Engine Controls - Basic Testing Porsche 911 Carrera 964

INTRODUCTION

The following diagnostic steps will help prevent overlooking a simple problem. This is also where to begin diagnosis for a no-start condition.

The first step in diagnosing any driveability problem is verifying the customer's complaint with a test drive under conditions problem reportedly occurred.

Before entering self-diagnostics, perform a careful and complete visual inspection. Most engine control problems result from mechanical breakdowns, poor electrical connections or damaged/misrouted vacuum hoses. Before condemning the computerized system, perform each test listed in this article.

Note. Perform all voltage tests with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure.

VISUAL INSPECTION

Visually inspect all electrical wiring, looking for chafed, stretched, cut or pinched wiring. Ensure electrical connectors fit tightly and are not corroded. Ensure vacuum hoses are properly routed and not pinched or cut. See VACUUM DIAGRAMS article in the ENGINE PERFORMANCE Section to verify routing and connections (if necessary). Inspect air induction system for possible vacuum leaks.

Compression

Check engine mechanical condition with a compression gauge, vacuum gauge or an engine analyzer. See engine analyzer manual for specific instructions. Always disconnect the engine speed/reference sensor for compression tests. If this is not done, dangerous high voltages and insulation damage to the following may result: ignition coil, high-voltage distributor, and ignition leads.

ENGINE COMPRESSION

Exhaust System Backpressure

Exhaust system can be checked with a vacuum or pressure gauge. Remove O2 sensor or air injection check valve (if equipped). Connect a 1-10 psi pressure gauge and operate engine at 2500 RPM. If exhaust system backpressure is greater than 1 3/4 - 2 psi, exhaust system or catalytic converter is plugged.

If a vacuum gauge is used, connect vacuum gauge hose to intake manifold vacuum port and start engine. Observe vacuum gauge. Open throttle part way and hold steady. If vacuum gauge reading slowly drops after stabilizing, check exhaust system for restriction.

FUEL PRESSURE

Begin basic diagnosis of fuel system by determining fuel system pressure.

1. Remove hot-air guide at left rear of engine compartment. Unscrew and remove cap nut from test connection of fuel distribution line. (Scheme 2) Connect fuel pressure gauge. Catch any spilled fuel with a shop rag. CAUTION: DO NOT allow sealing ball to fall out when removing cap nut.
2. Disconnect DME relay from fuse/relay block and jump terminals No. 30 and 87b together. (Scheme 2) Fuel pump should operate at specified pressure. See «FUEL PUMP PERFORMANCE»(/porsche/911-carrera/964-1988-1994/remont/testing-diagnostics/#engine-controls-basic-testing). After testing, remove test equipment and tighten cap nut to 108 INCH lbs. (12 N.m).

FUEL PUMP PERFORMANCE

Note. Digital Motor Electronic (DME) relay terminal numbers are marked on the relay socket.

FUEL PUMP VOLUME TEST

1. Remove hot-air blower at left rear of engine compartment. Disconnect fuel return line from fuel distributor. Catch any spilled fuel with a shop rag.
2. Connect a test hose from fuel distributor to measuring receptacle. Disconnect DME relay from fuse/relay block. Jump terminals No. 30 and 87b together. (Scheme 2) Fuel pump should operate. Allow fuel pump to operate for 30 seconds. Volume delivered into receptacle should be at least 1.8 pts. (.85 L).

Testing Fuel Pump Pressure & Volume. Scheme 2

SPARK

Check for spark at coil wire and at each spark plug wire using a high output spark tester. Check spark plug wire resistance, on suspected wire(s), from cap to plug connector. Resistance should NOT be greater than 4000 ohms.

IGNITION COIL PRIMARY RESISTANCE

1. Ensure ignition switch is off. Disconnect wires from primary terminals of ignition coils to isolate them from system.
2. Set ohmmeter at x1 scale. Connect ohmmeter leads to both primary terminals. Reading should be .4-.6 ohm. If reading is not as specified, replace ignition coil.

IGNITION SECONDARY RESISTANCE

1. Ensure ignition switch is off. Remove wire from coil tower. Set ohmmeter at x1000 scale. Connect ohmmeter leads to ignition coil positive terminal and coil tower.
2. Reading should be 5000-7200 ohms. If reading is not as specified, replace ignition coil.

DISTRIBUTOR CAP & SPARK PLUG LEADS

Resistance of distributor cap suppression resistors should be 1000 ohms (each). Resistance of spark plug lead suppression resistors should be 3000 ohms (each).

IDLE SPEED & IGNITION TIMING

Ensure idle speed and ignition timing are set to specification. Idle speed and ignition timing are controlled by DME engine management system and are not adjustable. Values given in IGNITION TIMING SPECIFICATIONS and IDLE SPEED SPECIFICATIONS are for verification purpose only.

IGNITION TIMING SPECIFICATIONS

IDLE SPEED SPECIFICATIONS

SUMMARY

If no faults were found while performing BASIC TESTING , proceed to G - TESTS W/ CODES article in the ENGINE PERFORMANCE Section.

Poses Online

See also:
• G - TESTS W/ CODES
• FUEL PUMP PERFORMANCE