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9-point checklist for diesel engines with low power

Johnathan CokerDiagnosing Problems


So, your diesel engine has low power. She just doesn’t climb the hills like she used to. I have seen this a lot throughout my years as a mechanic. When the driver comes in hot because his truck barely made it up a big hill.

This is a common problem with really many different causes depending on the symptoms. You might also say it is not pulling under a load. However, you describe it, loss of power in an over the road truck or a piece of equipment can be dangerous or even fatal depending on the circumstances.

What causes a diesel engine to lose power? These are the 9 most common causes of low power: Check engine light, Dirty fuel filter, Intake air leak or restriction, Plugged exhaust, Malfunctioning throttle pedal, Failing injectors, Low fuel pressure, Malfunctioning Turbo, and Internal engine problems


1. Check engine light

You might be thinking, well duh! 🙄 Well read on before you judge. A check engine light indicates that there is an issue with the systems or sensors on your system. If it is on, start here and see why.

As you will see in this article, there are problems that will not have a check engine if you have one that is great news. Determine what it is using your diagnostic software and then move on.

In case you were not aware, trucks today are equipped with many safety features. Of those features some are used to protect the engine from damage. In an attempt to do that newer vehicles, have what’s called a derate.

A derate is when the ECM has decided for you that you need to head in to the shop. Most of the time the MIL (malfunction indicator lamp) will be on, indicating that there is an issue. If this is the case head on in and see what code is causing te issue.

This is one of the more important checks because, even if you are not in derate a check engine light could indicate that you have another issue causing your power woes. Think about it, you could have a missing injector or stuck turbo actuator. The scenarios are endless. If that lamp is on this is the best place to start.

2. Dirty Fuel Filter

A very common reason for low power complaints is the fuel filter. After the filter gets dirty beyond its limit it will not operate effectively. Once it is clogged, it is going to cause you problems.

It is a good idea with any power complaint to go ahead and change the fuel filters. You never know what it has got into from the fuel. The tank could have had some water in it, or trash stirred up by something hitting the tank. There are all sorts of ways for trash and water to get in and that is way you have the filters in the first place.

Most diesels will have a primary and a secondary filter. I recommend always changing both together. You may be tempted to change just the secondary since it is easier to get to, but it is better to just do both. Especially when there has been a complain about power. If that fixes the problem, you should empty the filters and see what was causing the issues. You may need to drain the tank if there is a substantial amount of water or trash in it.


3. Intake air leak or restriction

The next great place to look if your check engine light is not on is the intake air system. In most diesels there is a turbo and it is sending compressed air to the engine creating boost and giving it that much needed power.

This air is cooled before it is introduced to the engine this is known as the CAC (Charge Air Cooler). It makes sure the engine can perform correctly and with the best efficiency. The CAC is very important against your engines worst enemy. Heat

When there is a leak in the induction system the engine will not put out as much horsepower and it will use more fuel. You can lose up to 20% power and lose a ½ mile per gallon.

  • You need to start at the turbo clamps.

If you are working on a vehicle, you have a lot of vibration and the clamps will loosen up over time. It is best to find the torque spec for the these and check them all. Make sure they are also lined up correctly. Sometimes when they are going back on, they don’t want to fall into place. Clamps will be the culprit of leaks more often than not.

  • Leak down test

If you are familiar with a CAC leak down test than you know this is a great way to find leaks in the intercooler itself. This requires a special adapter in order to charge the intercooler with compressed air.

When you charge the intercooler, the specific amount given by the manufacture. At that given PSI you can then allow it to sit for about 30-45 minutes to see if there is a decrease in pressure over that time.

  • Hoses

Carefully look at all the hoses. Look for any signs of rubbing or chaffing. A lot of times they can develop pin holes over time. If there is a silicone hose check to see it is not collapsing when the engine is running.

You can also have a restriction causing that lack of power. If your air filter is clogged or dirty or you have some sort of obstruction in the air intake system, your engine will not be able to breath and cause a dramatic amount of power to be lost. Make sure there is nothing on the filter and is not soiled to the point of clogging.

4. Plugged exhaust

This is an often-overlooked issue. A plugged exhaust system can create too much back pressure and cause a serious decline in the power of the engine. There is an amount of time based on the manufacture of the muffler to which it should be replaced. If it is very old, you should think about replacing it. Some signs your muffler or DPF (Diesel Particulate Filter) are clogged are:

  • Engine runs ruff
  • Bad smell from exhaust
  • Condensation from exhaust
  • Strange noise from exhaust

5. Malfunctioning throttle pedal

Another problem you could have if you are experiencing low power is a malfunctioning throttle pedal. You may think I should have included this in the check engine light section but I have seen many times an electronic throttle pedal has problems and not throw a code.

There are many ways the TPS (Throttle Position Sensor) can fail. Do not think this thing will just be on or off. A TPS will fail by not being accurate, have a dead area in the pedal, bouncing readings and the hardware itself broken or sticking.

It is best to hook up to the manufactures diagnostic software and take a look at the operation of the TPS. I know Cummins Insite has a test you can run, and you can look at it manually. You can move the pedal and see if the software displays the correct percentage. If you have the pedal 50% depressed the software should indicate that within reason.


6. Failing injectors

If you have an injector that is not operating correctly you will have a loss of power. Injector issues do no always throw engine codes. I have seen several times in my career an engine how 1 or even more bad injectors and never turn on the check engine light.

I you hear a miss in the engine it is best to have a closer look to see if that is the culprit stealing your power. Most diagnostic software if going to have an injector performance test or diagnostic test. You should run the test on all injectors to single out the issue. Don’t stop when you see one that is out of spec, keep the test up to the end to make sure that you don’t have more than one failing injector.

It is not unusual to have more than one bad injector. Depending on the vehicle or equipment’s age and maintenance history I would most of the time recommend replaced all 6 if you are having issues. You can also have those sent out for testing and even to be rebuilt.

7. Low fuel pressure

Another place to look in at the fuel pressure the engine has. Low fuel pressure will definitely cause a lack of power and with today’s high-pressure systems you can definably have some trouble here.

You should check all the lines in your fuel system. Make sure the lines are tight and not leaking. Any air introduced to the system will significantly reduce you power. With the newer high-pressure systems even a little looseness with cause them to suck air in.

In a lot of diesel engines this is known as a common rail. In these systems the injectors are fueled by individual lines. The system also has a relief valve to protect the system from having excessive pressure. These pressures can be as high as 30,000 PSI! The ECM controls the pressure based on information from several sensors in the system. Bad information for even one can causes all sorts of problems including low power.

There is also a type of injection called HEUI (hydraulic electronic unit injector) This was a technology invented by Cat. This is the system used in the 7.3L and 6.0L Power Stroke diesel engines. These came with a high-pressure oil pump which when your having issues there will cause an array of problems including low power. There are many other types of injection and they will all have their own set of issues.

If your engine is starved for fuel a couple other things to check for are

  • Porous fuel lines – the braided fuel line soften over time will start to allow air into to fuel and cause all sorts of issues.
  • Weak lift pump – This can cause low power and intermittent hesitations. Even if it sounds fine! It is hard to measure in some applications, but I have seen this on many engines.

8. Malfunctioning Turbo

The turbo can have issues and cause substantial power loss. Regular inspection of the turbine inside is needed. If any trash enters the turbo the damage can be catastrophic. If the turbine is chipped or scored it will not be as efficient and a loss of power will occur.

Also, with today’s emissions systems we have turbo actuators and VGT (variable geometry turbocharger). Any issues with these systems can cause low of power and perhaps derate. I would hope to see a stored engine code with these but, visual inspection is always recommended as well.

There are several diagnostic tests than can be run on the turbo and actuator. If you at all suspect the issue is here, run the tests and regroup.

9. Internal engine problems

The last place I want the problem to happen would be in the internal engine. Problems here can be difficult and expensive to fix but there are several things that can happen to cause low power that are inside the valve cover.

  • Broken rocker
  • Poorly adjusted valves or injectors
  • Bent/broken push rods
  • Bad valves
  • Worn rings/cylinder/liners
  • Compromised head or head gasket
  • Failed lifter
  • Engine brakes malfunction
  • Bad cam roller
  • Damaged cam shaft
  • Engine timing misaligned

Note: You may have more than one problem

If you found an issue and fixed it and when you got back out on the road and it still didn’t have the power, it needed you may need to get back in there and go back to diagnostics. It is very common in my experience to have more than one issue causing low power. Do not be surprised if you find a loose clamp and a missing injector. A lot of times a smaller issue won’t be noticed until a domino effect happens and two or even three things happen effecting your power from the engine. Keep this in mind when you are diagnosing the problem with your engine.

Learn Diesels

Johnathan Coker is a ASE and EVT Certified Mechanic. He is married with two kids and lives in sunny Florida. He loves taking stuff apart and teaching others how he messed it up.

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