Engine Controls - Basic Testing Porsche 911 Carrera 993

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 complaint with a test drive under the conditions the 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 using a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless otherwise instructed 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. Inspect air induction system for possible vacuum leaks.

MECHANICAL INSPECTION

Compression

Check engine mechanical condition using a compression gauge, vacuum gauge or engine analyzer. See engine analyzer manual for specific instructions. Always disconnect engine speed/reference sensor for compression tests, otherwise dangerous high voltages and insulation damage to ignition coil, high-voltage distributor and ignition leads may result.

ENGINE COMPRESSION

Exhaust System Backpressure

Check exhaust system using a vacuum or pressure gauge. Remove heated 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 using a vacuum gauge, connect vacuum gauge hose to intake manifold vacuum port and start engine. Observe vacuum gauge. Open throttle part way and hold steady. If vacuum slowly drops after stabilizing, check exhaust system for restriction.

FUEL PRESSURE

1. Remove hot-air guide at left rear of engine compartment. Disconnect fuel return line from fuel distributor. (Scheme 6) Connect fuel pressure gauge. Catch any spilled fuel using a shop rag.
2. Disconnect DME relay from fuse/relay block. Jump terminals No. 30 and 87b together. (Scheme 6) Fuel pump should operate at specified pressure. See «FUEL PUMP PERFORMANCE»(/porsche/911-carrera/993-1993-1998/remont/testing-diagnostics/#engine-controls-basic-testing).

FUEL PUMP PERFORMANCE

Scheme 6

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 6) Fuel pump should operate. Allow fuel pump to operate for 30 seconds. Volume delivered into receptacle should be at least 1.8 pts. (.85L).

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 Coil 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.

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 suspect wires from cap to plug connector. See DISTRIBUTOR CAP & SPARK PLUG LEADS .

IDLE SPEED & IGNITION TIMING

Ensure idle speed and ignition timing are set to specification. Idle speed and ignition timing are controlled by Digital Motor Electronics (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 TESTS W/CODES article.

Poses Online

See also:
• TESTS W/CODES
• FUEL PUMP PERFORMANCE
• DISTRIBUTOR CAP & SPARK PLUG LEADS