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Diagnosis: A Weak Diesel

Often, a lack of power sneaks up on a diesel like gray hair does on a person when one day it becomes very apparent. This problem can also include an increase in fuel usage and degradation in idle quality.

The cause can be very elusive since the engine sounds fine and compression and cylinder leak-down tests prove inconclusive and just show normal wear.

When you cannot find anything wrong but you know there is something, think out of the box and evoke the following tips.

For a diesel to run properly, it requires the necessary amount of fuel. If the delivery is weak, then the engine will not be itself.

Most think of a clogged fuel filter or corrupted water separator as the culprit. They can be valid suspects, but mechanics consider the problem with aerated fuel. This is fuel that is not a solid stream but, instead, is mixed with air. Think of it like an air-bound hydraulic brake system.

When air is introduced into the fuel, it not only displaces the combustible fluid but also causes an ebb and flow of the amount getting to the injection pump or common rail system. Splashing of the fuel in the tank along with any returned fuel can cause aeration. It can also be the root cause of a change in performance when the fuel level is very low. This is a normal phenomenon that an efficient lift pump has the ability to negate.

If you suspect fuel aeration, the easiest way to diagnose the cause would be to install a sight glass between the lift pump and the engine. If there are excessive bubbles, you’ll then need to backtrack. 

Air can be introduced via a cracked or degraded pickup tube in the fuel tank, a loose fuel line fitting, or a cracked or loose water separator or any other region in the pathway that it travels. 

To fix the cause, temporarily bypass each section of the fuel line until the bubbles disappear.

Missing boost

A turbocharger is a key component of most diesels. For it to perform properly, the exhaust must have no leaks prior to the turbocharger’s hot side (turbine), and the air feed to the engine from the cold side (compressor) must have integrity. An exhaust leak will limit turbine speed and boost, while an intake leak will allow boost to escape to atmosphere.

A pinhole in an intercooler core along with a poorly sealing wastegate and loose or degraded hoses on the induction system can also bleed off a good deal of boost. Many engines suffer from cumulative minor leaks on both sides of the turbo system. The result is a lazy and fuel-hungry engine.

If there is no boost gauge on the engine, you can temporarily install one to check operation.

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