Diagnosing with a Temperature Gun

The ability to quickly determine the temperature of a part without any disassembly is a wonderful aid when things go wrong. All it requires is an infrared non-contact temperature gun. In some cases, heat will be an indication of a problem while in others it will be a lack of temperature.

Engine misfire: To determine which cylinder is misfiring on either a gas or diesel engine, measure the temperature as close to the exhaust port of the cylinder head as possible. Compare this reading with the other cylinders. If the cylinder is not burning the fuel it will be cooler than the others.

Confirmation of engine coolant gauge: Measure the temperature at the sending unit for the gauge. It should read within reason of the gauge. If not, the sending unit/gauge/wiring is the problem.

Engine oil temperature: Shoot the oil pan and if possible, the oil filter. There will be a small amount of heat loss, but it will be close enough for diagnostics.

Blockage in radiator / intercooler / oil cooler / heater core: Follow the liquid or air flow into the heat exchanger. It should drop in temperature as it travels through it. Look for either excessively high or low temperature spots to find a blockage.

Excessive brake drag: Measure the drum or rotor temperature to find a dragging brake shoe or pad.

Stuck thermostat: When stuck closed the temperature will be high on the engine side of the thermostat and low on the outlet. If stuck open it will be almost the same on both sides of the flow in and out.

Tires: When balancing tire pressure on a machine that is in the sun keep in mind that for every 10 degrees F change the pressure is skewed by one psi. For example, the ambient temperature is 60 degrees, and you want 32 psi in all tires. The temperature of the tires in the sun is 90 degrees F. Those will need to read 35 psi on the gauge to be 32 psi when they drop to 60 degrees F.

Bearings: A bearing will run hotter when it is starting to fail. Find a noisy bearing on any machine by taking its temperature.
The possibilities are endless if you just apply some American farmer ingenuity.

Octane versus Cetane

Ready for a pop quiz? OK, what is the difference between octane and cetane? A widespread misconception is that the terms refer to almost the same aspect of the respective fuel. Both indeed identify a characteristic of combustibility, but they are as different as soybeans are from corn in terms of nitrogen scavenging.

The definition of octane is the fuel’s ability to resist combustion from pressure or heat and wait for the arcing of the spark plug… the higher the octane, the better the ability to resist auto-combustion. A low octane fuel will be prone to auto-combust via heat or pressure.

When gasoline auto-combusts, multiple flames are expanding in the bore instead of one. There is a flame as the result of auto-combustion along with one from the arcing of the spark plug.

When the two flames collide, it is heard as knock or ping. The octane rating of gasoline has nothing to do with its energy content. Often the components used to raise the octane steal some of the potential energy when measured in Btu. Cetane number is the exact opposite. It is the fuel’s eagerness to ignite from the heat created in the bore by the compression of the air.

The higher the cetane number, the shorter the ignition delay and the better the ignition quality. As with octane, cetane provides an audible qualifier. Higher cetane fuel will create a quieter combustion event that enjoys less diesel knock than a fuel with a low number.

As an aside, the engine will also run smoother, be more responsive, and usually return better fuel economy. When designing a gas engine, the goal is for it to be octane tolerant, meaning it can have a normal combustion event on low octane fuel. The aim of a diesel engine is to not be cetane sensitive The design should promote a short amount of ignition delay, even with low cetane fuel.

For most life from any engine, it is essential to understand the difference between the two fuels and how they are rated.