Reasons that require engine repair (Nissan Murano Z50)

Low oil pressure can be caused by several reasons that will require engine repair. If you decide to repair your engine due to low oil pressure, pay special attention to the following:

• condition of the main bearings and main journals of the crankshaft;
• condition of the connecting rod bearings and connecting rod journals of the crankshaft;
• clearance in the camshaft bearings and on the camshaft journals;
• condition of the oil line plugs and condition of the oil line.

Condition of main bearings and main journals of the crankshaft

Damaged main bearings can cause low oil pressure or even a knocking noise. The sound will resemble a high-frequency dull knock. Replacing main bearings is part of any engine overhaul.

When the main bearings and/or main journals of the crankshaft are worn, the clearance between the bearing (liner) and the journal increases. Increasing the clearance allows oil to easily escape from the space between the bearing and the journal. This oil leakage 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 crankpins

Bad connecting rod bearings cause dull knocking sounds, some higher in pitch than main bearing knocks but lower in frequency. The knocking sound of connecting rod bearings appears only when the working mixture in the combustion chamber is ignited. The main bearing knocking sound occurs every time a spark jumps in any of the spark plugs.

Like main bearings and main journals, connecting rod bearings and journals can cause a loss of oil pressure if the clearance between them becomes too large. The noticeable increase in clearance allows engine oil to easily escape from the space between the bearing and the crankshaft journal. Oil leakage causes a decrease in oil pressure. The only way to fix this problem is to regrind or replace the crankshaft and replace the bearings.

Camshaft bearings and camshaft journal clearance

Often overlooked by inexperienced repairmen, excessive clearance between the camshaft journals and the camshaft bearing can also cause low oil pressure, just as large clearance in the connecting rod or main bearings can. It is sometimes believed that because camshaft bearings are not subject to the same loads as connecting rod and main bearings, they require no more than a simple visual inspection. Nothing could be further from the truth. On most overhead cam engines, bearings are virtually impossible to replace without completely disassembling the engine. Rebuilding an engine without thoroughly checking the bearings and camshaft journals runs the risk that the rebuilt engine will still suffer from low oil pressure. Always check the bearings and camshaft journals thoroughly.

Condition of the oil line plugs and the oil line itself

If the oil lines are clogged with deposits from poor maintenance, or if the plugs at the ends of the oil lines are leaking, the engine will suffer from low oil pressure. When rebuilding an engine, take the block and cylinder head to a workshop and have them hot cleaned. This ensures clean oil lines through which oil can flow freely. Knocking 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, high-frequency sound coming from deep within the cylinder block. The name "high pitch" refers to the musical pitch of the tapping sound. The term "high frequency" refers to the number of knocks that occur during one engine cycle. If the frequency of knocking is approximately the same as the number of flashes in the cylinders, then the main bearings require attention.

Knocking of connecting rods

The knocking sound of connecting rods can be very similar to the knocking sound of main bearings. Its tone can vary from higher than that of the main bearings to lower. The main difference is the lower repetition rate. This happens because the connecting rod itself is subject to load, unlike the main bearing (high pressure) only when a flash occurs in the corresponding cylinder. The connecting rod knock frequency will correspond to one knock per one engine operating cycle. This is explained by the fact that a cylinder with a faulty connecting rod operates only once per engine cycle.

The problem of analyzing bearing knock by sound remains difficult to solve until you are very familiar with the engine you are working on and can say to yourself, "Is this bearing knock high pitched or low pitched, high frequency or low frequency, different from the knock that is coming from another component if it is faulty?".

Valve knocking

On the engine described in this manual, valve noise can often be eliminated by adjusting the valve clearances. There is an opinion that carrying out major repairs to valves causes deterioration in the sealing ability of piston rings. As with other opinions, there is an element of truth in this one. When the valves' sealing ability increases, the pressure in the combustion chambers increases so much that it becomes close to the value it had on a new engine. And the piston rings are no longer new. They start to leak compression pressure. The result is that the valve action appears to be causing the piston rings to deteriorate. In reality, what is happening is that the rings have now replaced the valves as the weakest link in some chain.



The conclusion from the above is the following: if the engine has significant wear and requires valve grinding, then it probably needs a major overhaul. This is quite a bold statement, and in some cases no repair is required, such as when a valve lifter has failed due to a manufacturing defect.

Camshaft wear

Some engines have chronic problems with camshaft lobes wearing out.

Sometimes this happens because the camshaft designers have chosen the wrong material for the camshaft. Often camshaft failures are the result of poor maintenance or lack of lubrication.

Before deciding whether the camshaft cams will require a complete engine repair, or whether a simple replacement of the camshaft and valve tappets will be sufficient, consult a car repair shop.

Low compression

As an engine wears over time, the ability of each cylinder to withstand and create pressure decreases. This may be due to several factors. First, the valves no longer seal as well as they did when the engine was new. After tens of thousands of kilometers, piston rings do not seal as well as they did when the engine was new. Over time, the cylinders become slightly oval in shape, and the piston rings no longer seal as well as before. When an engine reaches 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 sealing valve. Another is a damaged cylinder head gasket. The best way to differentiate between 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 components.

• Never use low quality parts.
• Always check thoroughly for additional damage when performing the main work. A catastrophic failure of one part can cause failure of other parts. It may also be an indicator of other nearby faults.
• Never repair your friends' cars.

Oil leaks

Relatively new engines do not require major repairs simply because small oil leaks are found on the garage floor. However, older engines may have so many leaking areas that the only practical and logical way to repair it is to remove it and rebuild it. When a car owner decides to remove the engine to fix leaks, it makes sense to overhaul the engine.

Oil burnout

Two problems can cause excess oil to burn: oil leaking past the piston rings and oil leaking past the valve guides. Each malfunction has its own symptoms and solutions.

Oil penetration past the piston elbows

The most common symptom associated with piston rings that are wearing out and allowing oil to pass past them is blue smoke when accelerating. Before suspecting the rings, there are a few other things to check.

If the crankcase pressure is too high, it can force oil past the piston rings into the combustion chamber. If the crankcase pressure is too low, oil can enter the intake tract. When either of these happens, the oil is burned when the air-fuel mixture ignites. While these symptoms are identical to those of worn piston rings, this problem is fairly easy to fix.



The engines are equipped with a positive crankcase ventilation (PCV) system. This system's purpose is to capture hydrocarbon-containing gases passing past the rings from the combustion chamber and redirect them back into the combustion chambers for reburning. When the PCV system becomes clogged, the pressure in the engine crankcase begins to drop. This pressure drop is the result of intake manifold vacuum pulling gases from the crankcase, and air failing to replace the gases in the crankcase due to clogging of the PCV system. As a result, when pressure drops, the intake tract draws in increased amounts of oil through the ventilation system. If the PCV valve itself is clogged, crankcase pressure increases and oil passes by the oil scraper rings during the intake stroke.

Another common cause of oil burnout, which is somewhat similar to oil passing past the piston rings, is sticking or "sticking" of the oil scraper ring. a stuck ring can be released using high-detergency oil. Also, a large number of oil additives can clean piston rings. Consult with your auto repair shop staff regarding which oil to use for these purposes.

Oil penetration through valve guides

If blue smoke comes out of the exhaust pipe when engine braking, it is probably the result of worn valve guides. When engine braking occurs, the pressure in the intake manifold decreases greatly and the vacuum increases. A higher vacuum causes the combustion chamber to empty. Excess clearance between the valve stem and the guide bushing allows combustion chamber vacuum to draw air from the top of the cylinder head, under the valve cover. This air is saturated with oil. Oil is drawn in along with the air and burned in the combustion chamber.



A malfunction associated with oil penetration into the valve guides can be easily corrected without a major engine overhaul. However, piston rings, main and connecting rod bearings, camshafts and camshaft bearings may also show the same wear as the valve guides. You need to see if you're just trying to eliminate the symptom or if you're making the engine capable of running for tens of thousands of more kilometers.

Checking for cylinder leaks

Checking cylinders for leaks is a higher level test than checking compression, although its purposes are the same. It requires more equipment. When the piston is at TDC on the compression stroke, connect a cylinder leak tester to cylinder number 1. Measure the leak percentage. Additionally, use a piece of rubber hose and place one end of it against your ear to determine where the leak is. Insert the other end of the hose into the open hole of the fuel injection system throttle body. If a lot of air comes out of there, then you have a damaged intake valve. If you hear a hissing sound coming from the exhaust pipe, the exhaust valve is damaged. Look at the coolant in the radiator. If there are a lot of bubbles in it, then you have a damaged cylinder head gasket, a crack in the head or in the block itself.

Oil analysis

Of the various ways to determine if an engine needs major repairs, engine oil analysis is one of the most accurate. Contact a car repair shop. Some workshops may perform oil analysis or may contact locations that have the equipment to perform such analysis. An oil analysis will determine how much bearing and crankshaft material is contained in the oil. If the amount of these materials is high, this indicates the need for engine repair. This procedure costs money, but can help avoid major unexpected repair costs.