LO 10-5 Identify and describe the types, functions, construction features, and applications of diesel engine lubrication system components

1.8. Oil Cooler

Oil Cooler

An oil cooler is a heat exchanger used to remove excess heat from engine oil Figure 38. Virtually all diesel engines use oil coolers because more cooling of pistons and other internal engine parts is performed using lubrication oil. Engine oil can easily become overheated while absorbing heat from pistons, turbochargers, and other internal parts, particularly when the engine is heavily loaded. Oil cooling nozzles or drilled passageways in the connecting rods direct lubricating oil to the underside of the piston crown to prevent heat damage and ring seizure Figure 39. The result is higher heat loads transferred to the lubricating system in a diesel engine. Engine oil should never exceed 250° F (139° C). Many electronic engines use oil temperature sensors to shut down or derate engine power if oil temperatures exceed 250–260° F (139–143° C). When under load, normal engine oil temperature should not exceed the temperature of the coolant by more than 30–40° F (−1.1 to 4.4° C). This means an engine operating at 190–205° F (88–96° C) should have a maximum oil temperature of 220–245° F (122–136° C).

FIGURE 10-38: The lubrication system often uses oil modules incorporating filters, pressure control valves, and oil coolers into a single base.

FIGURE 10-39: The cooling nozzle is aimed at the underside of the piston crown. A notch in the piston prevents contact with the oil cooler nozzle.

Oil coolers are usually thermostatically controlled to prevent either oil or coolant to flow through the cooler until the oil or coolant has warmed up Figure 40. This feature prevents prolonged circulation of poorly lubricating, highly viscous cold oil during warm-up Figure 41. Ideally, oil should warm up quickly and before coolant reaches operating temperature to easily circulate into narrow operating clearances. When the oil or coolant reaches operating temperature, oil diverts into the cooler where it releases excess heat to the cooling system Figure 42. The cooler may also be equipped with a pressure-relief valve that allows oil to bypass the cooler if it becomes plugged with unfiltered oil contaminants. The bypass valve opens if a substantial pressure drop occurs across the cooler.

FIGURE 10-40: Oil circuits of a late-model Mack MP8 engine. Note the location of various control valves, the oil thermostat, the oil galleries, and the number of parts requiring lubrication.

FIGURE 10-41: Engine coolant circulates around the outside of this plate-type cooler.

FIGURE 10-42: Location of an oil temperature thermostat and oil pressure-regulating valve for the lubrication system in an oil cooler-lubrication system module.

Oil Cooler Failures
Engine coolant will usually remove heat from an oil cooler. Plate- and tube-type oil coolers are the most commonly used designs (Figure 43 and Figure 44). Defective coolers or poor maintenance of the cooling system can lead to cooler corrosion, which will cause oil to enter the engine coolant. Oil enters the cooling system because the oil pressure is higher than the pressure in the cooling system. Metal cooler tubes and plates can easily corrode if the pH of the cooling system becomes either too acidic or too alkaline, depending on the plate metal. A suspected leaking cooler can be removed from the engine and pressurized with air to confirm that it is leaking because other engine defects can also push oil into the cooling system. Cooling systems must be drained and flushed after a cooler is replaced if oil has contaminated the coolant.

FIGURE 10-43: A tube-type cooler and oil cooler bundle using internal tubes and plates.

FIGURE 10-44: An oil cooler used on an International diesel truck engine. Oil passes through the bundle tubes and coolant circulates around the cooler bundle.

Another problem with coolers is that they can become contaminated and plugged with crankcase debris. To ensure that high oil flow takes place through the cooler, manufacturers generally send unfiltered oil through a cooler before it enters the oil filters. In that situation, the cooler functions like a primary oil filter. Soot, oil sludge, and debris, such as pieces of gaskets, silicone sealant used as a gasket material, and dirt, can clog cooler passageways. For this reason, a cooler bypass valve is needed to prevent oil starvation if the cooler becomes plugged. Engine bearing failures will also fill the cooler with metal debris, which requires the cooler to be reverse flushed with solvent before the cooler is returned to service after an engine repair or overhaul. One symptom of a plugged cooler is a sudden drop in oil pressure during engine warm-up. Because coolers normally do not allow oil through the core until the oil or coolant is warmed up, oil pressure is normal until the cooler thermostat diverts oil to pass through the cooler. If the cooler is clogged, oil pressure suddenly drops after engine warm-up.

Technicians Tip     Oil coolers are often installed in series with other components dependent on coolant or oil flow Figure 45. Restricted coolant flow due to a partially blocked oil cooler can cause problems for those downstream devices. Unless a technician is vigilant in understanding how the coolant or oil flow circuits are arranged in an engine, failures in downstream components are often difficult to diagnose or have repeat failures after replacement. One common example is EGR cooler failures. Coolant flow through a coolant-restricted oil cooler also restricts coolant flow to an EGR cooler if it is connected in series. In that situation, coolant will boil in the EGR cooler and thermally stress it, quickly causing a failure in the EGR cooler.

FIGURE 10-45: The oil cooler of this Detroit Series 60 engine is located above the oil filters. Note the coolant pipe attached directly to the water pump outlet.