Is Your Temperature OK
| 31 May 2008
If you have a first generation Mustang, you know that engine temperature can sometimes be a big question mark. If it’s boiling over, you know something needs to be replaced or repaired. But what if the radiator isn’t boiling over and yet the temperature gauge is giving you a reading that you question? Maybe it reads very hot and you think it’s wrong. Or maybe you’ve driven the car for several miles, the engine should be up to temperature, but the gauge has barely moved from the cold position? What do you do and how do you know if engine temperature is in the correct range?
You’ve checked the temperature gauge and it appears to be working properly, you’ve replaced the temperature sending unit, you’ve replaced the voltage regulator for the instrument cluster, you’ve changed the thermostat, and maybe you’ve even replaced the water pump and the radiator. Still this 40-year old automotive icon refuses to show the “right” temperature on the gauge. If it’s not a restomod, you’d probably like to keep the original look which means keeping the original gauge in the original instrument cluster. That means installing an auxiliary gauge isn’t an option. But you won’t be comfortable until you know what the operating temperature really is. Well, there’s a measuring instrument that can help you out.
The instrument won’t give you minute to minute temperature monitoring but it will let you take snapshots of the temperature. The snapshots will either give you piece of mind or let you know that you need to do more work to correct temperature that’s out of range. The instrument is a non-contact infrared thermometer. There are a number of them on the market and the cost varies widely. A reliable one for automotive testing can be purchased for around $150. If you buy one, make sure it has laser focusing because the infrared beam is most accurate when it’s focused correctly. The one I use is a Raytek AUTOPRO ST25. As you can see in the picture, it looks like a caricature of a handgun and it operates by pointing, squeezing the trigger (switch), and focusing. The temperature is reported on an integral digital display.
OK, you bought the instrument and now that you have it in your hand it’s time to decide where to read a representative engine temperature? You can focus on the temperature sending unit where it screws into the intake manifold and that will tell you what the sensor is “seeing.” But what you really want to know is the closest approximation of actual coolant temperature where the coolant exits the engine. The best place to get that reading is at the thermostat housing after the engine is warmed up and the thermostat is open. You can tell when the thermostat opens by the temperature of the upper radiator hose. When it gets hot, the thermostat is opening. If it doesn’t get hot but there’s no question that the engine is getting hot, you have a thermostat stuck in the closed position. It will need to be replaced before you do anything else.
When the hose starts to get hot, give the engine a few minutes to stabilize and then focus the infra red beam right on the thermostat housing. Avoid focusing on a shiny bright surface such as a stainless steel hose clamp or a chrome plated housing because the infrared circuitry is probably calibrated to read the emissivity of a flat black surface. There won’t be a significant difference between flat black and other relatively dark colors but bright shiny surfaces can distort the reading. If there’s a question in your mind put a piece of flat black electrical tape on the housing and then focus the beam on the tape.
Obviously, the infrared thermometer won’t fix a problem but it will sure let you know where you stand when the temperature gauge may not be telling the truth. And there are a number of other uses for the thermometer.
Suppose you have an engine with a steady miss and you want to locate the dead cylinder. You can pull spark plug wires one at a time to see which cylinder is on vacation but you’d rather not risk being burned or shocked. Take the infrared thermometer and focus on the exhaust manifold at the point where it attaches to an exhaust port in the cylinder head. Read the manifold temperature at each exhaust port. The one with the “coldest” reading is probably connected to the dead cylinder. When you’ve identified the cylinder, you can concentrate on diagnosing the problem. You may have a bad spark plug, a bad plug wire or something more serious but now you know where to focus.
A short list of other uses for the thermometer might include track temperature, tire temperature, and brake temperature for open track and driving school cars or track temperature at the dragstrip. And around the house, other uses could include temperature distribution from room to room, freezer temperature, refrigerator temperature, and electrical outlet temperature.