The Stovebolt Site Forums The Shop Area Engine Shop Temperature gremlin

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

Check the wire between the dash and the sending unit for a bare spot in the insulation. It might be that you're getting an intermittent ground that is pegging the gauge occasionally.
Jerry

Jerry, thanks. I'll run a temporary replacement wire.

Well, the wire was good. Same results with the temporary wire. I'm going to run a leakdown test on the engine tomorrow. Maybe the gauge is accurate and I have a head gasket that has just started to fail, enough to raise the coolant temp so that there is a noticeable difference at the gauge needle but not so bad that it is overheating. Even though the mechanical gauge shows the engine running in an acceptable range, I never used it on this engine in the past so I don't know where the dash gauge needle would be when the mechanical gauge measures 180 to 195. The engine is original and it has never been out of the truck, so a leakdown test will be good anyway. With 67K miles, I know I'm on borrowed time when it comes to a rebuild.

Matt

Well, I'm going to leave the gauge alone for now. When I did the leak down test, I had a small amount of air passing into the cooling system. I'm hoping it's just a bad head gasket and not a crack in the block or the head.

The engine has made it sixty years without a rebuild. Not bad. Compression readings front to back are 120, 118, 42, 40, 98, 110. I'm hoping that doesn't indicate a crack in the head or the block between #3 and #4. We'll see when I tear it down.

I have a replacement 261 built and ready as a replacement. My plan was to eventually switch out the two, but I've been procrastinating.

Matt