's Auto Repair Manual 1964-1971 states that all 390's should use BF 42 plugs and the plug gaps should be .034 inch.
42% = 42/100 = 21/50.
gap is 42