Maintenance Q&A
Clogged filters and falling fuel mileage

By Paul Abelson, senior technical editor

Q. I have a 2009 Freightliner Columbia with a Detroit Diesel engine. It has almost 450,000 miles. My check engine light came on a few months ago. The dealer said the codes showed that the oil filter was plugged. I had the filter and the oil changed even though it was only 19,000 miles since the last change, and the recommended change is at 25,000. I use premium motor oil and Freightliner brand filters.

A few months later, again with less than 20,000 miles on the new oil, the light came on. It had the same code. I brought it back to the dealer and he said that as long as my oil pressure was OK – it was – all he could do is change the oil and filter again. The oil analysis said the oil was still good.

Does that mean I have to change to a 20,000 mile schedule? That’s one extra oil change a year, and it means I have to waste good oil. What can I do to make the oil last longer?

A. The sensor is detecting reduced oil flow through the filter, indicating a possible obstruction. You may have coolant in your oil. As it flows through the paper element in your filter, the paper absorbs the water and swells. After absorbing enough water, the swelling is enough to obstruct the oil flow so the sensor turns on the light.

After we spoke, you sent a sample of your used oil for a more detailed analysis when the light came on the next time. While there was no excessive water in the oil and the wear metals were still good, it read high for sodium, potassium and boron. Those indicate a leak in the exhaust gas recirculation (EGR) cooler.

The water in the coolant was either absorbed or cooked off in the oil sump. The leak may have been moderate, but the swelling and restriction in the filter are cumulative. You had the EGR cooler replaced by the dealer and the problem seems to be resolved. You are back to your 25,000-mile oil change interval and your last analysis reading was normal. Another potential source of coolant in oil could be leaking injector cups. If that is the cause, use stainless cups as replacements. They cost more, but last much longer.

Q. I’ve been getting 6.81 mpg since my 2010 International ProStar’s MaxxForce 13 engine was broken in. Since then, my summer-to-summer fuel mileage has dropped to 6.77. It’s not that much, and some of my friends tell me that the variation is normal. But as you see, I keep records to two decimals, and this is significant. It’s almost 0.1 percent, and for me, that’s 85 gallons a year, or $336 at $3.95 a gallon. I can do a lot more with $336 than give it to the oil companies.

My question is, is this normal? I perform all maintenance on schedule and I can’t remember having my fuel mileage drop like that in just three years. Are my friends correct or is this something I should be worried about?

A. First, I admire the way you keep records, look at them, and are concerned with small variations in results. That prevents them from becoming big disastrous ones. It’s good management.

You indicated you compare by seasons. Sine weather can have a great effect on fuel mileage, comparing spring to spring or summer to summer takes climate out of the equation. I’ve seen vendors use before-and-after testimonials to sell mileage improvement products. The “before” was always in winter and the “after” was spring into summer.

But back to your mileage. There are several possible causes. Because of the age of your truck, just under three years, your diesel particulate filter (DPF) may need a good cleaning. The periodic regeneration, igniting fuel over a catalyst to generate temperature high enough to burn any accumulated soot and convert it to carbon dioxide, does not clean out the buildup of ash that takes place. Even an engine that seems to burn no oil between changes will burn a little oil from the cylinders, piston and valve guides.

Oil contains trace amounts of metallic compounds that burn to form an insoluble, incombustible ash. That ash gets trapped in the DPF where it builds up and restricts the exhaust, requiring additional energy to get the exhaust through the filter. The result could be the reduced fuel mileage you’ve experienced.

Other possible causes can be anything that consumes power to overcome drag, such as a dragging brake, wheel-end bearings with excessive pre-load, or out-of-alignment wheels. These problems generally have noticeable side effects such as an excessively worn brake shoe, hot wheel hubs or irregular tire wear. Since you had no other symptoms, my guess is that it’s your DPF.

Cleaning a DPF is a tricky job, best left to professionals. The bodies are very fragile, porous ceramic. If mishandled, they can be easily damaged. Your dealer is (or should be) trained in how to remove one from your truck and how to handle it once it is removed. If he doesn’t have the costly equipment to do the job himself, he has access to a specialized service that has the tools and the skills.

You can’t just tap the end on a counter and watch the ash fall out. If you do, you’ll turn a several-hundred-dollar cleaning job into a several-thousand-dollar replacement DPF. LL