The temperature gauge on my '57 with a 235 usually runs about 1/3 of the way toward hot after the engine is warmed up. About a month ago, it shot all the way to the hot side and pegged before I could safely pull over. It happened so quickly that I could actually watch the needle move. When I opened the hood, everything seemed fine. No steam, none of the smell that you get from an engine that is too hot, no ticking or 'heat noises', hoses all felt fine, coolant level was fine, etc. It ran as normal as I continued to drive it, but the gauge stayed way over to the hot side.
I flushed the cooling system and all was good. Coolant flowed freely out of the rear drain on the block. Water pump is good. Changed the thermostat for good measure. Problem remained.
I figured the sending unit might have gone bad, so I changed it out. No luck. Changed it a second time using an AC Delco. Still no luck. I'm not one to just throw parts at a problem, but the sending units are relatively inexpensive (except the AC Delco) and it seemed logical to think a faulty sending unit was causing the problem.
I borrowed the mechanical gauge from my engine run stand and temporarily connected it so that I could monitor the engine while I drove the truck. It indicated the temperature to be holding steady at 180 to 195 once the engine is warmed up, even on a 110 degree day. I use a 160 thermostat. Thinking that the thermostat might not be completely opening, I tested with it installed and with it removed. I get the same results either way with the mechanical gauge.
When I use an infrared thermometer, the temp at the sending unit and at the front of the head is about 200, input to the radiator is about 170, output from the radiator is about 155.
My experience with the temp gauges is that either they work or they don't. I've never had one suddenly start reading differently. I'm thinking that the problem is either that I'm using the wrong sending units (or I keep getting bad ones) or that my gauge has gone bad. I have a spare gauge stashed away somewhere, but I haven't tried it yet.
Any thoughts before I dig out the spare gauge?
Matt
