Maintenance Q&A
Injection questions and failing radiator caps

By Paul Abelson, senior technical editor

Q. I recently retired my Cat-powered Pete 359. From now on it’s just for American Truck Historical Society events and parades. I got a Peterbilt 389 with a bit more than 500,000 miles for a good price, and it should last the rest of my career. Since Cat gave up on trucking, I figured I’d be better off with a Cummins. It is a 550-horsepower ISX.

With my 3406 Cat, I pulled my fuel injectors every year. They gave me clues about what was happening inside my engine. I checked for carbon near the tips and once found a tip that was blown off. There’s nothing in the Cummins engine manual about pulling the injectors. Should I still pull my injectors when I winterize? If not, why not?

A. In the days before electronic engines, when injection pressures were about 3,000 pounds per square inch, injectors used to load up with soot, essentially carbon residue mixed with unburned diesel and motor oil. It would bake onto injector tips, altering the spray pattern.

That’s a result of incomplete combustion. As it built up around the injector holes, it interfered with the flow of fuel. The more it interfered, the more incomplete the combustion. It was a vicious cycle that could be broken only by pulling the injectors, putting them on a cleaning machine, and running concentrated detergent through them. You wouldn’t want to run that strong a chemical through your engine while it was running. If it got into the engine oil and circulated through the engine, it could weaken the oil’s ability to lubricate and could possibly destroy bearings.

Since those days, a great deal of progress has occurred.

Most trucks today have fuel-water separators to keep water out of injectors where it could – when instantly raised to combustion chamber temperature – convert to steam with enough explosive force to blow the tip of the injector away from the body.

Injector nozzle holes have gotten finer to better atomize the diesel.

Injection pressures have increased tenfold or more. Some engines in development inject at more than 35,000 psi.

Sensors are monitoring what is happening in fuel systems as never before. If fuel flow through an injector isn’t what it should be, or if blockage is raising injection pressure at an individual injector, the sensors know. It is entered in the ECM. If anything changes to a critical level, the driver message center issues an immediate alert. Otherwise, when a computer is plugged into the data port at the next preventive maintenance, the exception is brought to the technician’s attention.

Removing, testing and cleaning injectors are a significant investment. The testing and cleaning equipment is beyond the reach of most individual operators. Just the downtime represents a substantial cost. There is no incremental benefit when compared to what your ECM will tell you for free. You also run a risk of tearing an O ring or damaging an injector.

The Technology and Maintenance Council would have a Recommended Practice if it were a necessary procedure, even for older trucks. Just to be sure, I checked the latest electronic version of the RP Manual. There was no mention in either the Maintenance RPs or the Engineering RPs.

That is not to say that injectors don’t need proper care and feeding. You should make sure you get a computer report at every preventive maintenance, even if everything checks out. To avoid injector problems, be sure to use good quality fuel. Cheap fuel may be attractive, but be sure you know why one truck stop’s fuel is cheaper than another’s. It may be because the stop has fewer amenities, or it may be that they search for bargain fuel that may be old and oxidized or otherwise contaminated. To sell at lower prices, some truck stops cut back on checking tanks for water or changing fuel filters regularly.

Major brand truck stops have high turnover, often getting several loads of fresh fuel per day.

Avoid fueling if you see a delivery tanker dropping a load of fuel, or if one is leaving the facility. Deliveries stir up any sediment in the storage tank, increasing the chances of pumping contaminants into your tanks. Maintain your on-board fuel filters according to your engine builder’s recommendations. Pay attention to filter efficiency. Filters remove contamination down to 5 or 10 microns. A micron is a millionth of a meter, or 0.000039 of an inch. Filtering fuel that finely will cut abrasive wear in injector tips to a minimum.

Finally, a fuel treatment will prevent soot buildup on injector nozzles. I recommend using Howes, Lucas, Power Service, Nalco or any other nationally known brand on a regular basis. They’re not just for fighting winter problems. They are effective cleaners and lubricants, and help the entire fuel system.

Q.  I had my truck at the shop, and the service writer told me I needed a new radiator cap. I’ve been an owner-operator for almost two years, and I never heard of this. Is he trying to pad the sale or could I really need a new cap?

A. First, my compliments to your dealer for checking the pressure cap. Many don’t, even though it only takes a few minutes.

At almost two years, your truck is probably past due. A pressure cap is essential to keeping your cooling system from boiling over. Water boils at 212 degrees. A 50/50 mix of ethylene glycol and water boils at 223 degrees at atmospheric pressure. Engines normally operate at 180 to 210 degrees, leaving you only a 13-degree safety margin.  

If your engine and cooling system are both under a heavy continuous load, such as when climbing long, steep hills on a hot day, with your air conditioner running, engine coolant temperature can reach 225 to 240 degrees.

In the mountains, air pressure is less and the boiling point is somewhat lower. However, the pressure cap continues to give you that buffer zone needed to keep the antifreeze/water mix from boiling.

Pressure caps rely on a calibrated spring to hold the cap closed. When that pressure is exceeded, the cap allows coolant to escape into an overflow tank. When pressure drops, the cap closes. Over time, the springs fatigue. Most need replacement at one to two years normal service.

Testing is done with a tool that fits on a radiator or has a fitting for a cap. Pressure is raised using a hand pump with an air gauge. It takes a few minutes and could save you a great deal of grief. LL