Car Analogy

Ok, I think you can rule out temp in the cabin. Doesn't sound like it could have been hot enough inside the car to do anything. That leaves underhood heat.

So isolate your troubleshooting to underhood areas that could be getting too hot.

zeroptzero

Many Fords suffered from this exact problem years ago, it was related to a relay that was heat soaked and then shutting down. I don't know what part would suffer like that on the S2000 but the symptoms are identical to the Ford issue.

Since your car is stock, I assume the engine heat shield is still in place around the battery. Battery fluid/water can boil if it isn't protected from header heat.

MR.T

Inspect/replace the main relay.

https://www.s2ki.com/s2000/topic/104...while-driving/

Could be wiring too, but this guys story sounds almost identical to yours.

MR.T

Welcome to the club by the way.

Greyout

Thanks for the link, and thanks for the help everyone.

When I looked under the dash, the main relay was just laying there, not clipped down to anything. I think its worth a shot to replace it.

Greyout

UPDATE

So the problem is solved, and the fix is interesting enough that I wanted to share.

I got in about 500 miles since I last posted, and I was just about ready to be 100% satisfied that the new relay fixed it. After driving home from work, I was home for about 30 minutes and then was going to head back out again.

I hopped in, turned the key, hit the start button... it ran for about 1 second and DIED and it, again, WOULD NOT START!!

The way it sputtered to a stop, and then wouldn't start, just screamed fuel. Interestingly though, I could hear the pump prime. More interestingly though, I realized I was hearing the pump prime very loudly, louder then I normally do.

With past cars, when I would try to troubleshoot a non-start, I'd listen for the fuel pump, and if I heard any kind of noise back there, I'd cross fuel off my list and proceed to look for an electrical problem. With this car, it was no different.

However, the S2K was new to me, so I was not used to the normal fuel pump sound. In previous days of dealing with this problem, I'd hear the pump run, very obviously, and just think the pump was fine. I thought the pump was kinda loud, but just attributed it to being a 2 seat convertible, and not having much sound deadening between the cabin and fuel tank.

After several weeks of owning the car, and starting in my quiet garage, I have now become used to a normal fuel pump sound.

Today, when it failed to start, I actually had the thought as I reached for the start button 'hmmm, that was a loud fuel pump prime'. No start.

I tried to start it a few more times, and each time I would hear the extra-loud prime. It was so loud, I could actually hear the pump start running again over the sound of the starter.

In hindsight, I am not sure why I didn't notice this before. Its really loud. I'm going to blame this on the fact that each time I've dealt with this before, its been on the side of the road or in some other loud environment.

Soooo, like before, I let it sit 20-30 more minutes to cool off, and tried it again. This time, I hear the pump prime QUIETLY. Fires run up and runs smoothly.

So, my fuel pump is failing, and I've ordered a new one.

I just thought it was very interesting that when my fuel pump was in its failure mode, it was still making a sound, which I think would cause many people to scratch fuel off their list of problems. It stumped myself, a mom-and-pop shop, and a dealership.

So, lesson learned - a bad fuel pump is not always a quiet fuel pump.

Car Analogy

My technique for confirming fuel and or mixture issues is to try some starter fluid. If it starts up easily with the starter fluid, but won't stay running unless ypu keep giving squirts, its a fuel issue.

MR.T

Sounds like you've already ordered the pump, but to properly diagnose an fp problem, tapping the fuel rail is the proper way to go about it.
Just running a pressure gauge temporarily into the cabin would be enough. Then you could actually see what the fp was doing during these non-starts.

Hope this takes care of your issue though and thanks for updating. Don't forget to let us know how it pans out.