Diesel Fuel Problems: Filters, Air, Algae and Cold
Most no-start and low-power complaints on a diesel trace back to fuel delivery. Here is how to work through it on the shoulder.
A diesel that cranks fine and will not fire, or fires and runs rough with no power, is usually a fuel problem before it is anything else. Fuel delivery is also the easiest system to check without pulling anything apart, which makes it the right place to start.
Here is the order that finds it.
Start with the obvious
Is there fuel in the tank you are drawing from. On a truck with two tanks and a crossover, a plugged crossover line or a valve in the wrong position means you can be sitting on three hundred gallons and starving.
It sounds too simple to check. It is also a real cause of no-starts more often than anyone admits.
Filters
A plugged primary or secondary filter restricts flow, and the symptom is power loss under load first, then hard starting, then nothing. If the truck runs fine at idle and dies pulling a grade, filters are the first suspect.
Filters plug faster on dirty fuel, and they plug suddenly if the tank has picked up debris or growth. A filter that was fine last week can be finished today if something got stirred up.
Air in the fuel
A diesel will not run on air. Any leak on the suction side between the tank and the transfer pump lets air in, and that produces exactly the symptoms of a fuel starvation problem — hard starting, rough running, stalling.
The frustrating part is that a suction-side leak does not drip. It pulls air in rather than pushing fuel out, so there is nothing to see. Look at filter housing seals, fittings, the primer, and any line that has been disturbed recently.
After running out of fuel or changing a filter, the system generally needs priming and bleeding before it will start. Some engines self-prime and many do not.
Algae and water
Diesel fuel supports microbial growth at the water interface in a tank. The result is a black sludge that plugs filters, and it plugs them repeatedly until the tank is dealt with rather than just the filter.
If you are changing filters more often than makes sense, the tank is the problem. Water in the fuel does its own damage to injection components, which is expensive in a way filters are not.
If you change a filter on the shoulder and the truck runs for twenty minutes and does it again, stop changing filters. Something in the tank is feeding them.
Cold weather gelling
Diesel gels when it gets cold enough, and the paraffin comes out of solution and plugs filters. Panhandle winters are mild most days and we do get nights that will do it, and trucks running north out of here see worse.
Symptoms are a truck that ran fine last night and will not start this morning, or one that loses power as it cools down. Treated fuel and a working fuel heater are the prevention. Once it has gelled, it needs warming.
Working through it
- Confirm fuel in the tank you are drawing from, and check crossover and valve positions
- Check for water in the separator and drain it
- Look at the filters — restriction, contamination, when they were last changed
- Look for air — leaks on the suction side, loose fittings, a disturbed line
- Check whether the system needs priming
- Consider tank contamination if filters keep plugging
What not to do
Do not keep cranking. You will kill the batteries and cook the starter, and you will still have a fuel problem when the tow truck arrives.
Do not add anything to the tank you are not sure about. Some additives help with gelling and water. Others do nothing for the problem you actually have and complicate diagnosis.
Restriction gauges tell you before it stops
A lot of trucks have a fuel restriction gauge and nobody looks at it. It is the same idea as an air filter restriction gauge — it shows the filter loading up before the engine starts losing power.
If yours has one, reading it during a pre-trip converts filter changes from reactive to scheduled.
Fuel quality on the road
You do not control what is in the tank at every stop. What you can control is draining the water separator regularly, changing filters on schedule rather than on symptoms, and paying attention when a truck starts behaving differently after a particular fill.
A truck that runs fine for months and starts plugging filters after one stop is telling you something about that stop.
If you are sitting with a diesel that will not run and you have worked through the obvious, call (850) 495-0366. Dispatch answers around the clock and the truck comes to you with filters, primers and the tools to find where the air is getting in.