General Engine Diagnosis Study Notes

General Engine Diagnosis

Task 1 - Verify Customer Concern and/or Road Test Vehicle

  • Determine needed action when a customer presents an engine performance or drivability concern.
  • Modern engines are more efficient than those from the Model T era, utilizing smaller displacements, advanced management systems, turbochargers, and superchargers.
  • Today's engines produce more horsepower and fewer emissions compared to classic Muscle Cars.
  • Diagnostic responsibilities include assessing concerns despite complexities like Variable Valve Timing (VVT) and Variable Engine Displacement.

Basic Engine Functions

  • Modern engines retain core functions similar to early models:
    • Spark ignition four-cycle internal combustion engine
    • Compress air/fuel mixture
    • Ignite mixture with spark plugs
  • Advanced diagnostic tools facilitate exploration of engine assembly, such as:
    • Scan tools
    • Digital Volt-Ohmmeters (DVOM)
    • Digital Storage Oscilloscopes (DSO)
    • Thermal imaging
    • Borescopes

Customer Interaction

  • Often, technicians do not meet directly with customers; this role is filled by service writers or counter staff.
  • Importance of properly training service personnel to gather detailed customer feedback regarding vehicle issues.
  • Suggested to ride with customers to better understand and pinpoint concerns.

Key Information to Gather from Customers

  • Specific details about when problems occur:
    • Overnight vehicle inactivity
    • Engine temperature impacts
    • Driving conditions: highway vs. city traffic

Recommended Starting Steps for Diagnosis

  1. Verify the Concern: Check if the Check Engine Light (CEL) is illuminated.
  2. Check for Diagnostic Trouble Codes (DTCs):
    • OBD II regulations require the Powertrain Control Module (PCM) to test its functions and provide accessible information for aftermarket diagnostics.
  3. Technical Service Bulletins (TSBs):
    • OEMs issue TSBs to address issues; checking these can help identify problems that have been analyzed and resolved by factory engineers.
    • Example: PCM reprogramming may be necessary for unrecorded misfires.
  4. Visual Inspection:
    • Look for signs of neglect or potential sources of problems:
      • Corroded or loose battery cables
      • Damaged or improperly routed wiring harnesses
      • Condition of air filter and coolant levels

Task 2 - Research System Operation

  • Investigate by checking DTCs and any freeze frame data to help diagnose issues like misfire or fuel concerns.
  • Utilize OE-level service information for determining appropriate repair procedures.
    • Use resources such as:
      • Mitchell 1
      • AllData
      • MotoLogic
      • Idenifix
    • Access to National Automotive Service Task Force resources (www.nastf.org) for OEM service sites.
  • Importance of avoiding misleading information from informal online searches; accuracy is paramount in diagnosing concerns.

Common Causes of Engine Poor Performance

  • Identify problems under the categories prioritized in the following order:
    1. Engine Mechanical Issues:
      • Sealing and compression of air/fuel mixture
    2. Volumetric Efficiency:
      • Engine's ability to allow air intake and exhaust
    3. Ignition System Failure:
      • Quality of spark delivery at the correct timing
    4. Fuel Delivery Issues:
      • Proper timing and atomization of fuel delivery

Task 3 - Diagnose Crank/No-Start or Hard Start Conditions

  • Important to address DTCs before further diagnostic actions.
  • Evaluate battery condition:
    • Visual checks for damage or corrosion
    • Ensure tight cable connections and correct battery type.
  • Battery Types:
    • Conventional flooded lead acid
    • AGM (Absorbed Glass Mat)
    • EFB (Enhanced Flooded Battery)
    • Lithium (Lithium Ion or Lithium Iron Phosphate)

Testing Battery Condition

  1. Check Open Circuit Voltage (OCV): Must be no less than 12.5 V for good SOC.
  2. Use a battery tester to verify it can deliver power to starter and other 12V systems.
  3. Use DVOM for a quick assessment of battery and charging system:
    • Measure OCV
    • Record MIN/MAX readings during engine start cycle.
    • Identify acceptable battery performance based on specifications for battery type.
  4. Measure AC ripple to assess alternator performance using DVOM.

Conditions to Diagnose

  1. No Crank Condition:
    • Engine does not turn over; listen for starter clicks.
    • Check solenoid and starter setup.
    • Perform starter current draw tests:
      • High draw indicates problem, while low may hint at wiring issues or weak battery.
  2. Crank/No-Start Condition:
    • Engine spins but does not start.
    • Analyze spinning speed and look for compression issues.
    • Check for Crankshaft Position Sensor (CKP) signal and proper ignition at cylinders.
    • Assess fuel pressure and delivery using mechanical gauges.
  3. Hard Start Condition:
    • Engine cranks normally but is difficult to start.
    • Investigate fuel pressure loss, volumetric efficiency issues, and any symptoms of clogged exhaust.

Task 4 - Inspect for Fluid Leaks

Fuel Leaks

  • Often detectable via the smell of gasoline; check hoses, lines, and connections.

Oil Leaks

  • Commonly due to worn seals, gaskets, or oil pressure sending units.
  • Excessive crankcase pressure symptoms:
    • Oil fouled spark plugs
    • Blue-gray exhaust smoke
  • Inspect crankcase ventilation system for functionality to prevent pressure buildup.

Coolant Leaks

  • Inspect visually for leaks; pressure test cooling system.
    • Look for coolant in the oil indicating internal leaks.

Task 5 - Isolate Engine Noises/Vibrations

Engine Noise Diagnosis

  • Common sources of noise: Crankshaft and valvetrain issues.
  • Muffled metallic sounds may indicate piston slap or excess clearance issues.
  • Address noise frequency and causative factors to isolate problems.

Task 6 - Diagnose Excessive Oil/Coolant Consumption

Diagnosing Exhaust Smoke Variants

  1. Black smoke:
    • Indicates too rich air/fuel mixture.
  2. White smoke:
    • Usually coolant entering cylinders, often from head gasket failure.
  3. Blue-gray smoke:
    • Suggests burning oil from valve seals or piston rings.

Oil Consumption Diagnosis

  • Verify oil usage, and if excessive, inspect for leaks and perform road tests to determine consumption reasons based on driving conditions.

Task 7 - Perform Vacuum/Pressure Tests

Manifold Vacuum Test

  • Monitor vacuum readings at idle for proper sealing and performance.
  • Adjustments needed for altitude effects on readings.

Cranking Vacuum Test

  • Assess cranking vacuum for strength and compression indications.

Task 8 - Perform Power Balance Testing

  • Conduct tests while shorting each cylinder to diagnose issues with engine contribution.
  • Use exhaust gas analysis as an adjunct to this testing methodology.

Task 9 - Perform Compression Tests

Cranking Compression Test

  • Essential for pinpointing leaks and gauging overall cylinder health.

Wet Compression Test

  • An additional technique to highlight compression issues via oil application if testing indicates weakness.

Task 10 - Perform Cylinder Leak-Down Tests

  • Essential for pinpointing leaks by identifying escape points for air pressure under TDC conditions.

Task 11 - Test PCV System Components

  • Regular evaluation of PCV systems prevents oil leaks and engine performance issues.

Task 12 - Engine Diagnostics with Tools

  • Use tools effectively to diagnose engine issues along the lines of mechanical, volumetric, ignition, and fuel system evaluations.

Suggested Techniques

  1. Engine Mechanical:
    • Perform various compression tests and leak-down tests.
  2. Volumetric Efficiency:
    • WOT testing for evaluating air intake.
  3. Ignition:
    • Assess ignition signals and secondary outputs for performance.
  4. Fuel Systems:
    • Fuel pressure versus volume assessments for diagnostic precision.