Vacuum, Pressure & Atmospheric Pressure Vacuum is any pressure lower than atmospheric pressure. Measure in inches of mercury ( Hg) or centimeters of mercury (Cm Hg). Pressure is any pressure that is greater than atmospheric pressure. Measured in pounds per square inch (PSI) or Kilograms per square centimeter (Kg/cm2)
Types of Engine Vacuum Venturi vacuum is found in the venturi on carbureted engines only. Ported vacuum is just above the throttle plate. Manifold vacuum is always below the throttle plate.
Vacuum Gauge Used to measure manifold vacuum Measured in inches of mercury (Hg) Great screening tool for engine problems Must be connected to the intake manifold
Vacuum Diagnosis Chart
Compression Test Allows technician to pumps air out of the cylinder through the spark plug hole. Engine is cranked through at least four compression strokes. The cylinders are measured and compared to determine leakage.
How to Perform a Compression Test Remove the spark plugs. Screw the appropriate adapter into the spark plug hole and connect the gauge. Disable the engine so it will not start. Connect the remote starter button to the engines starting system. Crank the engine through at least 4 compression strokes. Do this for all the cylinders.
Common Leakage Points Intake Valve Head Gasket Exhaust valve Rings
Interpreting the Results All cylinders should be with 25% of each other from the highest to the lowest reading. This indicates a mechanically sound engine. To determine this take the highest reading times 75% this will give you the lowest acceptable reading x.75% = _____ lowest OK
What to do About a Low Cylinder 1. You need to determines if your problem is the piston rings or the valves. Do a wet test or a cylinder leakage test. Theres a big difference in cost between a valve job and a complete overhaul. 2. If adjacent cylinders low: head gasket or warped head. 3. Coolant in cylinder.
Cylinder Leakage Tester Use this tester to pin- point specific malfunctions in the cylinder. This tester pumps air into the cylinder. You listen to where it leaks out.
Cylinder Leakage Set-up Connect air supply to the tester and calibrate the tester. Remove the spark plug, attach the adapter and get the cylinder at TDC between the compression and power strokes. Connect the air supply to the cylinder and listen for where the air escapes.
Evaluating the Results If the leakage is 20% or less the cylinder is OK. If the leakage is greater than 20% check the following: Air escaping from the tail pipe. Burnt exhaust valve. Air escaping from the throttle body. Burnt intake valve. Air Escaping from the oil filler. Bad piston rings.
Cylinder Power Balance Locates a misfiring cylinder. Manually Scopes have built-in power balance test. Cars computer will run its own cylinder balance test. Need a scan tool.
Methods of Performing a Power Balance Test 1) Remove each plug wire one at a time listen for drop in engine RPM. If there in no drop the misfiring cylinder is located. This test is tough in the catalytic converter. 2) Remove the fuel injector wire to each cylinder one at a time. This way the converter wont be over loaded with fuel. 3) This test is built in on many engine analyzers. All 3 of our analyzers have it. 4) With many new cars, this test can be run with the scan tool.
Camshaft Verification Refer to manufacturers service information. OHC SOHC DOHC
Single Overhead Cam
Dual Overhead Cam
Exhaust Color Normal exhaust is clear. White smoke (steam) indicates coolant entering the combustion chamber. Black smoke indicates excessive gasoline (rich mixture) entering the combustion chamber. Blue smoke indicates oil entering the combustion chamber.
Diagnosing Engine Noises Ticking, clicking or clatter. Usually indicates valve train problems: Camshaft Lifters Pushrods Rocker arms Timing chain or gears Knocking Usually indicates lower end noises: Rod bearing worn (rod knock) Main bearing worn (main bearing knock) Loose piston (piston slap) Loose wrist pin (double knock) p. 146
Diagnosing Engine Noises Stethoscope helpful in isolating noises but a big screwdriver, dowel rod or heater hose can also work. Sort out bearing noises such as alternators and water pumps.
Crankcase Pressure Test This test checks the amount of gases passing the piston rings (also called a blow-by test). The tester connects in place of the PCV valve. The engine is run and the amount of blow-by it measured. This test is weird, but it really shows if an engine has bad rings.
Measuring Oil Pressure This test accurately measures engine oil pressure. Remove the cars sender unit and attach the shop pressure gauge. Can indicate a variety of engine problems. This is where the oil pressure sender is removed from.
Interpreting the Results Causes of low oil pressure. Worn engine bearings. Worn or defective oil pump. Defective pressure sender. Low oil level. Diluted oil.
Checking for Oil Consumption Only two ways to lose oil: leak or burn. Burning oil. Look for blue smoke and check oil consumption. Leaking oil. Perform a oil leak inspection. Visual Dyes
Cooling System Inspection Check the following: Coolant level. Coolant condition. Anti-freeze protection. Hoses and belts. Check the water pump. Radiator fins. Pressure test for leaks. Test cap. Determine thermostat condition. Check fan for proper operation.
Loss of Coolant Leaks Loss to combustion chamber
Block Test This test is used to determine a cracked head, block or head gasket. This test is performed by taking vapor from the top of the cooling system and running it through a blue liquid. If the fluid stays blue the engine is OK. If the fluid turns yellow the coolant is leaking into the combustion chamber.