Engine Starting and Running Troubleshooter

Introduction

Troubleshooting starting and running problems can be difficult without awareness of all possible causes. This guide lists symptoms for 944 starting and running problems and common causes of those problems. It is intended to help map out a troubleshooting plan.

Running Problems

1. Rough Idle

a. Cold start rough idle that clears as engine warms

This can indicate a failing engine temperature sensor. When the engine is started cold, the temperature sensor signals the DME control unit to provide a richer mixture. A failing sensor sends a higher-than-actual temperature signal, causing the DME to lean the mixture prematurely. As the engine reaches operating temperature, the enrichment is no longer needed and the engine runs smoother.

b. Rough idle with arcing visible in the dark

A rough idle can be caused by bad plug wires, distributor cap, or rotor. Check plug wire condition by inspecting the wire runs with the car running in a very dark area. Arcing to the cylinder head or fuel rail is visible with faulty wires.

c. Rough idle or cut-off at low RPM, but normal at full throttle

The cause could be a failing O₂ sensor. The O₂ sensor is out of the loop at full throttle and will not affect high-RPM running.

d. Rough idle, possibly correctable by cleaning the air flow box contacts

A failing air flow box damper (mechanically or dirty electrical contacts) can send errant signals to the DME. Cleaning the circuit board inside the air box with a good contact cleaner sometimes corrects this temporarily. Access requires cutting through the caulk holding the cover in place and resealing with non-corrosive caulk after cleaning.

e. Idle fluctuating several hundred RPM

This can indicate a sticking idle stabilizer valve. Cleaning is usually only a temporary fix; the valve should be replaced. A faulty O₂ sensor can also cause RPM fluctuation at idle — unplug the O₂ sensor and check whether idle still fluctuates.

2. Difficult Starting

a. Hard to crank after sitting for several hours

The fuel return regulator or fuel pump check valve may be faulty, allowing the fuel rail to depressurize. The fuel pump must then build pressure from scratch before the car will fire.

b. Hard to start when warm

A leaking fuel pump check valve, fuel return regulator, or individual fuel injectors can allow fuel to drain back to the tank, depressurizing the fuel rail. Warm fuel in the rail can flash, causing vapor lock.

To check: connect a fuel pressure gauge at the fuel rail. Start the car, let fuel pressure stabilize, then shut down and monitor pressure. Pressure should not drop below 15 psig with the engine off. If it drops, clamp the rubber section of the fuel return line to the tank — if pressure holds, the return regulator is suspect. If not, clamp the fuel pump discharge line — if pressure holds, the pump check valve is suspect. Individual injector leakdown can also cause the same symptom.

c. Starts easily cold but hard to restart when warm

A failing DME relay can cause this. A failing relay operates correctly when cool but develops high resistance at elevated temperature, preventing it from picking up after a heat-soak stop. As the relay cools, it functions normally again.

d. Hard to start, fires momentarily then dies

Could be a problem with the distributor cap or rotor. The rotor set screw is notorious for backing out, allowing the rotor to spin on the end of the camshaft. When this happens, the car will not fire at all, or will fire momentarily when the rotor happens to align with the correct cylinder.

e. Hard to start when cold, tachometer does not jump during cranking

Could be a problem with the engine speed or reference sensors. One sensor provides TDC signal and the other provides engine speed in RPM. The DME uses these signals to time the ignition coil pulses and fuel injector firing. Sensor failure sometimes results from debris buildup on the sensor tips that reduces the gap between the sensor and flywheel — cleaning the sensor tips may restore function.

f. Will not start regardless of operating temperature

Possible causes include:

• DME Control Unit
• DME Relay
• Security System Control Unit
• Fuel Pump
• Fuel Pump Fuse
• Distributor Cap, Rotor, Coil, or Primary Coil Wire
• Engine Speed and/or Reference Sensors

A fuel pressure gauge, spark tester, and voltmeter are essential tools for narrowing down the cause.