What Makes A Diesel Engine Run Rough?

Excess lubricating oil in the engine cylinders causes this problem during combustion. As a result, the surplus oil is burned and blue smoke is produced. The engine will normally start rough, but after about 30 seconds, the idle will settle out.

What can cause a diesel engine to run rough?

James Dunst, Bell’s resident master mechanic, explains the most common mechanical issues that he is questioned about in this series. He goes over the causes, what can be done about them (if anything can be done), and any other questions a technician would ask in order to properly diagnose and cure such an issue.

The Common Problem: Rough Running Engine

A properly functioning engine should be quiet and run smoothly. If your engine begins to behave in a way that makes you think it’s “running rough,” it’s most likely due to one of a few frequent causes. To rule out a simple explanation for the engine’s poor performance, a mechanic would look into these typical problems first.

Let’s take a moment to discuss how the mechanic should assess the problem before we look at each of these.

Checking the computer for diagnostic codes

Before replacing parts or performing service on an engine with a rough idle, there are a few things that must be done. Your vehicle will almost certainly feature an engine management computer if it was manufactured in 1981 or later. A variety of engine compartment components are monitored by the computer. When a problem occurs in a computer-monitored system, codes specific to that system are set and stored in the computer’s memory. The check engine light on the dashboard will glow as a result of this. A hard fault occurs when a light is on continuously and is the easiest problem to locate because the problem is present at the time. If the light flashes on and off, the fault is intermittent, implying the problem only existed for a short time before disappearing. Both of these scenarios will cause the computer to save codes for subsequent inquiry.

These diagnostic codes will last for fifty engine warming cycles in most vehicles. The codes will be removed by the computer if the problem does not recur. Avoid mechanics or shops that change parts without first performing adequate diagnostics. Checking for computer codes initially is critical since it may lead to the problem with the rough running. And this is exactly what you should expect a skilled technician to do if you approach them with a problem like this.

Mechanical reasons for a rough idle or engine miss

High-mileage automobiles with over 100,000 miles are the most prone to develop engine miss issues (although it can be less in some cases), and a compression test should be performed to rule this out. If one or more cylinders have low compression, it could indicate a more serious problem that has to be addressed right away.

A compression test is often performed by removing all spark plugs, inserting a compression gauge into the spark plug hole, and turning the engine over three times. Make a note of the reading and repeat for all of the cylinders. If any of the readings change by more than 20%, a second test is required to identify whether the variance problem is caused by a fault with the piston rings or valves.

How can a mechanic (or you) figure out what’s causing the problem? Next, put a little amount of oil into the cylinder with the low compression reading and repeat the compression test on that cylinder. The piston rings are the source of the problem if the compression rises from the prior test reading. The oil aids in the improvement of the seal, compensating for the loss of compression due to worn piston rings in the first reading. As a result, the compression in the second test would be higher.

If the compression does not increase the second time around, the issue is most likely a burned valve.

So now you know which type of mechanical malfunction is causing the issue.

If either fault is discovered, the engine must be disassembled in order to be repaired. When removing the valve covers, take special care to ensure that all of the valves are making the same amount of downward motion. Some engines have a problem where the camshaft lobes wear out and prevent the valve from opening, which can lead to misfiring. A good mechanic will be able to diagnose and repair the problem at this stage. However, it is unlikely to be an inexpensive cure.

Rough idle caused by a vacuum leak

A network of vacuum hoses may be found in most engine compartments, and they can wear out at any time. They become brittle and rigid with time, in other words. It’s just one of those things that happens over the course of an engine’s lifetime. A lean air/fuel condition will ensue if any of these hoses spring a leak, resulting in a rough idle from the subsequent engine misfire. Depending on the magnitude and location of the leak, the misfire could be in one or more cylinders.

Intake manifold gaskets, vacuum brake boosters, and vacuum supply tanks can all produce vacuum leaks. How can you know if a vacuum leak is the source of the problem? When driving a car with a slight vacuum leak, you’ll notice that everything feels okay at higher speeds or RPMs, but the engine runs rough when it’s idle. At idle, the engine may appear to rev up and down on its own, but it’s more probable that it’ll just rev up excessively and stay there. In any case, that’s a clear sign of a vacuum leak.

A mechanic will look for a lean code set in the engine computer to establish the presence of a vacuum leak.

This makes sense because a vacuum hose leak allows more oxygen into the system than is necessary. As a result, the reading is lean — not enough gasoline, too much air. If the air/fuel combination is incorrect, the engine will not run correctly.

Once a mechanic has determined that the problem is caused by a vacuum leak, they must determine where the leak is.

It would be beneficial if you could locate it on your own. Who knows, maybe you’ll be able to fix it then. When trying to figure out where the leak is coming from, the first thing you should do is listen for a hissing sound in the engine compartment. It could be something as simple as a dislodged vacuum hose; in this case, all you have to do is reinstall it. If you think a component such as a vacuum brake booster or a vacuum supply tank is causing the leak, crimp the hose to the suspected component with needle tip pliers. The idle will smooth out if the component being checked is the source of the problem.

Finally, if you suspect the intake manifold gaskets are leaking, spray a substance like WD40 along the intake manifold’s edge while the engine is running. When you spray the WD40 on the gaskets, the idle will smooth out or change.

Rough idle caused by dirty fuel injectors

You can expect more than a choppy idle if unclean fuel injectors are the source of the problem. The most common cause of poor gas mileage is dirty fuel injectors. If the injectors are clogged, the engine’s lack of performance will be even more apparent when the vehicle is accelerated, resulting in a larger fuel consumption.

The best way to diagnose this problem is to take it to a repair shop with an exhaust gas analyzer. Carbon monoxide and hydrocarbon levels will be elevated due to restricted injectors. This is because anything that obstructs an optimum fuel spray causes the fuel to burn inefficiently, resulting in increased carbon monoxide in the exhaust. Because complete combustion produces water and carbon dioxide as byproducts, we know this. Incomplete or incorrect combustion occurs when the fuel does not burn fully, resulting in greater carbon monoxide and hydrocarbon measurements. Because raw fuel enters the exhaust system when an engine misses, you obtain greater hydrocarbon readings from the exhaust. Both of these diseases are caused by restricted injectors.

If dirty injectors are the issue, a mechanic will clean them with an injectable cleaning concentrate. This is a problem that begs to be addressed through prevention. The easiest approach to avoid problems like this in the future is to use an injector cleaner gas additive to prevent them before they happen.

Rough idle caused by carburetor problems

When we talk about carburetors, we’re referring about older automobiles with more mileage as well as small engines. When the engine gets up to temperature, a significant volume of black exhaust smoke is one of the signs of a carburetor malfunction. A properly functioning carbureted system should not emit large amounts of black smoke; this is an indication that something is wrong.

When the engine is warmed up, the first thing to check is the choke to make sure it is fully open.

If the choke is open, the issue is most likely an internal one that necessitates a carburetor rebuild.

A float that has been damaged is an example of this.

Floats in carburetors have been damaged in many cases due to the ethanol in today’s fuel.

How can I make my diesel run quieter?

How to Make a Diesel Engine Quieter

  • Install a sound-dampening hood mat under the hood of the car to absorb engine sounds.

How do I know if my diesel is misfiring?

Rough running, a decrease of power and fuel economy, and the unwanted check-engine light flashing on the instrument panel are all common signs. Many system faults can be the cause, but let’s look at the three most common sorts of misfires.

How do I know if my diesel fuel pump is bad?

Seven Signs That Your Fuel Pump Is Failing

  • Engine with Sputtering. If your engine starts to stutter after you’ve reached highway peak speed, your fuel pump is telling you something.

What diesel additive should I use?

The best additive in the game is Diesel Extreme. This one raises the cetane rating of diesel by seven points (improving the fuel’s combustion performance once again), as well as cleaning and lubricating injectors and other essential fuel system components. Diesel Extreme also aids in the removal of impurities and excess water from fuel.

How do you prolong the life of a diesel engine?

With regular and cost-effective maintenance, diesel engines can last the life of your car or boat.

It’s critical to change the engine oil on a regular basis if you want your diesel engine to survive a long period.

This is because the oil produces a very small’cushion’ between the moving metal engine components when the engine is running.

Friction is the adversary of engine longevity because it promotes early wear and component failure, such as bearings.

A diesel engine’s engine oil will become polluted with carbon and small metal particles over time.

The oil will also lose efficiency as a result of the extra effort it must undertake at excessive heat.

As a result, oil should be replaced on a regular basis, never later than the manufacturer’s recommended service interval.

To ensure that your diesel engine lasts as long as possible, I recommend changing your oil more frequently than the manufacturer recommends.

The reason behind this is because manufacturers may increase servicing times in order to entice people to purchase the car.

This is especially common with engines installed in road vehicles, such as cars.

The oil may be fine to leave in longer under ideal conditions, such as extensive highway driving, however many automobiles are stuck in traffic or only make short trips.

Short trips are particularly harmful to automobiles because the diesel engine does not have enough time to warm up to its proper operating temperature.

Because the engine has not yet warmed up, the oil will not be performing at its best, resulting in increased wear.

On a car that is supposed to only require oil changes every 12,000 miles, I replace my diesel engine oil every 6000 miles. At roughly 6000–7000 miles, I start to notice a difference in smoothness, which indicates that it’s time for a change.

If I performed a lot of town driving, I would definitely change it at 5000 miles because most of my driving is long distance highway.

Check out this link to my page on oil changes for suggestions on changing the oil in a diesel engine.

Another vital step in ensuring the longevity of your diesel engine is to replace the fuel filter on a regular basis.

Your marine diesel engine may have two fuel filters to filter particles of various sizes, whereas your road vehicle is likely to have only one.

Fuel can be contaminated by dirt, small creatures, and even water, therefore it’s crucial to filter it.

To allow the gasoline to get through, these injectors contain a single, or many, very small holes at their ends.

The injector might become blocked if dirt is forced through it via the gasoline.

A clogged injector will cause the engine to perform poorly and will necessitate injector replacement, which is highly costly.

The oil and the engine, as previously stated, require time to warm up to their typical working temperatures.

As a result, running an engine at high RPM before it has warmed up will shorten the engine’s life.

This is due to increased friction generated by oil that is not at the proper temperature.

To produce maximum power and torque, diesel engines do not need to be cranked as high as petrol engines.

Engine components such as the alternator will benefit from lower engine rpm.

This is due to the fact that they will spin at a slower rate, putting less strain on bearings, belts, and other components.