Have you ever put together a project car, or installed an engine or a set of gauges only to find that the coolant temperature reads too high? It’s happened a lot to us, and it’s rarely the gauge that is at fault. One quick way to double-check right away if the gauge or sender is at fault is to use an infrared temp gun on various parts of the cooling system to observe the heat flow out of the engine and through the radiator. If it’s a lot different than the gauge, you’ll need to dig a little deeper. We recently got some tips for troubleshooting temperature gauges from the experts at Classic Instruments. Here’s what Jake at Classic Instruments had to say:
“We get calls from time to time from customers who are having problems with a temp gauge that's reading too high. Here are four common causes to check before you assume the gauge itself is faulty:”
Bad gauge ground. Ensure the gauge has a good chassis ground wire attached to its terminal. A bad ground will cause the gauge to use the sender as its ground point and cause the gauge to read inaccurately high.
The sender probe is not completely submerged in the engine’s coolant. This is caused when thread adapters are used to convert the sender thread size to the intake manifold or head thread size. The adapter keeps the probe from getting all the way into the coolant. Classic Instruments has temperature senders with 1/8-, ¼-, and ½-inch NPT standards as well as a 12mm thread for use on GM LS engines.
Air pockets in the cooling system. This will cause the temperature gauge to read high and then come back down when the thermostat opens and pushes the air pocket past the sender probe. The gauge pointer will continue to rise and fall as the air pocket moves past the sender.
Close proximity to external heat sources. This occurs mostly when the sender is installed in the engine head where it is close to the exhaust. The high temperature of the exhaust can artificially raise the reading of the temperature gauge by 10-15 degrees. If possible mount the temperature sender in the intake manifold or try shielding it from other heat sources.