I have a 1990 Miata with 200,000 miles. I have been having a problem with what I think is valve clatter mostly upon start-up after a run at highway speeds. The clatter would last for a few seconds after a start-up when the car sat a few minutes after a highway run (like when I stopped for gas). The problem would not occur if my Miata was allowed to cool. I believe valves and injectors are clean because I run 44K by BG Products through it every spring.

First I tried an oil additive that was supposed to clean the valve area. Then we ran 20/50 oil in the summer. What seemed to work until recently was 5/40 synthetic oil with STP (I could get 3000 miles out of an oil change without the problem). I just had the oil changed and I am getting the clatter worse than I have in a long time.

I am looking for ideas to try now. Thinking of just changing the oil again to see if that helps. Looking forward to your suggestions.