ENGINE MECHANICAL EVALUATION TESTING AN ENGINE’S MECHANICAL CONDITION VISUAL INSPECTION uOIL LEAKS – external uCOOLANT LEAKS – at heads or hoses uVACUUM LEAKS – cracked or broken vacuum lines uENGINE SUPPORT MOUNTS uWATCH FOR SIGNS OF BLOWBY NOISES u Many mechanical problems produce characteristic sounds which can help us diagnose the cause of the problem. u We can use a stethoscope to pinpoint the source of the noise. STETHOSCOPE NOISE ANALYSIS u Ring Noise u Piston Slap u Piston Pin Knock u Ridge Noise u Rod-Bearing Noise u Main or Thrust Bearing Noise u Tappet Noise FUNCTIONS OF OIL VISCOCITY LUBRICATION SYSTEM u Engine oil has 3 important jobs. u It lubricates bearings and moving parts. u It cools those hot moving parts. u It picks up and absorbs tiny dirt particles that can scratch polished bearing surfaces, plug passageways and jam up moving parts. LUBRICATION SYSTEM u Oil pumps do not make pressure. u They are designed to circulate a specific volume of oil through the engine. u The passage ways are a specific size and the clearance between bearings and the rotating shafts is very small, (between 0.0015” & 0.002”) LUBRICATION SYSTEM u What creates the pressure is the resistance created by these small clearances (restriction). u Oil changes its viscosity as it changes temperature. u The viscosity of the oil can also affect oil pressure. u Cold (thick) oil will cause high oil pressure readings. OIL PRESSURE TEST u Hot (thin) oil will give lower oil pressure readings. u Engine oil contaminated with gasoline loses its lubricating qualities and is much thinner than it should be. u Oil contaminated with water or antifreeze (coolant) is greyish and foamy. (like whip cream) OIL PRESSURE TEST u TEST FOR POSSIBLE INTERNAL WEAR. u WORN BEARINGS u WORN OIL PUMP u DEFECTIVE OR JAMMED PRESSURE RELIEF VALVE OIL PRSSURE TEST u REFER TO Shop Key Pro in Engine Mechanical folder for oil pump specifications. u Specification will give pressure value at a set engine speed WITH THE ENGINE AT FULL OPERATING TEMPERATURE. OIL PRESSURE TEST u Example: 2003 Honda Accord with V-6. Engine Oil Pressure should be minimum 10 psi at idle, and 71 psi at 3000 rpm. With engine at operating temperature. OIL PRESSURE TEST u Locate the oil pressure switch or sender on the engine (usually near the oil filter). u Remove switch or sender and connect pressure gauge at the threaded hole. u Start engine and allow it to reach temperature. Take reading at idle and at 2500 or 3000 rpm. OIL PRESSURE TEST u Compare results with specifications. u Remember that the idle speed pressure value is the minimum allowed. u The higher speed specification has a range (eg. 38 psi to 62 psi) which is considered “normal”. COMPRESSION TEST u An internal combustion piston engine needs 4 things in order to run. u They are AIR, FUEL, SPARK, and COMPRESSION. u If one of these things are missing, the engine can not run. COMPRESSION TEST u Compression ratios associated with gasoline engines are from 8.0/1 up to 12.5/1. u On a compression gauge, this should show as a pressure reading of between 145 to 175 psi. u Maximum variation should be no more than 15%. COMPRSSION TEST u If the compression is too low, the fuel can not burn properly and the engine will run poorly or not at all. u The loss of compression could be caused by worn piston rings and cylinders, leaking inlet or exhaust valves, a defective cylinder head gasket or cracked cylinder head. COMPRESSION TEST u A crack in the cylinder wall could also be the culprit. u A compression test can detect a compression leak but it can’t always pinpoint the cause. PROCEDURE u Check battery. We need 12.6 V u Run engine until it is warmed up. u Stop engine and remove all spark plugs. u Remove intake air ducting to throttle body. Prop throttle wide open! u Disarm ignition system. u Disarm fuel system. u Install compression gauge in spark plug hole of #1 cylinder and crank engine with starter for 4 to 6 rotations. u Squirt oil down spark plug hole and repeat test. Record results. u Write down pressure reading of first and last compression pulse.. Release pressure in gauge and install gauge in spark plug hole of #2 cylinder. u Crank engine with starter the same number of turns as cylinder #1. u Repeat the process for each cylinder. u Remember that we must crank the engine for the same number of turns for each cylinder. CYLINDER LEAK DOWN u The compression test can spot a cylinder that has a compression leak, but it can’t isolate the cause. u A cylinder can leak compression through the piston rings, through the valves (bent valve stems or worn sealing surfaces), or through the head gasket. CYLINDER LEAK DOWN u We could also have a cracked head or upper cylinder. u The cylinder leak down test can pinpoint the exact cause of the leak. u By pressurizing the cylinder with compressed air (100 psi), we can check for the source of the leak. CYLINDER LEAK DOWN u If we see air bubbles in the radiator, the head gasket is leaking or the head is cracked. u If we hear air escaping through the open throttle body, the intake valve is leaking. u If we hear air escaping through the valve cover (oil filler cap), the rings are leaking. CYLINDER LEAK DOWN u And if we hear air leaking through the tail pipe, the exhaust valve is leaking. PROCEDURE u Warm up engine. u Remove all spark plugs. u Turn engine by hand until #1 piston is at Top Dead Centre Compression. u Connect tool to #1 spark plug hole. u Apply 100 psi shop air pressure to the cylinder. PROCEDURE u I will demonstrate the full procedure in the shop. ENGINE VACUUM u Vacuum or low pressure in the intake manifold, is created by the downward movement of the piston drawing the air out of the manifold, faster than the air can pass the restriction of the throttle plate. ENGINE VACUUM u If the cylinder has a good seal, the air pressure in the intake manifold will be about 5 psi less than atmospheric pressure. We call this pressure drop VACUUM. ENGINE VACUUM u Barometric pressure is a tool used to predict weather conditions. Normal barometric pressure at Sea Level is about 29.9 inches of Mercury. u We write this as 29.9 in. Hg. u The Metric Value is 101.3 kpa. u 100 kpa = 14.7 psi ENGINE VACUUM u A Vacuum Gauge is used to measure the pressure drop inside the intake manifold. u The vacuum reading from an engine that is in good condition should be between 17 in. of Hg and 22 in. of Hg. VACUUM TEST u By connecting a vacuum gauge to the intake manifold and running the engine, we can check the VOLUMETRIC EFFICIENCY of the engine. u We can also check for vacuum leaks around the intake manifold. VACUUM TEST u Volumetric efficiency is affected by vacuum leaks, ignition timing, carbon deposits in the intake ports, weak or broken valve springs, leaking valves, worn cam shaft lobes, or a blocked exhaust system. READING THE GAUGE u Late or “Retarded” ignition timing will cause a steady needle on the gauge, slightly below the normal 17 in. Hg. Normal ignition timing of between 8° BTDC and 12° BTDC will give the correct reading of between 17 and 22 in. Hg. Late or Retarded timing means the spark plug is firing at a point closer to Top Dead Centre. u The engine will run, but poorly. Idle will be slower than normal, power will be poor and fuel consumption will increase dramatically. u A vacuum leak at the intake manifold, caused by defective intake gaskets, (at the head or throttle body, disconnected or broken vacuum hoses) may cause a no start condition or a no idle condition. The gauge needle will be steady at 7 in. Hg or less. u A weak or broken valve spring will cause the needle to drop to almost zero every time the affected cylinder is on the intake stroke. u Burnt or sticking valves will also cause the needle to jump around, but the affected cylinder will have some compression, so the needle will not drop to zero. u A plugged exhaust system will show near normal intake vacuum for a few seconds, but as the exhaust gases trapped in the pipes increase, the vacuum reading will slowly decrease. If the exhaust gases can’t escape, the flow of air into the cylinder on the intake stroke will be restricted..