At a new low?
Troubleshooting lost power isn't as tough as you might think

By Jeff Barker, Land Line contributor

It happens to the best of us. After we have driven a truck for a while, we begin to notice how much longer it can take to get up to speed in certain situations.

Whether it’s a sluggish feeling while accelerating from a dead stop or going down a few more gears than before when pulling a familiar hill, an engine that isn’t performing at peak efficiency will definitely get your attention.

When the power you can realistically expect out of your truck isn’t there, it is time to find out what’s wrong. Lower power problems aren’t just about getting around slower; they can also be detrimental to the fuel mileage.

First, you need to know what to expect out of a particular truck. Being acquainted with a truck’s characteristics before noticing a loss of power is one thing, but a recently purchased truck is another story.

You can’t have the same expectations from a recently purchased truck that may have been spec’d with a 13-liter engine and higher (numerically lower) drivetrain gear ratios as from another that has a 15-liter engine and the same or lower (numerically higher) drivetrain ratios. While engines have horsepower and torque ratings, the gear ratios in the drivetrain will affect the overall performance of a truck.

Here’s a rundown of the top 10 most common causes of power loss:

1. Bad fuel: If you just fueled up and notice a major loss in power, then the fuel might have a significant amount of water in it. Draining the water separator on the base of your fuel filter assembly until fuel runs out will tell you if water was the problem.

If your fuel filter suddenly clogged up, then it may have been from debris in the storage tank where the fuel was purchased. That can become worse if you fuel up while the fuel tanker is dumping fuel into the storage tank and the sediment in it gets stirred up.

2. Clogged fuel filter: Over time, debris in the fuel tanks will eventually find its way into the filter and restrict fuel flow.   

Most trucks built since the mid-1990s use Davco-style fuel filter assemblies with elements housed in clear plastic. As the filter clogs, the fuel level inside the housing rises. Replace the fuel filter if it’s clogged and keep a spare.

3. Debris in the fuel tanks: Paper towels, shop rags, gloves and plenty of other things can end up in a fuel tank. Once something finds its way to the bottom of the pickup tube, the transfer pump won’t be able to draw fuel. This will be evident if your fuel filter is dry.

4. Debris in the return line check valve: If the spring-loaded plunger of the return line check valve is held open with debris, it will cause a loss of fuel pressure in the cylinder head. That keeps injectors from getting the proper amount of fuel.

Another symptom is an engine needing to be cranked for a long period before it will start. The return line check valve is located toward or at the rear of the cylinder head on most engines.

5. Fuel lines: On an older truck, the aging rubber or nylon fuel lines could be coming apart internally and obstructing the fuel flow. Another common problem is a leak on the suction side between the fuel tanks and the transfer pump, which allows the fuel system to draw air in with the fuel.

6. Bad and/or worn fuel injectors: Bad and/or worn fuel injectors: After about 500,000 miles of over-the-road service, or at a lower mileage if the engine has high idle time, the injector spray tips are usually eroded and can’t provide a good spray pattern. Have a reputable engine shop replace them.

7. Clogged air filter: Diesel engines draw a lot of air. As the filter clogs up, it becomes progressively harder for the turbocharger to spool up. Once an air filter is clogged, it can choke an engine to the point where it will start blowing black smoke out of the exhaust or even begin to draw oil out of the crankcase.   

It’s important to visually check an air filter at least once a week (and more often if the truck operates in dusty or dirty environments). Replace the air filter if it’s dirty. Never use compressed air to clean an air filter. It will damage the element.

8. Valvetrain out of adjustment: Most engines need to have an “overhead” or valvetrain adjustment done every 250,000 miles. If it’s ignored for too long, then the camshaft and other valvetrain components could end up with abnormal wear.

9. Loss of turbo boost pressure due to a leak in the air-to-air intercooler or the plumbing: If you’re hearing an unusually loud whistling or hissing noise coming from under the hood as you try to accelerate under power, you may have a torn or split rubber boot in the plumbing or a leaking air-to-air intercooler.

10. Worn engine: If your engine has an excessive amount of blowby, it’s likely to need an in-frame rebuild. Once an engine is worn out, it can’t produce the power it did before. The power loss is gradual over time, but can be revealed on a dynamometer. LL

Jeff Barker is an OOIDA member and a former certified diesel mechanic. He can be reached at truckmaintenancestuff@yahoo.com.