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Engine Controls - Basic Diagnostic Procedures: Other Chevrolet TrailBlazer I

Testing & Diagnostics 2 illustrations ~596 words

Cylinder Compression Pressure

Check engine mechanical condition with compression gauge, vacuum gauge, or engine analyzer. See engine analyzer manual for specific instructions. For compression specifications, see ENGINE COMPRESSION SPECIFICATIONS table. Install and tighten spark plugs to 13 ft. lbs. (18 N.m).

ApplicationSpecification - psi (kPa)
Normal Compression Pressure215 (1482.4)
Minimum Compression Pressure(1)
Compression Variance(2)
(1) Lowest compression reading should not be less than 70 percent of highest compression reading. No cylinder compression reading should be less than 100 psi (689.5 kPa). (2) Information is not available from manufacturer.
(1)Lowest compression reading should not be less than 70 percent of highest compression reading. No cylinder compression reading should be less than 100 psi (689.5 kPa).
(2)Information is not available from manufacturer.

ENGINE COMPRESSION SPECIFICATIONS

Cylinder Combustion Pressure Leakage

  1. Disconnect negative battery cable. Remove spark plugs. Install Cylinder Head Leakdown Tester (J-35667 A). Measure each cylinder on compression stroke, with both valves closed.
  2. Apply air pressure to cylinder leakdown tester. Record cylinder leakage readings for each cylinder. Any cylinder with 25 percent leakage or more requires service.
  3. Inspect following primary areas in order to properly diagnose a leaking cylinder. If air is heard from intake or exhaust system, remove valve rocker arm cover of suspect cylinder head. Ensure that both valves are closed. Inspect cylinder head for a broken valve spring or remove and inspect suspect cylinder head. If air is heard from crankcase system at crankcase oil filler tube, remove piston from suspect cylinder. Inspect piston and connecting rod assembly. Inspect engine block. If bubbles are observed in radiator, remove and inspect both cylinder heads. Inspect engine block.
  4. After performing test, remove leakdown tester. Install spark plugs and reconnect negative battery cable.

Exhaust System Backpressure

Before replacing any components, check exhaust system for restrictions. Use vacuum gauge or low pressure (0 5 psi) gauge to check exhaust system.

If vacuum gauge is used, connect vacuum gauge hose to intake manifold vacuum port and start engine. Observe vacuum gauge. Partially open throttle and hold steady. If vacuum gauge reading slowly drops after stabilizing, exhaust system should be checked for restriction. If using low pressure gauge, connect gauge in following manner

  1. Check At Oxygen Sensor - Remove oxygen sensor. Install backpressure tester in place of oxygen sensor. After test is completed, coat oxygen sensor threads with anti seize compound before installation.

Preliminary Checks

Before performing following tests, check battery condition, engine cranking speed and for adequate fuel in tank. Perform diagnostic system check following any repairs. See DIAGNOSTIC SYSTEM CHECK -- ENGINE CONTROLS under SELF DIAGNOSTIC SYSTEM in SELF DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

FUEL SYSTEM PRESSURE RELEASE

WARNINGRemove fuel tank cap and release fuel system pressure before servicing any fuel injection related components. After releasing fuel system pressure, a small amount of fuel may be released when servicing fuel lines, fuel pump or fuel connections. Cover fuel system components with a shop towel before disconnection. DO NOT allow fuel to contact engine or electrical components.
CAUTIONFuel system is under pressure. Pressure must be released prior to servicing fuel system.

Remove fuel pump relay from underhood fuse/relay block. Crank engine. Allow engine to start and stall. Crank engine for an additional 3 seconds to ensure release of any remaining fuel pressure. Disconnect negative battery cable in order to avoid re pressurizing fuel system. Reinstall fuel pump relay. Tighten fuel filler cap.

IGNITION SYSTEMS

Note. For additional ignition system and component diagnosis, see IGNITION SYSTEMS in SYSTEM & COMPONENT TESTING - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

CAMSHAFT POSITION SENSOR

For testing procedures and specifications, perform appropriate diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

CRANKSHAFT POSITION SENSOR

For testing procedures and specifications, perform appropriate diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

Scheme 2

Scheme 2: ELECTRONIC IGNITION SYSTEM
Scheme 2: ELECTRONIC IGNITION SYSTEM

Scheme 3

Scheme 3
Scheme 3

IDLE SPEED & IGNITION TIMING

Idle speed and ignition timing are controlled by PCM. Incorrect idle speed are normally caused by dirty throttle plate or vacuum leaks. Ensure all vacuum components are functioning properly.

BRAVADA

For wiring diagrams, see ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS article in ELECTRICAL.

ENVOY & ENVOY XL

For wiring diagrams, see ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS article in ELECTRICAL.

TRAILBLAZER & TRAILBLAZER EXT

For wiring diagrams, see ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS article in ELECTRICAL.

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