Last issue I tested your knowledge to see if you could identify the parts of an engine that were circled. Now I would like to point out what some of the lesser-known or often-neglected parts do and how they are important. 1. Turbo oil feed line: This line takes cooled, filtered and clean oil to the turbo charger. The turbo charger is flooded with oil so the quickly-revolving parts can work properly.

I’ve never seen this tube clogged, but it can get brittle and stiff. A brittle hose can rupture and spray hot oil all over the engine and, more importantly, starve the turbo charger of the oil it must have. Even a short time without oil will cause the turbo charger severe damage, requiring replacement.

2. Engine coolant thermostat: This small part – this engine has two – opens and sends coolant to the radiator for additional cooling. The engine works best at 180 to 200 degrees. If the engine gets hotter than that, the thermostat will kick in and start to cool the engine. When replacing the thermostat, make sure the spring and sensor always point down towards the engine. The thermostat should be changed every few thousand hours or according to engine manufacturing specifications.

3. Coolant system bypass: On this engine, the coolant system is always circulating. You might wonder why the thermostat is needed then it’s always being bypassed. Simply put, the bypass system always runs to keep the heat, even so there aren’t hot pockets. Like stirring a pot of stew, by circulating the fluid, the bottom doesn’t burn and you have even temperatures throughout. Same with the engine; a constant circulation of coolant keeps temperatures even.

4. Secondary fuel filter: Sometimes called the final fuel filter, it is exactly what you’d think. After the fuel passes through the first filter, it passes through the secondary filter. This is a safety precaution to keep the fuel clean. Many people don’t service the filters as often as needed. When you change one, you should change the other. The fuel filters should be changed at least every 250 to 300 hours.


5. AFC – Air fuel control: This is hooked into the fuel delivery system. It monitors the air being brought into the engine by the turbo charger and then measures the proper amount of fuel to put into the pistons. If there is low air pressure, then less fuel is sent to the engine. This is mainly an emissions check, but it also keeps your fuel efficiency up. Some people forget that a lack of air doesn’t just cause a black puff of smoke, but also the unburned fuel drops down the piston and collects in the oil. These pieces are there for a reason.

6. Oil filter bypass: This helps ensure you always have oil going to the engine and turbo charger. When the filter becomes plugged, the oil will run unfiltered to the rest of the engine. Dirty oil is better than no oil ... but not much better. The carbon that is cleaned up by the oil will cause problems later on, so you will want to stay on top of changing your filters on time. Consult your manufacturer’s specifications on how often you should change your oil filter and get an oil analysis to make sure your are changing your oil at the right intervals for your operation.

7. Inlet sediment fuel bowl: This little bowl is often overlooked, and rarely cleaned, even though it’s the first line of defense for your fuel system. The sediment bowl is connected directly to the fuel tank and is designed to catch sediment that may have entered the tank. The fuel is then pumped to the fuel filters. Since it is a filter of sorts, it too should be checked and cleaned periodically. Technicians will usually look at this bowl when the machine has a hard start or runability problems. PD

Mechanic’s tip: What are the four functions of oil?
It lubricates, cools, cleans and seals.

Jim Schlund
  • Jim Schlund

  • Retired Diesel
  • Mechanics Professor
  • College of Southern Idaho