Key Points and Common Pitfalls in Heavy-Duty Truck Maintenance and Repair

Heavy-duty trucks are essential for transportation, but their maintenance and repair require careful attention to detail. This article will outline some critical aspects and common mistakes to avoid in maintaining heavy-duty trucks, ensuring better performance and longevity.

1. Misconceptions about Engine Oil Usage and Replacement

Engine oil plays a vital role in lubricating, cooling, and cleaning the diesel engine. However, many truck drivers focus only on checking and topping up the oil level, neglecting the quality of the oil and the need for timely replacement. Over time, contaminants such as soot, carbon deposits, and sludge from incomplete fuel combustion mix with the oil. In newly repaired or overhauled engines, these contaminants increase significantly. Simply adding more oil without replacing it can lead to severe accidents like bearing seizure or shaft failure. For example, a transport company's truck experienced a bearing seizure due to long-term neglect of oil replacement, resulting in significant repair costs and delays in transportation.

Even when drivers recognize the need to replace oil, they may fail to thoroughly clean the oil passages. This leaves residual contaminants in the oil pan and oil lines, which continue to damage the engine. These contaminants circulate with the oil, increasing wear on internal components, reducing efficiency, and shortening the engine's lifespan.

2. Incorrect Use of Grease

Grease is commonly used for lubrication and sealing in heavy-duty trucks, but improper use can cause problems. For instance, some mechanics apply grease on the cylinder head gasket to enhance sealing. However, this practice is counterproductive. When the cylinder head bolts are tightened, grease can be squeezed into the water and oil passages. During engine operation, the high temperature causes some grease to burn in the cylinder, while the rest remains on the mating surfaces of the cylinder head and block, creating gaps. High-pressure gases can then escape through these gaps, damaging the gasket and causing leaks. Additionally, prolonged exposure to high temperatures turns grease into carbon deposits, accelerating the gasket's aging. For example, a truck experienced reduced power and leaks shortly after repair due to grease on the cylinder head gasket, which was resolved only after proper replacement and installation.

Another common mistake is applying grease to tire bolts and nuts to prevent rust. This practice is risky because the friction angle between the threads decreases when lubricated, reducing the self-locking effect of the bolt connection. This can cause nuts to loosen or even lead to tire detachment during driving, posing a significant safety hazard.

3. Misunderstandings in Cylinder Liner and Piston Installation

Many people assume that new cylinder liners and pistons are interchangeable standard parts and can be installed directly. However, both components have dimensional tolerances. If the largest cylinder liner is paired with the smallest piston, the clearance will be too large, resulting in insufficient compression and difficulty in starting. Therefore, it is crucial to verify the size group codes of the cylinder liners and pistons during replacement to ensure they match the standard specifications. Additionally, before installation, the cylinder-piston clearance should be checked to prevent the use of counterfeit or substandard parts. For example, a repair shop installed mismatched cylinder liners and pistons in a truck, causing abnormal engine performance until the correct parts were installed.

4. Importance of Checking Plunger Stroke Clearance

During the adjustment of plunger-type fuel injection pumps, many technicians overlook the inspection of plunger stroke clearance. This clearance, which is the distance the plunger can move upward after being lifted to the top dead center by the camshaft, is a key indicator for adjusting the fuel injection pump. It is closely related to the wear of the plunger and sleeve. When these components wear, the plunger needs to travel a greater distance to start fuel delivery, affecting the accuracy of fuel timing. Changes in the adjustment screws or shims can also reduce the stroke clearance. Therefore, checking this clearance is essential before adjusting the fuel injection pump.

The inspection method depends on the specific structure of the fuel injection pump. Some pumps allow measurement by rotating the camshaft and removing the delivery valve and seat, while others require lifting the plunger to its highest point using a screwdriver and measuring with a feeler gauge. The standard plunger stroke clearance is approximately 5mm, and the minimum acceptable clearance after wear should still be above 5mm. Neglecting this check can lead to inaccurate fuel timing, affecting engine power and fuel economy.

5. Critical Direction for Measuring Cylinder Clearance

When measuring cylinder clearance, the direction of measurement is crucial. Due to the special structure of aluminum pistons, the clearance around the cylinder is not uniform. Therefore, the measurement should be taken in the direction perpendicular to the piston pin hole on the piston skirt. This direction experiences higher lateral pressure during the piston's reciprocating motion and is more prone to wear. Measuring in other directions can lead to incorrect assessments of cylinder clearance, resulting in improper fit between the piston and cylinder wall, which can affect engine performance.

6. Incorrect Heating Method for Piston Installation

When installing piston pins, the piston and pin are designed with an interference fit, requiring the piston to expand due to heat. However, some mechanics use direct flame heating, which causes uneven heating and can deform the piston. It also leads to carbon deposits on the piston surface, reducing its lifespan. Additionally, the metallurgical structure of the piston can be damaged if it cools naturally after being heated to high temperatures, significantly reducing its wear resistance and service life. The correct method is to heat the piston slowly and evenly in hot oil to avoid direct flame heating. For example, a truck experienced severe piston deformation and engine vibration due to direct flame heating during repair, necessitating piston replacement and increasing repair costs.

7. Mistakes in Bearing Shell Repair

Bearing shell repair can be challenging for inexperienced mechanics. Since mastering the scraping technique is difficult, some choose to use sandpaper to grind the bearing shells to increase contact area with the crankshaft. This is not advisable because the hard abrasive particles in sandpaper can embed themselves in the soft bearing alloy. When the engine operates, these embedded particles accelerate crankshaft wear, significantly shortening its lifespan. For example, a truck's crankshaft experienced severe wear shortly after sandpaper grinding of the bearing shells, leading to engine failure and costly repairs.

8. Misconceptions about Bolt Tightening Torque

Many components in heavy-duty trucks require specific tightening torques for their bolts. However, some users believe that the tighter the bolt, the safer it is. Over-tightening can cause bolts to break or strip threads, leading to failures. For example, over-tightening cylinder head bolts in a truck resulted in broken bolts and poor cylinder head sealing, causing air leaks and engine performance issues. Therefore, it is essential to follow the vehicle's manual for the correct tightening torques during maintenance.

9. Incorrect Handling of Radiator Boil-Over

Overloading, poor heat dissipation, or low coolant levels can cause the radiator to boil over. When this happens, the engine should be stopped immediately, and allowed to cool naturally. Adding cold water directly can cause the cylinder head and block to crack due to sudden temperature changes. For example, a truck driver added cold water to an overheated engine while climbing a hill, resulting in a cracked cylinder head and major engine damage that required extensive repairs and significant financial loss.

By understanding these key points and avoiding common pitfalls, heavy-duty truck owners and mechanics can ensure better maintenance practices, leading to improved vehicle performance, reduced downtime, and lower repair costs.