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Diesel Tractor Running Rough? The Fix Is In The Smoke Color!

Diesel Tractor Running Rough? The Fix Is In The Smoke Color!

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Paul Brown

If your diesel tractor is running rough, it can be difficult to know why or how to fix it. I’ve been doing a lot of research on this lately and here’s what I’ve learned.

What Does It Mean When a Diesel Tractor Is Running Rough? When a diesel tractor is running rough, the three most common causes are:

  • Incorrect fuel-to-air ratio
  • Fuel is not being burned effectively
  • excess oil

You can quickly identify the cause based on the color of smoke coming out of the exhaust pipe.

Many people who own a diesel tractor would be surprised to know that diesel tractors are a fairly recent phenomenon, not becoming popular until the 1960s and 1970s (source).

Before diesel engines, tractors were typically powered by gasoline, kerosene, or liquified propane.

There are several reasons that a diesel tractor may run rough. If you’re interested in learning more about what causes a diesel tractor to run rough and what you can do in this situation, read on.

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What Color of Smoke Do You See?

It’s very common for an old diesel engine to be a rough running one. In many cases, this means that the engine will idle more often than optimal. When this happens, you will observe a “hiccup” noise.

Generally, you will be able to figure out what is wrong with your diesel engine by taking note of the color of smoke that is coming out of the exhaust.

If your diesel engine is in good condition, you shouldn’t see any smoke at all.

Black Smoke

This is the most common situation. It means that there is an imbalance in the air to fuel ratio. Basically, some of the fuel was not properly burned and turned into soot.

This can be caused by faulty air filters, faulty glow plugs, faulty injectors or injector pumps, or bad EGR valves. These particular issues will make it hard to start your engine, particularly when it is cold outside. However, they are typically easy to fix.

Usually, in this situation, the engine will only be rough for a short period while you are starting it up but at some point may begin losing power. For more details, see What to Do When a Tractor Is Blowing Black Smoke and Has No Power

White Smoke

If you see white smoke, this means that the fuel that is entering the combustion chamber is not being properly burned. Common causes of this situation are problems with the fuel injection system, proper injector pump timing, and low cylinder compression.

If you’re seeing white smoke, the engine will likely idle roughly when it is cold and hot, and the problem will occur on and off.

When diesel engines get older, the fuel injection system can malfunction, causing the injector to squirt instead of mist, as it should. When the timing becomes very off, it can lead to wear and tear on the cylinder walls and piston rings, which can result in wasted combustion.

Note: If the tractor is stalling, read this guide.

Blue Smoke

If you see blue smoke coming out of the exhaust pipe, this means there is excess lubricating oil within the cylinders of the engine. When it burns, it comes out as blue smoke.

This means that the engine oil is going places where it shouldn’t; in a healthy engine, it won’t be getting into places where it can burn like this.

If this is the case, you will typically observe rough startups, with everything smoothing out after approximately 30 seconds.

Common causes of blue smoke in a diesel tractor include:

  • faulty injector pumps or lift pumps
  • worn-out piston rings or cylinders
  • head gasket failures
  • worn-out valve stem oil seals
  • turbocharger problems.

What Causes a Diesel Engine to Run Rough?

Diesel engines are very fuel-efficient, not to mention durable and powerful. However, they are also prone to problems in cold weather and idling (source).

When the weather gets cold, there are a few things that can make it hard to start a diesel engine. The oil in the crankcase and the motor oil in many cases will get thicker.

The colder the thicker oil will often create an increased drag, which will slow down the cranking speed to the point where you may not be able to start the engine.

Additionally, battery output will go down, making it so that there are fewer amps available to crank the diesel engine.

What exactly you should do when the engine is idling roughly depends on the exact problem. The following are some examples of some of the specific problems that can occur and how you should address them.

More Common Causes

There are many potential issues that can be the cause of rough idling in a diesel engine. Here are more common issues that should be checked:

  • Dipstick and engine oil. If the oil is too thick, you might need to switch to one of a lighter viscosity for your winter driving. If the weather is extremely cold, you might want to change it to a CG-4 rated synthetic motor oil.
  • Minimum cranking speed. The RPM that is needed to start the engine is not always going to be the same, but you will need a minimum of 100 RPM when it is cold and 180 RPM when it is hot if you have a 6.2L or 6.5L diesel engine with a stanadyne rotary injection pump.
  • Battery and charging system. Make sure that the battery is sufficiently charged, the output of the charging system is adequate, and that the cable connections and amp draw are functioning well.

If you find that everything is okay in all of the above areas, you can start troubleshooting for other potential problems.

Fuel Issues

Diesel oil is made of heavier hydrocarbons than regular gasoline. These compounds cause it to solidify when temperatures drop down to 10-40°F (-12.2° to 4.4°C). In some cases, wax crystals can also form in the water/fuel separator, which can block the system.

If this is the case, take the tractor to a warm place so that it can thaw out. Replace the water/fuel separator if needed. You can then either drain the tank and refill it with diesel fuel, or add an appropriate fuel conditioner additive to the tank.

If you want to prevent this occurrence, you can install a fuel heater.

Fuel contamination is also common. Condensation that forms when it is cold outside, meaning water gets into the fuel tank, is a common cause. Alternatively, certain microbes can get into the diesel fuel and form unwanted residues that can clog up the system. 

If this is what happens, you need to drain and clean your fuel tank and use a biocide when replacing the fuel.

Glow Plug Problems

Many diesel tractors will have glow plugs to help with starting up the engine when it’s cold. These are powered by a relay and timer that will direct voltage to the plugs until the timer runs out. However, sometimes the voltage continues to go to the glow plugs until they burn out.

You can check your glow plugs by measuring their resistance and continuity. If there is too much resistance or not enough continuity, the glow plug is not doing its job.


As popular as diesel tractors have become, they can malfunction in very inconvenient ways because of different issues that can arise with the engines.

However, the good news is that you can fairly easily diagnose the issue most of the time. You just need to pay attention to the color of smoke that you see and go from there.

Of course, what you need to do after that varies quite a bit, based on what exactly the problem could be. If you can’t figure it out yourself, you’ll likely get the right help when you seek out a professional’s opinion.