Comprehensive diagnostic procedure for the Rochester fuel injection system on the 1966 Corvette 327ci V8, including system pressure testing, injector function verification, and electronic control troubleshooting.

Warnings

Tools required

Parts

• Fuel filter × 1 — Use OEM specification
• Fuel injection system O-ring kit × 1 — Rochester FI O-ring set

Preparation

1. Park vehicle on level surface and apply parking brake
2. Ensure fuel tank is at least 1/4 full for accurate testing
3. Allow engine to cool to ambient temperature if previously run
4. Verify battery is fully charged (12.6V minimum) as weak battery causes false diagnostic results
5. Gather all service documentation including wiring diagrams for Rochester fuel injection system
6. Position fire extinguisher within immediate reach
7. Remove engine air cleaner assembly and set aside
8. Inspect all visible fuel lines and connections for leaks, damage, or deterioration before proceeding

Procedure

1. 1

   Visual Inspection and Preliminary Checks
   Perform thorough visual inspection of the Rochester fuel injection system. Check all vacuum hoses for cracks, proper routing, and secure connections. Inspect all electrical connectors at injectors, fuel injection control unit (mounted on firewall), and sensors for corrosion, looseness, or damage. Verify throttle linkage moves freely through full range without binding. Check condition of fuel filter (mounted on frame rail) and inspect all visible fuel lines for leaks, kinks, or deterioration. Document any obvious issues before proceeding with testing.
2. 2

   Relieve Fuel System Pressure
   Locate the fuel pressure test port on the fuel meter body atop the intake manifold. Using extreme caution, slowly loosen the test port screw while holding shop rags around the connection to capture fuel spray. Allow pressure to bleed down completely (this may take 30-60 seconds). Tighten test port screw finger-tight. This step is critical before any fuel line disconnection.
   ⚠️System pressure can exceed 1000 psi. Stand clear of the relief point and ensure rags are positioned to capture spray.
3. 3

   Connect Fuel Pressure Test Gauge
   Install fuel pressure gauge adapter to the test port on fuel meter body. Ensure gauge is rated for at least 100 psi and has proper threading for Rochester FI system. Tighten connection securely but do not overtighten. Route gauge hose away from hot engine components and moving parts. This gauge will remain connected throughout multiple tests.
4. 4

   Static Fuel Pressure Test
   Turn ignition key to ON position (do not start engine). Fuel pump should run briefly (2-3 seconds) and build pressure. Observe gauge reading: correct static pressure is 85-95 psi. If pressure is below 80 psi, suspect weak fuel pump, restricted fuel filter, or faulty pressure regulator. If pressure exceeds 100 psi, pressure regulator is likely stuck closed. If no pressure builds, check fuel pump operation and electrical supply. Record actual pressure reading.
   ⚠Do not crank engine during this test. Static pressure measurement requires pump operation only.
5. 5

   Running Fuel Pressure Test
   Start engine and allow to idle. Observe fuel pressure gauge: running pressure should be 78-88 psi at idle (approximately 7-10 psi below static pressure). Rev engine to 2500 RPM and verify pressure remains stable within this range. Pressure drop greater than 15 psi under load indicates fuel pump weakness or fuel starvation. Pressure that rises above 95 psi at idle indicates vacuum line to pressure regulator is disconnected or regulator diaphragm is ruptured. Record idle and high-RPM pressure readings.
6. 6

   Fuel Pressure Leak-Down Test
   With engine at operating temperature and idling, turn ignition OFF and immediately observe pressure gauge. Pressure should drop to approximately 65-75 psi within first 5 seconds as regulator closes, then hold steady for at least 5 minutes. Rapid pressure decay (dropping below 50 psi in under 2 minutes) indicates leaking injector(s), faulty check valve in fuel pump, or internal pressure regulator leak. Note time required for pressure to drop to 50 psi if applicable.
7. 7

   Manifold Vacuum Test
   Connect vacuum gauge to manifold vacuum source (any available port on intake manifold, not ported vacuum). Start engine and observe vacuum at idle with transmission in neutral: reading should be 17-21 in/Hg steady. Low vacuum (below 15 in/Hg) indicates engine mechanical problems, vacuum leaks, or incorrect ignition timing. Erratic needle fluctuation indicates misfiring cylinders or valve train issues. These conditions will cause incorrect fuel injection operation regardless of FI system health. Record vacuum reading and needle behavior.
8. 8

   Injector Electrical Signal Test
   With engine running at idle, use test light or multimeter to verify power supply at each injector connector. Each of the 8 injectors should show battery voltage (12V+) on one terminal and pulsing ground signal on the other terminal. Probe the ground wire and observe pulsing (test light should flicker rapidly or multimeter should show fluctuating DC voltage). Absence of pulse at any injector indicates wiring problem or faulty control unit. All injectors pulse simultaneously on this system (batch-fire design). If noid light set is available, install inline with one injector connector to visually confirm pulse pattern.
9. 9

   Test Manifold Pressure Sensor Operation
   Locate manifold pressure sensor on firewall near fuel injection control unit. With ignition ON and engine OFF, measure voltage at sensor signal wire: should read approximately 4-5 volts. Start engine and verify voltage drops to 1.5-2.5 volts at idle (reflecting manifold vacuum). Disconnect vacuum hose from sensor and verify voltage returns to 4-5 volts (simulating wide-open throttle condition). Reconnect vacuum line. If voltage readings are incorrect or do not change with vacuum, sensor is faulty and must be replaced.
   ⚠Use extreme care when backprobing electrical connectors. Damage to connector pins will cause intermittent issues difficult to diagnose.
10. 10

    Test Air Temperature Sensor
    Locate air temperature sensor in intake manifold near throttle body. With ignition ON and engine cold (ambient temperature), measure resistance across sensor terminals: should read 2000-3000 ohms at 70°F. Start engine and allow to warm up, then re-measure: resistance should decrease to 200-400 ohms at full operating temperature. Alternatively, measure voltage drop across sensor with control unit connected: should be approximately 3-4 volts cold, dropping to 1-2 volts hot. Sensor that does not change resistance with temperature is faulty.
11. 11

    Test Throttle Position Sensing
    Verify throttle position switch operation located on throttle body. This is a simple on/off switch that closes at idle and opens off-idle. With ignition ON, measure continuity across switch terminals with throttle closed: should show continuity (less than 1 ohm). Open throttle slightly and verify continuity breaks (infinite resistance). If switch does not operate properly, adjust or replace. This switch signals idle condition to control unit for proper fuel delivery.
12. 12

    Check Ignition Timing Relationship
    Connect timing light to number 1 cylinder spark plug wire. Start engine and verify base timing is set to factory specification (typically 6-8 degrees BTDC for California emissions, 8-10 degrees BTDC for federal). Incorrect timing causes incorrect manifold vacuum readings which directly affects fuel injection calculations. If timing is incorrect, adjust distributor to proper specification before concluding fuel injection diagnostics. The fuel injection system assumes proper ignition timing for correct operation.
13. 13

    Analyze Diagnostic Results
    Compile all test results and cross-reference to identify root cause. Common failure patterns: Low fuel pressure with good vacuum indicates fuel delivery problem (pump, filter, regulator). Good fuel pressure but poor running with low/erratic vacuum indicates engine mechanical issues, not FI system fault. Good pressure and vacuum but missing injector pulses indicates control unit or wiring fault. Normal electrical signals but rough running with good pressure/vacuum suggests clogged injector nozzles requiring cleaning or replacement. Refer to service manual troubleshooting charts with your specific test results to pinpoint faulty components.

Reassembly

1. Relieve fuel pressure again before disconnecting test gauge
2. Remove fuel pressure gauge and reinstall test port screw, tightening securely
3. Reconnect any disconnected sensors, vacuum lines, or electrical connectors
4. Reinstall engine air cleaner assembly with all ducting properly secured
5. Start engine and inspect all connection points for fuel leaks
6. Allow engine to reach operating temperature and verify normal operation

Verification

• Engine should start easily and idle smoothly at 700-800 RPM with no hesitation
• Fuel pressure should stabilize at 78-88 psi at idle and remain steady during driving
• No fuel odor should be present indicating all connections are sealed
• Engine should respond crisply to throttle input without stumbling or hesitation
• No check engine light or warning indicators should be illuminated
• Perform short test drive to verify proper acceleration and fuel delivery under load

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