- the condition of the main bearings and main journals of the crankshaft;
- condition of connecting rod bearings and connecting rod journals of the crankshaft;
- clearance in the camshaft bearings and on the camshaft journals;
- the condition of the oil line plugs and the condition of the oil line.
Condition of main bearings and main journals of the crankshaft
Damaged main bearings can result in low oil pressure or even a thud. The sound will resemble a high-pitched thud. Replacing the main bearings is part of any engine overhaul.
When the main bearings and/or main journals of the crankshaft have been worn, the clearance between the bearing (liner) and the neck increases. Increasing the clearance allows oil to easily escape from the space between the bearing and journal. This output of oil causes a decrease in oil pressure. The way to eliminate this problem is to grind the crankshaft and / or replace the crankshaft and bearing shells.
Condition of connecting rod bearings and connecting rod journals
Failed connecting rod bearings result in thuds, some higher in tone than main bearing knocks, but lower in frequency. The knock of connecting rod bearings appears only when the working mixture in the combustion chamber is ignited. The knock of the main bearing occurs every time a spark jumps in any spark plug (gasoline engines) or fuel is being injected from the injector (diesel engines).
Like main bearings and main journals, connecting rod bearings and connecting rod journals can cause oil pressure to drop if the clearance between them increases too much. A noticeable increase in clearance allows engine oil to easily exit the space between the bearing and the crankshaft journal. Oil leakage causes a decrease in oil pressure. The only way to eliminate this malfunction is to regrind or replace the crankshaft and replace the bearings.
Camshaft bearings and camshaft journal clearance
It is often overlooked by inexperienced repairmen, but excessive clearance between the camshaft journals and the camshaft bearing can also cause low oil pressure, as is the case with large clearance in the connecting rod or main bearings. It is sometimes believed that because camshaft bearings are not subjected to the same stresses as connecting rod and main bearings, they require nothing more than a simple visual inspection. Nothing is further from the truth. On most overhead cam engines, the camshaft bearings are almost impossible to replace without completely disassembling the engine. Repairing an engine without a thorough inspection of the bearings and camshaft journals has the risk that the repaired engine will still suffer from low oil pressure. Always check bearings and camshaft journals carefully.
Condition of the oil line plugs and the oil line itself
If the oil lines are clogged with deposits from insufficient maintenance, or if the plugs at the ends of the oil lines are leaking, then the engine will suffer from low oil pressure. This keeps the oil lines clean, through which oil can pass freely.
Knock of main bearings
Not only does increased bearing clearance lead to low oil pressure, but low oil pressure is usually accompanied by knocking sounds. This knock is a high-pitched and high-pitched sound from deep within the engine block. Name «high pitched» refers to the musical tone of the pounding sound. Name «high frequency» refers to the number of knocks that occur in one engine cycle. If the frequency of knocks is about the same as the number of flashes in the cylinders, then the main bearings require attention.
Waste of time and wasted effort can be avoided by analyzing a sample of engine oil. Some workshops can analyze a sample of engine oil from your engine. The results of the analysis will tell you how much bearing and crankshaft material is in the oil. If the amount of these materials is high, then this indicates the need for engine repair.
Knocking rods
The sound of connecting rods can be very similar to the sound of main bearings. Its tone can vary from higher than that of main bearings to lower. The main difference is the lower repetition rate. This is because a particular connecting rod, unlike the main bearing, is subjected to a load (high pressure) only when a flash occurs in the corresponding cylinder. The frequency of the connecting rod knock will correspond to one knock per engine cycle. This is due to the fact that a cylinder with a faulty connecting rod only works once per engine cycle.
The problem with analyzing bearing knocks by sound remains intractable until you are very familiar with the engine you are working with and can say to yourself: «Is it a high or low tone, high or low frequency bearing knock that is different from the knock that comes from another part if it is faulty?».
Valve knock
Many engines have manually adjustable valves. On these engines, valve noise can often be eliminated by adjusting the valve clearances. Even for engines with hydraulic valve lifters, valve knocking does not always require engine repair. It is believed that a major overhaul of the valves causes a deterioration in the sealing ability of the piston rings. As with other opinions, there is an element of truth in this. When the sealing ability of the valves increases, the pressure in the combustion chambers increases so much that it becomes close to the value that was on the new engine, and the piston rings are no longer new. They start leaking compression pressure. The result is that valve operation seems to cause piston ring deterioration. What is happening in reality is that the rings have now replaced the valves as the weakest link in a chain.
The conclusion from the above is the following: if the engine has enough wear and tear of the valves, then it probably needs a major overhaul. This is a pretty bold claim, and in some cases no repair is required, such as when a rocker arm has failed due to a factory defect.
Camshaft wear
Some engines have chronic camshaft cam wear problems.
Sometimes this is due to the fact that the designers of the camshaft chose the wrong material for it. Often camshaft failures are the result of poor maintenance or lack of lubrication.
Before deciding whether wear on the camshaft lobes will require a complete engine overhaul or a simple replacement of the camshaft and valve lifters will suffice (if there) consult an auto repair shop. If this type of engine has a reputation for being prone to this kind of cam damage, it's probably best to replace the camshaft. On the other hand, if a given engine is not prone to this kind of malfunction, then a worn camshaft cam may be an indicator of another hidden malfunction; the engine needs to be overhauled.
Low compression
As an engine wears out over time, the ability of each cylinder to hold and build pressure decreases. This can be due to several factors. First, the valves no longer seal as well as they did when the engine was new. After running tens of thousands of miles, the piston rings don't seal as well as they did when the engine was new. During operation, the cylinders become slightly oval in shape, and the piston rings no longer seal as well as they used to. When the engine comes to this state, it is considered «tired».
Low compression in one of the cylinders is usually the result of one of two defects.
One of them is a burnt, bent or poorly sealed valve. Another is a damaged cylinder head gasket. The best way to separate these faults is to use a cylinder leak indicator.
Low compression in one of the cylinders is usually the result of damage to one of the parts.
Never use low quality parts
Always check carefully for additional damage when doing basic work. A catastrophic failure of one part can cause other parts to fail. It may also be an indicator of other nearby faults.
Oil leaks
Relatively new engines don't require a major overhaul simply because small oil leaks show up on the garage floor. However, older engines can have so many leaks that the only practical and logical way to fix it is to take it apart and rebuild it. When the owner of the car decided to remove the engine from it to eliminate leaks, it makes sense to overhaul the engine.
Oil burnout
Two faults can cause excess oil to burn - oil leaking past the piston rings and past the valve guides. Each malfunction has its own symptoms and solutions.
Oil penetration past the piston knees
The most common symptom associated with piston rings that wear and allow oil to pass by them is blue smoke when accelerating. Before suspecting rings, there are a few other things to check.
If the crankcase pressure is too high, it can force oil past the piston rings and into the combustion chamber. If the pressure in the crankcase is too low, then oil can enter the intake tract. When either of these occurs, the oil burns out when the air-fuel mixture ignites. Although these symptoms are identical to those of worn piston rings, this problem is fairly easy to fix.
The engines are equipped with a forced crankcase ventilation system (PCV). The purpose of this system is to capture gases containing hydrocarbons passing by the rings from the combustion chamber and direct them back to the combustion chambers for re-burning. When the PCV system becomes clogged, the crankcase pressure begins to drop. This pressure drop is the result of intake manifold vacuum pulling gases out of the crankcase, and air is not replacing crankcase gases due to clogging of the PCV system. The result is that when the pressure drops, the intake tract draws an increased amount of oil through the ventilation system. If the PCV valve itself is clogged, then crankcase pressure rises, and oil passes by the oil scraper rings during the intake stroke.
Another common cause of oil burnout, which is somewhat similar to oil running past piston rings, is sticking or «occurrence» oil scraper ring. «stale» the ring can often be loosened with a high detergent oil. Also, a large amount of oil additives can clean the piston rings. Consult with the workshop staff about what kind of oil to use for these purposes.
Oil penetration through valve guides
If blue smoke comes out of the exhaust pipe when braking the engine, this is probably the result of worn valve guides. During engine braking, the pressure in the intake manifold decreases greatly, and the vacuum increases. Higher vacuum causes the combustion chamber to become empty. The excess clearance between the valve stem and guide sleeve allows the vacuum of the combustion chamber to draw air from the top of the cylinder head, under the valve cover. This air is saturated with oil. The oil is drawn in with the air and burned in the combustion chamber.
A problem with oil intrusion into the valve guides is easy to fix without an overhaul of the engine. However, piston rings, main and connecting rod bearings, camshaft and camshaft bearings may also have the same wear as valve guides. You need to see if you're just trying to fix the symptom or make the engine capable of running for another few tens of thousands of miles.