We continue our discussion from The Fine Points of Engine Maintenance: Part 1 - Fuel Filters and Electrical Systems with two other important areas - adjusting your overheads and the cooling system.

Adjusting Your Overheads

Your diesel engine has four valves in each cylinder - two intake valves and two exhausts. These are operated from a camshaft, which is rotated via gears by the crankshaft. The design of the cams is such that the two kinds of valves will be opened and closed at exactly the right time to optimize the breathing of the engine. Some older engines, mostly those built before 2010 have additional camshaft lobes, or even a separate camshaft - as was the case with the older Cummins ISX - that operate(s) the unit injectors of the fuel injection system. Volvo had a special high-performance kind of unit injector until 2017 and it was operated by separate cams on the same shaft with the cams for the valves.

Most engines today have overhead camshafts and operate the valves and, if applicable, the injectors via a rocker lever. There is also a bridge that allows a single intake or exhaust rocker to operate the two intake or exhaust valves. The sole modern exception is the MX13 engine built by Paccar, which has a camshaft high in the block and pushrods to operate the rockers. This was done to maintain a lower profile and allow a more aerodynamic hood. The Cummins N14 also has pushrods.

All four-stroke engines have camshafts, rockers, and sometimes other parts (such as pushrods) to operate their valves. The rockers in diesels have roller-type cam followers to ride around the camshaft and operate the valves and/or injectors with minimal friction and wear. Although the system is extremely durable, it does not adjust itself the way modern passenger car valve actuation systems do, by using hydraulic lash adjusters.

It's extremely important that valve lashes, camshaft-operated unit injectors, and crankshaft are adjusted and rotated to exactly the right position, every time.

Below is an explanation of why proper positioning of valve lashes, camshaft-operated unit injectors, and crankshaft matters.

1. Valve Lashes

As you may know, this type of valve train is designed so there is a small clearance or "lash" maintained in the system. This is necessary so the valves will never hang open, because even a tiny bit in case expansion or contraction of parts or wear (like pounding into their seats) prevents them from fully closing. And although the system is extremely durable and wear changes this lash very slowly, especially after break-in, it does change. And, the problem is that the change will affect engine breathing.

Why? Well, if the system wears and the lash increases due to the pounding of the parts, the valves will then open a little too late, close too early, and then not open quite far enough. The engine turns so fast that even a short reduction in the open time is critical. Also, the valves are restrictive enough that even a slight reduction in the distance they open off the seat will significantly affect the engine's ability to draw in air and expel exhaust.

Even if the wear occurs so the lash decreases, which occurs because of the way the valve pounds into the seat, it's not ideal. This situation can, for example, cause the valve to still be slightly open when the piston is starting to rise rapidly at the beginning of the compression stroke, causing air to be expelled from the cylinder. This would kill low RPM torque. Also, valves that are too tight spend less time seated and transferring their heat to the head, which may cause them to overheat.

2. Camshaft-operated unit injectors

In the case of camshaft-operated unit injectors, the adjustment is every bit as critical. Proper adjustment ensures that the plunger (which is like a piston) inside the injector will be actuated by the right part of the injector cams on the camshaft. If this adjustment gets loose, the plunger will descend more slowly than it should, which will reduce the amount of fuel injected and the pressure.

In the case of Cummins engines with unit injectors, the relationship between the cam and the plunger is even more critical as the system must force the plunger to just reach the bottom of the injector, a fuel chamber called the "cup" and seal the holes in the cup shut during most of each four-stroke cycle. If the plunger does not descend all the way and leaves a small space at the bottom of the cup, fuel will coke up and clog the cup, and the end of each injection period is likely to see improperly atomized fuel, too. Cummins unit injectors are adjusted with a small torque wrench to ensure that the plunger seals off the holes in the cup properly. Detroit Series 60 injectors are adjusted with a small height gauge. The three unit pumps on 2017 and later Volvos are adjusted much as the valves are‚ check factory information for full details.

This is why maintenance of proper valve lash is critical. When the camshaft operates unit injectors, maintenance is even more critical.

Adjusting the valves (and injectors, if applicable) is a routine maintenance operation normally performed by a trained technician. It can be performed by a shade-tree mechanic, but it's important that the job be performed exactly right or serious damage will occur when the engine is started.

2. Crankshafts

The crankshaft must be rotated to exactly the right position for each adjustment. For this, you'll need full factory information on the proper positions.

Since the crankshaft turns twice to go through its entire cycle, the technician must also ensure that the crank is not 360 degrees off when an adjustment is made. This is done by looking at the position of various valves at the time, or the position of the camshaft, as specified in the factory instructions.

Thin gauges of proper thickness are used to adjust the lash, and the thickness often changes between the intake and exhaust valves because the exhausts run much hotter. The job would normally be done with the engine at normal operating temperature. You'll need a new gasket for the cam or valve cover, too, and you'll need to know the proper torque for the screws that attach it.

On all engines, a bridge that allows two valves to be operated by the same rocker must be adjusted first by using a gauge under one of the two valves it operates, using the procedure described next.

A locknut is loosened, an adjusting screw is rotated until the gauge just fills the gap between the valve bridge and rocker, (or the bridge and one of the two valves), and a slight pull is needed to slide the gauge in and out. Then, the locknut is torqued with a small wrench to the specified torque as the adjusting screw is held in place with a screwdriver.  The adjustment is then rechecked to ensure that the adjusting screw did not turn when the locknut was tightened. On some engines, like pre-2017 Volvos, the adjustment is made with shims of various thicknesses and a tool that rotates the rocker so shims can be inserted and removed.

Doing this is such a precise job that it should not be attempted unless the person performing it is experienced, has adjusted valves and similar parts before, and has ensured that he or she has all the required information - the sequence for rotating the crankshaft to the proper position for each adjustment, the clearances for the bridges, intake and exhaust valves, and the proper tools and specifications for injector adjustment.

The Frequency and Cost of Maintenance

The next question is: How often should this maintenance be done? Well, if the engine's valves seem to be creating a tapping noise and the last adjustment was done a long time ago, it should definitely be done. Factory mileage recommendations vary widely.

Most engine makers recommend an initial adjustment after break-in at about 150,000 miles, and readjustment every 300,000 miles after that. Overheads will be adjusted at a truck or engine dealer for something in the range of $250-400.

Cummins is unique in recommending the first adjustment at 500,000 miles, perhaps in the belief that their valve gear is especially good at maintaining the adjustment.

But, many fleets who are enlightened about maintenance may do this adjustment much more often, say at least once a year regardless of mileage. It certainly makes sense to do the adjustment at 100,000-200,000 miles after break-in as the lash will change faster with a new engine, and the adjustments will be more stable after that. Those who swear by more frequent adjustments say properly adjusted overheads mean a smoother operating engine that tends to produce less clogging of the Diesel Particulate Filter (DPF).

The Cooling System

Diesel cooling systems have always been a high maintenance item for trucks, and much more trouble than those in passenger cars. The reason is that diesels, in order to be completely rebuildable and to offer maximum performance, use what are termed “wet cylinder liners.”

Wet Cylinder Liners

The liners provide the cylinder walls‚ the surface that the piston rides up and down. The engine coolant flows directly around them, in contact with the metal of the liner.

These liners are replaceable cylinders, open at both ends, that are pulled out of the cylinder block when and if an engine is overhauled so the pistons and the surface they ride on can be completely replaced and brought to original specification without machining. The problem with these liners is that they are subject to vibration even though tightly clamped into the cylinder block, because of the high operating pressure of a diesel engine and the rapid pressure rise that occurs as the engine fires. These forces tend to create violent motion‚ said to be vapor bubbles that collapse violently after they form‚ as the vibration passes through the liner metal and into the liquid coolant. Regardless of the physics and chemistry, these physical forces cause corrosion and pitting of the outside surface of the cylinder liner, the surface the coolant is constantly in contact with. Pitting can get so bad that holes are created that pass right through the entire thickness of the metal, allowing coolant into the cylinder, and hot gases into the coolant!

Extended Life Antifreeze/Coolants (ELCs)

In the past, the cooling system was most often protected by adding Supplemental Coolant Additive (SCA) to the cooling system. Some companies still sell special filters that continuously dissolve this protective additive into the system. But, the additive is sacrificial, meaning it protects the metal by seeing the deterioration that otherwise would be occurring to the cylinder liner itself.

As a result, the level of the additive needs to be watched and the filter that provides the additive replaced periodically.

The additive also coats the outer surface of the liner, which makes it more difficult for heat to pass from the metal of the liner into the liquid coolant, slightly compromising the cooling effect. To help solve for this, Extended Life Antifreeze, or Extended Life Coolant (ELC) was developed, originally by Texaco, an oil company later purchased by Chevron.

The Frequency and cost of coolants

Today, diesels can be easily protected for up to 1,000,000 miles by using ELC as it's a coolant that uses a form of protection that is not sacrificial. The primary maintenance that is needed is the addition of an extender at either 500,000 or 600,000 miles.

The super antifreeze is relatively inexpensive at less than $20 a gallon, and even big trucks typically hold less than 15 gallons. That brings the cost of changing the coolant at an estimated $225-$300 with the labor. The extender costs about $25 and you just pour it in the radiator yourself.

maintenance best practices

There are a few additional tips and precautions to note:

• It's recommended to buy either ELC antifreeze that has nitrites, or nitrite-free XLC. Check your engine or tractor owner‚Äôs manual to see which is right for your particular engine. 
• Once you have the right antifreeze in your radiator, you need to make sure to carry some extra so you can refill the radiator as necessary with the same type and color (normally red) antifreeze. 
• When doing the original fill or adding, make sure to use a 50/50 mix of de-ionized water and antifreeze. Don't use tap water!
• Always buy and add pre-mixed coolant. The sole exception (except one other right below) is that it is allowable to use a 60/40 mix for extremely cold climates. This means with temps below -35F, the freeze point for 50/50 mix. You would then purchase straight antifreeze and de-ionized water designed for such use and mix to the proper proportions, using a hygrometer that can measure freeze protection.
• Over long periods the original 50/50 freeze protection can be altered. You need to occasionally test the strength of your antifreeze as the ratio between water and antifreeze may change slightly because a very small percentage of the water may escape right through the hoses while the antifreeze remains in the system. In this event, you simply drain a small amount of coolant out and add some e-ionized water approved for diesel cooling systems, or if that is unavailable. 50/50 mix, then check the percentage. Check your owner's manuals for advisable service intervals. Checking in spring to make sure there is enough water will help cool the engine during warmer weather. Checking anti-freeze strength in the fall is ideal for the cooler seasons, though if you have been adding only the proper mix, it should be OK. Maintaining the 50/50 ratio is best as this solution cools the engine more effectively than pure antifreeze or coolant that does not have enough water in it.
• When changing coolant, remember to use a chemical flush that will ensure the system is clean before adding the new coolant.
• Some engines come equipped with filters to remove cooling system impurities. Filters are often removed when ELC is installed. If you want a filter on your system even though you use ELC, make sure you use one that contains no chemicals, but is a purely mechanical filter, and then replace it as recommended.

So there you have it. The fine points of engine maintenance. We hope that you found this detail useful, and cant take advantage of staying proactive on caring for your equipment in order to keep you safe and successful.

Other relevant articles:

The Fine Points of Engine Maintenance - Part 1: Fuel Filters and Electrical Systems

Changing Your Diesel Engine Oil - Why and When?

Diesel Emission Control: Part 2 - Truck Maintenance Downsides