Miata overheating help!

From what I have read, I think the undertray is the the first thing to try, it developes a low pressure area behind the rad so there is flow through it. The fans can never cool enough for the engine under load.

If I remember correctly, one of mine had an aluminum coolant elbow on the driver's side that was full of corrosion.

+1 on the undertray. They can make a substantial difference.

One of the guys on MR2OC had a similar problem, he eventually found part of a shop rag in the inlet of the radiator. Also, how much do you trust that there is a new water pump in the engine?

On my last one, I replaced the coolant elbow that Woody mentioned with a one-part silicone hose from FM, that also made a massive difference, but that car wasn't running hot in the first place.

Also worth noting: for every five thermostats I buy, one is bad right out of the box. Might want to try another or test it at home on the stove in an old pot.

The under tray will certainly help. I'd definitely check that pipe in the lower radiator hose.

1.6s seem to be a little more marginal than the 1.8s when it comes to cooling. One thing you can do is wire the second AC fan to come on with the main fan, it can only help. A fan that's not spinning is a restriction.

Don't part the car out just yet. There could be an air bubble or two possibly in the system. It also can be that the water flow is not good, like a bad water pump or the under tray that you're missing.

The undertray helps, but I've heard its good for only about 5-10 degrees on coolant at speed. Enough to go from marginal cooling to insufficient cooling, but probably not to go from "good" to "awful". There's gotta be something else going on.

Verify your cooling system with all the hints listed above. You may also want to verify your timing. Too far off and it'll heat up quick under load.