General Guidelines

Troubleshooting Chart Symptoms/Causes

Troubleshooting Guide Booklet

Lambda considerations

Troubleshooting components

General Guidelines to CIS Troubleshooting

Before starting with the fuel system, assure that the electrical and mechanical aspects of the engine (ignition and cylinder compression, valve timing, etc.) are in order.

Since your car spends over 99% of it's driving time warmed up to operating temperature, it seems to make sense to carry out checks and adjustments so that they accommodate this mode of operation.  If you are having trouble starting the engine, although it seems like an oxymoron, your first step should be to check/adjust things so that the engine runs while warmed up. (Please note that this site is not intended to cover all the possiblities for a no-start engine.)

There are several checks you can make of CIS components that support the warmed up mode of running without ever starting the engine.

1. Fuel Pump Delivery Rate - (750 cc/min through '77, 1000cc/min through '83).
2. System Pressure - nominal 5.2 Bar, all models.
3. Control Pressure (Cold and Warm) - see WUR fuel pressures, model specific.

Checking items 2 and 3 requires a pressure guage setup. See menu item Testing > Pressures.

If these basic support functions are not within specification, you will doubtless waste time in trying to make other adjustments to correct poor running.

Once these things are in order:

√ Does the engine turn over with the starter?
√ Do the injectors spray fuel with fuel pump running and the airflow sensor plate slightly raised?

√ Does the engine cough and sputter and try to run?
√ Can you keep the engine trying to run by manipulating the throttle?
√ Can you keep the engine trying to run by manipulating the airflow sensor plate?

The answers to these questions should provide some hints as to the direction to follow.
As you can see with the above, the objective is getting the engine to run so that you can warm it up.  This write up is not intended to provide a direct route to get the engine running well enough to run unaided to warm up, but to provide a general path to follow. Check the chart below as a start.

Once the engine is warmed up and has settled in to an idle, get an idea of the approximate mixture setting from the simple ball-park check found in the menu item Testing > Mixture.



troubleshooting chart

Click here to go the Troubleshooting Guide (Pre-Lambda)

Lambda Troubleshooting Considerations

The addition of the Lambda system to the US 911 cars in 1980 was motivated by emissions regulations. Euro 911 models did not have meet the same regulations, and didn't use Lambda.

A WUR is still used on the Lambda cars for leaning out the mixture as the engine warms. However, the WUR used with the Lambda system no longer has a vacuum connection to the intake manifold. This means that a drop in manifold pressure, which was previously used to richen acceleration mixture, is no longer utilized. Warm up enrichment by the WUR is now augmented by that of the Lambda system. (A side note: Disabling the Lambda system in it's entirety is not a good idea. More later.)

The O2 sensor, sending a signal to the Lambda ECU, regulates the fuel mixture for the benefit of the catalytic converter. The ECU also provides a warm up mixture adjustment, and both acceleration and WOT (Wide Open Throttle) adjustments to the Fuel Distributor. For more theory on Lambda, see the Menu under CIS Description > Lambda. The following is information on checking operation of the Lambda system not found in the theory.

For the following table (applies to the '80 model), a dwell meter is connected to the Lambda test connector on the left side of the engine bay. On Lambda cars, the 3-pin test connector is on the left close to the CDI box. The test point is the green/white wire.

Dwell meter readings (4 cylinder setting)
(duty cycle)=(Meter reads on 90˚ scale)

Duty Cycle vs. Dwell Meter Reading
Duty Cycle   Dwell Meter
100%   =   90.0˚
87%   =    78.3˚
85%   =   76.5˚
65%   =   58.5˚
50%   =   45.0˚
20%   =   18.0˚

1980 Lambda Testing

Lambda checks

Throttle switch: For the 3-pin switch, pins 2 and 18 are closed at idle and open (on the one I checked) at ~ 5˚ rotation.  The connection at pins 3 and 18 is open at idle, and closed at 25˚ rotation.  If you adjust the switch for the 2-18 open point, you are adjusting also the 3-18 closure point. There is apparently nothing you can do external to the switch to adjust the two separately, due to the way the switch is made.

I believe the function of the the 3-pin switch is the same from ‘80 - ‘83. However, the model number for the '80 3-pin switch changed in '81. I don't know what the changes were.

 The two pin switch was added in ‘81.  It is adjusted to go from closed at idle to open at the slightest movement off-idle.

Troubleshooting Other Components

AAR and AAV:

The Porsche Workshop Manual has a troubleshooting procedure entitled "Automatic Choke" which is what Porsche considered the combination of the Auxiliary Air Regulator and the Auxiliary Air Valve. The procedure is done in the car with the components mounted, however removing vacuum lines and blocking the air flow with the engine running and noting the rpm changes.

Deceleration Valve:

The Deceleration Valve is closed until a vacuum of around 18" exists in the intake manifold. This is most easily checked with the valve removed from the engine and a hand vacuum pump connected to the small line. The two large lines should be closed until the specified vacuum is reached. The valve should hold the vacuum until released at the hand pump.


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