Hello fellow Buick enthusiasts, I am a new member to the forum even though I have often relied on all the help and advice posted by you guys,(and girls).My turn to pay it forward in regard to the many overheating issues that plague and perplex us. My 66 GS had been running hot at red lights. Went through all the obvious culprits that could be causing this to no avail. finally, I took my thermal temp. gun and checked the head temps.left side 200deg. rt side 300+. at first I thought it was a clogged muffler .Took it to the shop to put a new system in and found the riser valve in the exhaust manifold was rusted closed. The guy at the shop welded it closed....PROBLEM SOLVED! Hope this helps!