99 Honda Civic LX SOHC 1.6L engine overhauled 4 months ago. The deck of the engine block was resurfaced along with the cylinder head due to an overheating situation due to a defective coolant fan switch. I have a new radiator, thermostat < ECT sensor and fan comes on @ around 185 degrees. Engine will only overheat in traffic. Only when engine is cold, will set check engine light with a P0304 due to a misfire, when the engine runs for about 2 to 3 minutes misfire goes away. I have swapped spark plugs from #4 cyl to #1 cyl, and I still get P0304. As for the overheating, looses coolant in 3 to 4 days of driving. Pressure tested and no external leaks, and we did a block tester and turned out negative. Cylinder head gasket my next step, any suggestion out there??

 

Jump the fan switch and test drive with the fan on all the time.

If the misfiring is due to weak sparks, a bad plug wire is the most likely cause. Especially aftermarket wires.

With the engine cold, open the radiator cap to release any pressure then close it again. Squeeze the top hose, it should be soft. Now start the engine and rev it while you squeeze the hose. Nothing should happen until the engine warms up considerably. If you feel pressure build up right away there is a leak in the engine such as the head gasket.