'91 3.0L 6 Cyl. making crazy noise...

Ok, I really hope you guys can help! I have a '91 3.0L 6 Cyl., and the other day I filled the tank with probably not the greatest of fuels (it was Citgo Regular) and since then it has began acting VERY weird. The truck has no thermostat, due to an overheating problem with the previous owner, so we were going to try a lower temp one. Well, it is now making a noise that sounds like there is a penny ratteling inside the throttle! The other EXTREMELY weird thing, is that the temp gauge on the dash is fluctuating between off, and nearing red-line. Just changed the oil, and there was NO milky-ness, or foam. Checked fluids, and all was fine. I plan on flushing the cooling system and refilling with fresh. Any advice on this would be MUCH appriciated!

Flush the coolant system. Get a thermostat. Scan the computer for fault codes. Sounds like it might be pinging... Your MAF 'could' be dirty. Don't clean it with carb cleaner. We will need more details in order to give you better advise.

Pete