Misfiring occurs when the engine is idling
Note. Most often, engine misfiring occurs as a result of a malfunction of the power supply and / or ignition systems. If troubleshooting these systems fails to determine the cause of the problem, check the following options:
There are air leaks through the intake manifold gasket. Check the pipe joints with oil or a desiccant while the engine is running and listen for a hiss that indicates the source of the leak.
Valve adjustment out of order (engines 4.2 l). Check, if necessary, adjust the gaps.
The integrity of the cylinder head gasket is broken. Check the compression pressure in the cylinders, if necessary, replace the gasket.
There is a burnout of the valve plates or their seats in the cylinder head. Check the compression pressure, if necessary, overhaul the head.
Broken or worn piston rings. Check compression pressure, replace rings if necessary.
Weak or worn valve springs. Remove the cylinder head cover and check the condition of the valve springs.
Attention! To check the compression pressure, de-energize the ignition system and follow the instructions in the General Engine Settings Section (see chapter Settings and ongoing maintenance).
The valve mechanism makes unusual noises during operation
Valve clearances out of alignment (engines 4.2 l). Check and correct.
Weak or broken valve springs. Remove the cylinder head cover and check the condition of the valve springs. Replace if necessary.
Worn or damaged rocker arm assemblies. Replace defective components.
Faulty hydraulic pushers (engines 3.0 l). Replace defective pushrods.
Worn valve guides. Overhaul the cylinder head.
Cam lugs are worn out. Check the condition of the camshaft, replace if necessary.
Attention! Before removing the camshaft to inspect the cams, check the oil pressure and condition of the valve springs and, if applicable, the hydraulic tappets.
There are noises emitted by the bearings of the lower heads of the connecting rods
Oil supply interrupted. Check the oil level in the sump and the condition of the oil pump and pressure relief valve. Replace oil filter.
Excessive clearance in bearings. Replace the liners, check the condition of the connecting rod journals of the crankshaft, if necessary, perform their groove.
The oil filled is too thin, or it has been diluted with gasoline or water. Change the engine oil, filling it with a fresh one of the required grade. Try to identify and eliminate the cause of the liquefaction.
Oil pressure dropped. Check the serviceability of the pressure reducing valve, the condition of the spring. Also check the oil filter bypass valve for proper operation.
The connecting rod bearings have been misaligned. Correct the connecting rods, if necessary, replace the liners.
Note. When connecting rod bearings fail, a metallic knock usually occurs, the intensity of which is maximum when the vehicle is moving at a speed of 60 km / h with a closed throttle. Before proceeding with the disassembly of the engine in order to check the condition of the lower heads of the connecting rods, check the level and consistency of the impellent oil on the dipstick. Also remove the oil pressure switch and, screwing a pressure gauge in its place, measure the pressure.
There are noises emitted by the main bearings (obvious)
Loose flywheel. Tighten the bolts to the required torque.
Oil pressure dropped. Check the clearances in the main bearings. Check the condition of the oil pump and pressure reducing valve. Repair the pump if necessary.
Excessive axial play of the crankshaft. Replace main bearings.
There was an ovality of the necks of the crankshaft. Turn journals and replace bearings (on the repair size with reduction).
Oil supply interrupted. Add oil to the crankcase, bringing its level to the required value.
Note. A main bearing failure is indicated by a heavy but dull thud when the engine is running under load. When the flywheel fasteners are loosened, a dull or dull thud occurs when the engine is turned off. This is usually accompanied by vibration.
Axial play can be checked by moving the crankshaft by means of a lever, first back, then forward, measuring the absolute amount of movement.
If the oil pressure is normal, remove the main bearing caps and measure the gaps in the latter using a calibrated plastic wire (Plastigage set). Checking the crankshaft journals for ovality and other signs of wear can only be done with a micrometer after the shaft has been removed.
Excessive consumption of engine oil
There are oil leaks. Check the condition of the gaskets and seals. Replace if necessary.
Damaged or worn valve stem seals. Replace defective caps.
Excessive valve stem seat clearance in guide bushings. Replace guides and/or caps.
Worn or broken piston rings. Replace rings on all pistons. Piston rings too tight or stuck in their grooves. Replace rings, clean grooves.
Excessively worn cylinders, pistons and piston rings. Turn and/or honing cylinders, replace pistons and/or piston rings.
Incorrectly installed compression rings, broken or jammed oil scraper rings. Make a replacement.
Note. Before checking the engine for signs of oil leaks, it should be thoroughly cleaned and degreased. Warm up the engine to normal operating temperature. Lay on the ground directly under the engine clean (preferably white) a sheet of cardboard to identify sources of oil leaks.
If the valve stem seals are damaged, the oil begins to be squeezed out bypassing the valves into the combustion chambers. To check, let the engine run for a while at idle, then sharply open the throttle. If there is oil leakage around the valves, thick blue smoke will be emitted from the exhaust pipe.
Problems with pistons, piston rings and cylinders are usually accompanied by a loss of compression pressure, which should only be measured with a compression gauge.
Decreased engine oil pressure
The oil level in the oil pan has dropped. Check and add if necessary.
Too thin oil is filled, or its dilution has occurred. Change the oil and correct the cause of the dilution, if any.
The pressure relief valve is stuck or its spring is broken. Remove the cause of the jam or replace the broken valve spring.
Excessive clearance in bearings. Replace earbuds, or (if necessary) grind the crankshaft journals.
Excessive wear on oil pump components. Repair or replace the pump.
Note. If the vehicle is not equipped with an oil pressure gauge, remove the oil pressure switch and install a pressure gauge in its place. Measure the oil pressure both when the engine is cold and when it is hot. With a faulty oil pump or pressure reducing valve, the pressure will be low in both cases. however, if there is a defect in the bearings, the oil pressure on a cold engine will be quite high, while after warming up it will drop noticeably.
Engine won't crank
Stuck starter. Remove the starter. Check the status and (if necessary) Replace damaged drive and/or flywheel ring gear.
The engine wedges with simultaneous overheating. Remove the engine and disassemble it. Check the condition of the internal components, make the necessary replacements.
Moisture got into the cylinders due to the suction of water into the air intake path (e.g. while crossing a river).Remove the spark plugs and dry the cylinders.
Water enters the cylinders through the head gasket or cracks in the body of the head/block. Remove the cylinder head. If coolant leaks through the gasket, check the flatness of the mating surfaces of the head and block. Make a groove if necessary. If cracks are found, the head/block should be replaced.
There is damage to the crankshaft or components of the connecting rod and piston groups. Remove and disassemble the engine. Inspect components and make any necessary repairs/replacements.
The integrity of the valve disc is broken due to overheating or fatigue deformation. Remove the cylinder head and check the condition of its components. Make any necessary repairs or replacements.
Note. Frequent jamming of the starter drive on the flywheel ring gear can be caused by a bent armature shaft or damage to the teeth of the drive gear and/or flywheel ring. The condition of the flywheel ring gear can be checked through the starter mounting hole after the starter has been removed. If water enters the cylinders suddenly, during a water crossing or for another similar reason, deformation of the connecting rods may occur. However, if clean water gets into a stopped engine, it can be started without any special complications.
Remove the spark plugs and de-energize the ignition system as described in the Compression Pressure Test Section (engine settings) in Chapter Settings and ongoing maintenance.
Turn the engine over by hand, pushing as much water out of it as possible.
Then crank the engine over with the starter - try not to lean over the engine block as the water can be forced out through the spark plug holes with a decent amount of pressure. Check the engine oil, distributor, carburetor, air cleaner, fuel filter and fuel tank for signs of moisture. If no moisture is found, dry and replace the spark plugs and start the engine.
When the engine seizes as a result of overheating due to a drop in the coolant level, bearings, pistons, etc. definitely fail. Components. Although it is sometimes possible to start the engine successfully after cooling down and changing the oil and coolant, abnormalities such as increased oil consumption, a drop in oil pressure, increased engine noise, etc., usually soon appear, depending on the degree and nature damage.
When coolant seeps into the cylinders through the head gasket, or if the compression pressure in two adjacent cylinders is low, it is extremely important to carefully check the mating surfaces of the block and cylinder head for flatness. Through leaky gaskets or cracks in the head/block, enough water can seep into the cylinders to prevent the engine from cranking. Typically, this situation is accompanied by difficulty starting, misfiring, steam coming out of the exhaust pipe, as well as a drop in the coolant level.
Leaks of gases into the cooling system can be detected by removing the radiator cap and observing bubbles in the upper radiator reservoir with the engine running at fast idle or without removing the radiator cap for excessive overflow from the expansion tank.