Unofficial Montreal CPA exam study guide (automobile mechanics)
Disclaimer: This study guide is prepared without sponsorship by the Montreal CPA. This guide is based solely on the self-evaluation checklist made available to automotive technicians by the Montreal CPA available at:http://www.cpamontreal.ca/pdf/self_evaluation_automobile_mechanics.pdf in order to gauge competencies before undertaking their licensing exam. This is not a replacement for practical knowledge and experience, to pass the exam it is highly recommended to practice all questions repeatedly and on a car. The CPA is currently in the works of re-writing the certification exam. New information will be posted on this site in a timely manner. The questions have been researched as thoroughly as possible but are not guaranteed error free. Furthermore; enumerations do not represent exhaustive lists. You should be knowledgeable before consulting this study guide.
IC ENGINES
Explain the 4 strokes of an IC automobile engine; explain the position of the pistons and valves for each stroke.
Assumptions: no VVT, VVL, basic 4 cylinder engine, numbers in degrees of crank rotation
Admission – IO 339, IC 231 Piston is moving toward BDC
Compression – 231, 0 Piston is moving towards TDC
Power – 0, 143 Piston is moving towards BDC
Exhaust – EO 143, EC 15 Piston is moving towards TDC
IO – EC overlap 339 – 15
Note that actual degrees will vary by engine; this is only to illustrate valve overlap.
Explain how to time an engine with either a belt or chain drive.
Familiarise yourself with components to be timed such as balance shafts or oil pump.
Disconnect battery
Align all timing marks on the belt/chain sequence to be timed. If marks are not apparent or missing, make your own marks with cylinder #1 at TDC of compression stroke.
Remove crank or cam sensors if necessary
Loosen tensioner and remove belt/chain.
Clean sprockets to ensure dirt/oil free reassembly (belts only). Reinstall belt/chain starting from crankshaft moving away from tensioner so that all slack will be on tensioner side when engine is timed.
Explain how to adjust MLA valves
Consult the service manual for valve lash specification
Ensure engine is at room temperature
Disconnect battery
Position piston #1 at TDC at the compression to power transition to ensure valves are closed.
Using a Feeler gauge, measure valve lash for cylinder #1 intake and exhaust valves, compare to specifications and adjust if necessary. Turn the engine (360/n) (where n is the number of cylinders in the engine) degrees and adjust the next cylinder valves in firing order. Adjustments are made at cam base circle (part of the cam without lobe).
ie for a four cylinder engine with firing order 1-3-4-2 the procedure would be:
Cylinder #1 at TDC compression to power transition – measure and adjust intake and exhaust valve lash
Turn 90 crankshaft degrees and repeat procedure for cylinder # 3
Turn 90 degrees and repeat for cylinder # 4
Turn 90 degrees and repeat for cylinder # 2
Note: For 6+ cylinder engines, a torque angle gauge may be used.
Explain the steps to take in order to perform a compression test
1- Ensure battery is good and install charger
2- Disable ignition and fuel delivery (see procedure, may have to use scanner *Ford Escape)
3- Ensure throttle is 100% open
4- Remove ALL spark plugs
5- Connect remote starter, or call a buddy (turn off all accessories)
6- Install gauge and crank engine for four compression strokes
7- Wet test weak cylinders
8- Examine results
List causes for weak cylinders/ low compression
- Damaged head gasket
- Worn piston rings
- Damaged piston
- Damaged or burnt valves or seats
- Broken valve spring
- Worn camshaft
- Defective lifters, pushrods or rocker arms
Explain how to perform a cylinder Leak-Down test
Leak-Down test: measures the percentage of air lost in a combustion chamber and helps pinpoint the cause of low compression
Important notes: some engines are required to be at operating temperature for the test. However aluminum heads risk warping if spark plugs are removed when hot. Consult service manual for proper test temperature and procedure.
1– Place test cylinder piston at TDC of compression to power stroke
2 – Remove all spark plugs and radiator cap
3 – Install gauge and set correct pressure
4 – Record loss and compare to allowable limits
5 – If loss is excessive, locate leak by looking for bubbles in the coolant, air noise in adjacent cylinder, air escaping through exhaust valves or removing intake tube and listening at throttle body, or by removing dipstick and listening for blow by gases.
How to diagnose a head gasket leak prior to disassembly
Symptoms include
- Erratic vacuum readings usually between 7 and 20hg. at rhythmic intervals to coincide with one cylinder underperforming.
- If compression is low in one cylinder, install spark plug in adjacent cylinder(s) and retest. A higher compression shows a cylinder to cylinder head, head gasket or cylinder block crack.
- Look for white smoke from tailpipe (coolant vapours)
- Use a chemical combustion chamber tester (draws in and reacts to exhaust gases in coolant vapours)
- External oil and coolant leaks around head gasket.
- Soap can be used around outside of head gasket during leak down test to verify external head gasket leaks if there is weak compression and no symptoms of internal engine leaks.
Various OHC engine measurements
- Deck flatness (mating area between head and block) use a straightedge ruler and a feeler gauge. Position the straightedge diagonally across the deck and try to slip the feeler gauge between the ruler and the block. Switch diagonals and try again. The thickest feeler gauge must not be greater than the warpage tolerance. Continue on measuring around the perimeter. Use the diagram below as a guide
- Cylinder Taper is measured by inserting a telescoping gauge or a dial bore gauge into the cylinder and recoding measurements at the bottom of the cylinder bore and at the top just below the ridge. The difference between the two measurements is the cylinder taper.
- Cylinder out-of-round is measured by taking 2 more cylinder taper measurements at a perpendicular angle to the first ones. The difference between the measurements at the same height in the cylinder is the out-of-round.
- See the link below for a worksheet on cylinder taper and out-of round.
http://biosystems.okstate.edu/home/fharry/3211/900_Homework/CylinderTaper.htm
- Camshaft/crankshaft journal straightness (bore alignment) (saddle alignment) is checked by laying a straightedge ruler thru the bore centerline and trying to slide a feeler gauge of half the specified oil clearance between the journal surface and the ruler.
- Crankshaft end play is measured with a dial indicator installed at either end of the crankshaft with all journal bearings properly lubricated and torque to specifications. Use a screwdriver or small pry bar to move the crankshaft back and forward while observing the readings on the dial indicator
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- Bearing oil clearance is measured with a platigauge. A small strip is layed down on the bearing parallel to the crankshaft ,the bearing cap is then torqued to specifications and then removed to inspect the plastigauge. Compare the width of the squished plastigauge to the reference marks on the container for oil clearance measurements. See the photo below
Good videos for learning how to use a metric and sae micrometer.
Crankshaft components are listed here:
Measuring piston ring end gap is done with the piston ring in the cylinder. Install the piston ring in the cylinder then use the piston to push it halfway down the cylinder walls. At the midway point, measure the piston ring end gap with a feeler gauge and compare to specifications.
Cylinder wall preparation for new piston rings
To prepare cylinder walls for new piston rings it is recommended to deglaze them. Using a ball hone or very fine honing stones begin at the top of the cylinder moving the honing tool up and down in the cylinder to get a criss-cross pattern where the marking on the cylinder wall intersect each other at an angle between 50 and 60 degrees. This is done to guarantee a good seal and proper lubrication between the cylinder wall and piston rings. It is better to leave cylinder walls as is than to deglaze a cylinder this way if not done properly. Do not confuse this with cylinder boring; cylinder boring is done to restore taper or out-of-round to within specifications and will often oversize the cylinder and require new pistons. Although honing DOES remove a fine layer of metal from the cylinder wall, if done properly it DOES NOT significantly affect the cylinder diameter. Honing merely ensures good contact and lubrication between piston rings and cylinder wall. Finally refer to engine service procedures because some manufacturers recommend NOT servicing cylinder walls or have different methods of preparing cylinder walls for new piston rings.
PCV System
PCV (positive crankcase ventilation) system operates to recover blowby gases. Pcv valves are mainly responsible for engine breathing and preventing sludge build up. Inspection should include all rubber air hose conditions, engine air filter and, if applicable, auxiliary valve cover air filter; look for oil contamination. Replace any contaminated parts and PCV valve. If the PCV valve is in good condition suspect excessive blowby gases to be present; perform a compression test!
Functional PCV valve tests are:
- Llisten for a hissing sound at the PCV valve with the car running and valve removed from its seat.
- Block the PCV valve’s opening to check if vacuum is present (car has to be running).
- Shake the PCV valve and listen for needle movement, if no movement is heard replace the valve.
- With the valve removed, blow air thru the valve; air should flow freely in the direction of the intake manifold and exhibit much resistance when trying to blow the other way. You may also pinch the PCV valve’s vacuum line with the engine running and listen for a click (the PCV needle sliding inside the valve), if no click is heard replace the valve.
Cooling System
Finally test yourself by identifying all the major numbered components in the diagram below
Additional notes and observances
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Cylinder balance test (to show power contribution of each cylinder):
Connect vacuum gauge, disable one cylinder at a time for no more than 15 seconds while observing decrease in RPM and vacuum, all cylinders should show equal decrease in RPM when deactivated. USE SCANNER
If heads are shaved on a V or Boxer engine, both heads must be machined equally. A shim type Head Gasket must be used to restore original compression ratio and prevent head bolts from bottoming out and excess slack in timing belt/chain. Camshaft timing may also be affected because of the shorter length between crank and cam.