Engine Performance problem
1998 Ford Ranger 6 cyl Two Wheel Drive Automatic 98001 miles
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Randomly Spark plugs # 6 # 5 and # 4 coolant foul out and set codes 306, 305, & 304
No coolant in oil! – Slight loss of coolant. No external coolant leaks. Engine does not over heat, temperature stays normal when adding approx. 1 ½ quarts of coolant each week.
Change the spark plugs and erasing (resetting) codes engine runs fine for 3 to 4 days before
fouling any spark plugs.
Past experience would tell me the coolant is getting into the engine before the intake valves
as I do not have a heating problem.
4.0 OHV engine. Eighth digit VIM X

I would check headgasket and intake manifold gaskets