Bottom Line
Maintenance Q&A

Paul Abelson
Senior technical editor

Question: Thanks for a great article “Buying Cheap Fuel” in Land Line, February 2004 issue. I’ve been using baby food jars to check fuel quality and would appreciate any info or direction on the color of diesel fuel. Recently at a fuel station less that one year old, where the price of fuel is generally 5 to 10 cents less than average, the fuel has had a very yellow color. There are no signs of sediment, cloudiness or oxidization.

Answer: When diesel comes out of the refinery, it is fairly clear, at least before they treat it with the additives to protect the pipelines. As it ages, it oxidizes, starting from the moment it is refined. Yellow diesel is certainly acceptable. When it oxidizes badly, it turns brown and becomes quite dark. After you’ve done a few good tests, you’ll recognize bad fuel when you see it. As you have been, continue to look for sediment, signs of oxidation, water and cloudiness.

I did not mean to imply that all low-priced diesel is bad. Nor is diesel at full service chains always good. Human error occurs no matter what size the facility may be. Some low-price outlets save cost by not having large facilities. There are fewer fixed cost items (less overhead) to be absorbed into the cost of their fuel. If you are getting good fuel at a reasonable price, more power to you.

Question: I am still having a problem with lack of power, at least I think so, especially when 430 Detroits outpull me with heavier loads. Here are the truck specs: 1998 Freightshaker, 500/500 Detroit 12.7 L, 13 speed, 3.73 rears, 653,000 miles.

So far, the parts that I have replaced are a new charge air cooler and a new intake manifold gasket (both done by Penn Detroit). I put a new map sensor and air temp sensor in the manifold this past week. I have bought a new check valve and tank valves, which I am going to put in this weekend. I also want to replace all of the fuel lines, but I am going to have to put these on hold as I have to get the front rear replaced (went to Speedco to get oil changed, and a thrust washer split in two. A large chunk of metal too big to come out of the drain hole was found in the rear, so I am having a new one put in on Monday).

Here is what I noticed when pulling a hill. My turbo boost will go up to 26-27 psi and stay there; then it will suddenly drop to 22-23 psi for a couple of seconds, then go back up to 26-27 psi. It will fluctuate like this until I am done requiring full boost for a prolonged period, other then getting going on the flat.

Answer: There are a number of good ideas posted by some of our very knowledgeable members. They cover such things as leaky fuel lines, fuel water separators, air in the fuel system and sensors.

The Technology and Maintenance Council has a Recommended Practice, RP 337, “Troubleshooting Low Power Complaints.” The first thing it says is to make a visual check of the air intake systems. If all items check out, make sure fuel system pressures are within specifications. While doing this, check fuel system hoses and clamps. Check with your dealer or engine distributor to see whether there are any technical bulletins or upgrades for your engine.

One engine maker recently solved a number of low power complaints by going to 1/2-inch fuel line from 3/8-inch. Next, check the engine computer for any stored fault codes. It may indicate a faulty sensor.

Check grounds throughout the engine-management system. A chaffed wire or corrosion at a ground will send signals that will wreak havoc with the computer. Brief fluctuations of signal strength won’t show up as faults, but can cause erratic performance. Check the throttle position sensor for pedal travel signals.

Other items mentioned in the thread were return valves, air water separators and primary and secondary fuel filters. The poster mentioned that he found the filters partially plugged with a black substance, and there was a half-inch piece of plastic in a fuel line. These will affect performance, but they are the result of something else: a bad load of fuel, a deteriorating fuel line or poor housekeeping at a truck stop.

Low power problems are among the most difficult to diagnose because there are many possible causes, and they often start as intermittent problems. You’ll need to take the time to methodically go through the truck system by system, performing first visual, then electronic checks, and finally taking pressure and temperature readings.

Question: I am trying to get information on my air dryer. Can someone explain the operation of the air dryer? Not the part about drying the air, but what makes it release pressure, allowing the moisture to be released. The reason I am asking is that I had to replace my original air dryer about a month and half ago. Due to a delivery appointment, I had to purchase a rebuilt dryer from a repair shop and give them mine as a core charge. Since the rebuilt dryer was installed, I have not heard it release while idling or going down the highway. I have always drained the tanks but started draining them daily and it appears there is more water than usual.

Answer: Again, the question is from the Maintenance Forum. It is very unusual to have to replace an entire dryer unit. Usually only the cartridge containing the desiccant material needs to be replaced, and then only every year or two.

Even though the poster doesn’t need an explanation of how the dryer works, some readers may need the review to better understand how everything fits together.

The desiccant (air drying granules or pellets) adsorbs moisture from the air. Adsorption is not a spelling error. It is a process of attaching something (water) to the surface of another substance (the granules). It contrasts with absorption, the drawing of a fluid into a substance, as water into a sponge. Because the granules or pellets have a huge total surface area relative to their total volume, they can pull a great deal of humidity out of the air that flows over, around and through the particles in the cartridge.

Cartridges have to be replaced because tiny amounts of oil mist from the air compressor are trapped on the desiccant surface. They cannot be blown off, so the oil stays in place, preventing that area from adsorbing water. When, over time, enough oil has coated enough desiccant material so that performance is degraded, the only way to get enough good surface area is to replace the desiccant cartridge.

Water held on the desiccant’s surface can easily be blown off with compressed air. This usually happens when the air compressor has raised air pressure to the cutoff limit, and the compressor “unloads.”

Dried compressed air goes to the primary reservoir to do useful work applying brakes, filling suspensions and supporting seats. A small portion of the dry air goes to a regeneration air reservoir. When the compressor is no longer pumping air through the dryer, a valve in the dryer opens to the atmosphere. That allows the dry compressed air in the regeneration reservoir to release. The air flows back through the desiccant cartridge, blowing the accumulated water out of the cartridge and into the atmosphere. When the air pressure governor again loads the compressor and pressure builds in the dryer, the valve will close automatically.

This should happen every time the compressor unloads. If it does not, causes may be: an improper installation (or repair), a kinked or broken unloader line, a defective purge valve, a defective or damaged control line or a plugged governor exhaust port.

Paul Abelson can be reached at

Do you have a maintenance question?
You can write to Paul Abelson, senior technical editor, in care of Land Line Magazine, PO Box 1000, 1 NW OOIDA Drive, Grain Valley, MO 64029; or you can fax information to (630) 983-7678; or e-mail your question to Please mark your message Attention Maintenance Q&A. Although we won’t be able to publish an answer to all questions in Land Line, we will answer as many as possible.