ENG-15, Engine Starting and Running Troubleshooter


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Introduction

Troubleshooting starting and running problems can sometimes be quite difficult. This is especially true if you aren't aware if all the possible causes of a problem. This is a general guide which will list symptoms for 944 starting and running problems and common causes of those problems. It is intended to help you map out a troubleshooting plan.

Running Problems

  1. Rough Idle

    1. If the car starts when cold but, idles rough or does not want to stay running, this can indicate a problem with the engine temperature sensor. When the engine is started cold, the engine temperature sensor sends a signal to the DME control unit to provide a richer mixture. As the engine temperature sensor starts to fail, it sends a higher than actual temperature signal to the DME control unit. The DME control unit leans the fuel mixture thinking the engine is at normal operating temperature. This causes the engine to run rough. As the engine warms, it not longer needs the richer cold idle mixture and the engine runs smoother.

    2. If the car idles rough this can also be indicative of bad plug wires, distributor, or rotor. One way to check for bad plug wires is to inspect the plug wire runs with the car running in a very dark area. If the plug wires are bad you should see some arching to the cylinder head or fuel rail.

    3. If the car idles rough or cuts off at low RPM or idle, the cause could be a failing O2 sensor. The O2 sensor is out of the loop at full throttle and will not affect the way the car runs.

    4. A rough idle can also be caused by a problem with the air flow box. The damper for the air flow box can fail mechanically or the contacts inside the box can get dirty and send a errant signal to the DME control unit. This can sometimes be corrected by cleaning the electrical contacts inside the air box. This requires removing the cover on the air box which is held in place by caulk. You will need to cut through the caulk and gently pry the cover off. Once the cover is removed, clean the circuit board inside with a good contact cleaner. When you have finished cleaning the circuit board, reattach the cover using a non-corrosive caulk.

    5. If the car experiences a rough idle or idle fluctuates several hundred RPM, this can indicate a sticking idle stabilizer valve. Many people have removed and cleaned them but, this is normally a temporary fix and the valve should be replaced. A faulty O2 sensor can also cause RPM to fluctuate at idle. Unplug the O2 sensor and see if the idle still fluctuates.

  2. Difficult Starting
    1. If the car is difficult to crank after sitting for more than several hours, the fuel return regulator or fuel pump check valve could be faulty. This will allow the fuel rail to depressurize and it takes some time for the fuel pump to increase fuel rail pressure sufficiently to fire the car.

    2. If the car is difficult to start when warm, again the fuel pump check valve, fuel return regulator, or individual fuel injectors could be leaking. The fuel pump check valve or the fuel return regulator leaking allows the fuel to drain back to the gas tank depressurizing the fuel rail. When the car is warm, the fuel remaining in the fuel rail can flash causing vapor lock. There are several ways to check this. One is to connect a fuel pressure gauge at the fuel rail. Start the car, let it run until fuel pressure is stable, then shut down the car and monitor the pressure at the fuel rail. With the car not running, the pressure at the fuel rail should not decrease to less than 15 psig. If it starts to decrease, clamp the rubber part of the line from the fuel return regulator to the gas tank and see if it stops the pressure decrease. If not, clamp the line at the discharge of the fuel pump and see if it stops the pressure decrease. An individual fuel injector leaking can depressurize the fuel rail and cause the same problem.

    3. If the car starts easily when cold, but is difficult to start when warm a failing DME relay could be causing the problem. As the DME relay starts to fail, it operates at a higher temperature. If the car is stopped, the higher temperature of the relay creates a high enough resistance to keep the relay from picking up when the car is started again. When the relay cools and the resistance decreases, the relay will again function normally.

    4. If the car is very hard to start or only fires part of the time and then dies it could be a problem with the distributor cap or rotor. 944 rotors are notorious for having the set screw back out of the rotor allowing it 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 is in line with the correct cylinder to be fired.

    5. If the car is difficult to start when cold it could be a problem with the engine speed or reference sensors. One of the sensors sends a signal to the DME for TDC and the other an engine speed signal. The DME uses these signals to time the pulses to the ignition coil and the firing of the fuel injectors. If the sensors do not send the proper signal to the DME the normal result is a failure to get proper spark from the ignition coil. Sometimes their failure may be a result of debris building up on the ends of the sensors which reduces the gap between the sensor and the flywheel. If so, cleaning the ends of the sensor may get them working again.

    6. If the car is difficult to start or will not start at all regardless of operating temperature, there are a number of problems which could be causing this. Here is a list of possible causes:

      • 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 extremely useful in narrowing down the possible causes of the problem.

Clark's Garage 1998