10 Heavy Truck Diagnostics Terms You Should Know

10 Heavy Truck Diagnostics Terms You Should Know

There are hundreds of millions of drivers around the world, and all of them have vehicles that need maintenance from time to time. But there are far less mechanics than there are drivers, so unfortunately, the majority of drivers have a high likelihood of encountering a problem at some point that they can’t identify or explain.

“It wiggles, you know? Really fast, when I go fast.”
“There’s kind of a, I don’t know, chongakongakonk sound. Is that bad?”
“It feels like the tires are loud, and when I try to turn, there’s a…a shriek?”

While all of those may describe legitimately terrible trouble under your truck’s hood, they don’t really give a mechanic much more than a vague idea of what the problem might be. And that’s bad news for you, because if the technician fixing your heavy truck has to spend a couple of hours finding out what’s really going on – especially if your sound effects and explanation were the cause of the misunderstanding – your costs for labour are going to get much higher. 

Here are some of the top ten truck diagnostics terms that mechanics and technicians use all the time. Remember them for the next time you feel something off while you’re driving – it could save you serious headache!


1. Backfire: Often heard about in pop culture (and much less common nowadays than they used to be), a backfire is a gunshot-like sound that comes from the engine or tailpipe. They are caused by unburnt fuel in the exhaust system igniting and rapidly combusting, and are known to cause a loss of power and motion. See a video clip of an engine backfire here.

​2. Bottoming: This refers to excessive noise or harsh vibrations that are often felt through the steering wheel or passenger area, often when going over bumps. First place to look for the problem? The shock absorbers and other parts of the shock system, which help mitigate road imperfections while driving.

3. Bucking: Does your engine hesitate or “lurch”, as the transmission slips while trying to change gears? Then you’re familiar with bucking. The most likely cause is in the fuel line, as the engine isn’t getting the fuel that it needs to maintain power.

4. Dieseling: If you turn off your engine, but the truck continues to burn fuel and run for a moment afterwards, that is known as dieseling – because it’s due to an air-fuel mixture combusting in the cylinder without a spark, like in a diesel engine. It can signify timing issues, temperature moderation problems, or any of several other similar factors.

5. Hesitation: When you accelerate and feel a sudden loss of power, that’s hesitation. It can be traced back to low fuel pressure, slowed ignition timing, poor quality fuel, low voltages/weak sparks…the list goes on. 

6. Knocking: Also called “detonation”, this is a rapid rattling noise that can be heard upon acceleration, and is often caused by fuel burning unevenly instead of in one smooth, continuous action.

7. Misfire: A category of hesitation, these happen when fuel in one or more of the engine’s cylinders fails to ignite properly and interrupts the flow of power.

8. Shimmy: A pronounced side-to-side motion that transmits from the tires through the steering wheel. While the details can change for each case, the main culprit behind it all is an imbalance of some type – in the tires, in the treads, in the brake rotors, etc. When they are not symmetrical, the mismatch is very noticeable.

9. Sluggish: If you’ve ever floored the gas and the car barely made its way down the road, or felt very jumpy while accelerating, you’ve encountered a sluggish engine. It means it’s just not delivering the horsepower that it should be, based on your power input.

10. Surge: Like the opposite of a hesitation, a surge is a sudden (usually upward) change in a vehicle’s engine speed. Like a lot of these problems, inconsistent fuel delivery and pressure is usually to blame. 



Remember these terms for the next time you’re bringing your vehicle in for our money-saving preventative maintenance or repair, and it will help the mechanic pinpoint the problem faster – getting you back on the road more quickly, and making sure the root cause is fixed.