I see a 404 status code when I request your link: http://greatwallprojects.com/pingtest/
Please make sure that your application which runs on port 8081
responds to the request
GET/1.1 /pingtest/
In other words check if your app has an implementation for the /pingtest/
path.
You can always check with curl
what the direct answer from your node application is:
curl -i http://127.0.0.1:8081
Compare it with what you get from nginx.
While debugging nginx, looking at the logs is always helpful. Watch them in another console window with:
tail -f -n0 /var/log/nginx/*