[Ralph50]

My daughters 96 Lesabre-custom, 115,000 miles, 3.8L.
I got a code 301 misfire so I figured its time for a tune up. I replaced with AC delco plugs-OE-7 dollars each and AC delco wires. Seemed to run ok for about a week then when the car was started cold, 20F deg, and it appeared to have a loud lifter noise and I got the 301 code again. Today replaced the oil filter (fram 3387A) and oil (10w30) and used oil additive Lucas oil stablizer. I generally do not use these products but (what the H). I hear so much of the UIM problems so I pressure checked 15psi the coolant system. It lost about 1/2 lb pressure in 15 minutes. Is that OK? The oil was dirty and no foaming or milky substance. You might be thinking why is he checking the coolant system, cause when I add coolant to the bottle it is gone within a week. So I checked the system, hoses and bottle. I was thinking could coolant leaking around the lifters and cause them to stick?
So any ideas of what is causing the lifter noise?
Also I was thinking of flushing and replacing with the green antifreeze.

thx