This device is located in the fuel circuit between the fuel pump and the fuel filter. There is an internal spring loaded diaphragm which maintains some residual fuel pressure when the engine is shut off. A second purpose of the accumulator is to reduce the sharp pressure increase to the fuel distributor from the fuel pump when it first comes on. The accumlator delays the fuel pressure spike from microseconds to about a second. A third pupose is to dampen out some of the noise from the fuel pump. When the engine is shut off, fuel pressure will normally bleed off over time, and is not intended to remain over at the most an hour or so. Some easy to remember approximate numbers for testing purposes are 15 PSI remaining in 15 minutes after engine shutdown.

In '81, the internal configuration of the accumulator was changed to include a charging valve. The function of this valve was to allow fuel to start flowing out of the accumulator before the entire internal chamber in the accumulator was filled. The valve then allowed fuel to completely fill the chamber as the engine was running. This allowed fuel pressure to build more quickly.

Failure mode: leaking diaphragm

Air box:

The air box is made up of two sections. The top provides a recess for the air filter, a mounting surface for the sensor plate assembly, and for the throttle body. The throttle body opens into the closed bottom section. The lower outputs of the closed section feed the 6 runners for the intake ports of the individual cylinders. At the front (forward) side of the box is mounted the cold start valve, and the air intake from the aux air valve, aux air regulator, and deceleration valve. The 6 airbox output connections and air runners on the 78-79 engines are larger than both the earlier and later cars. In '80 an internal metal manifold was added to the air box. This manifold routed the fuel mist from the cold start valve to the inputs of each air runner to better distribute this mist to each individual cylinder. Prior to this, the cold start valve sprayed fuel into the open intake manifold part of the airbox. The later airbox will retrofit to the '74 - '77, but not the 78-79 without a different rubber connecting sleeve.

Failure mode: Air leaks. The air box is one major component that can contibute to a problem with CIS called "false air".
This is un-metered air entering the system, or air which didn't first go through the air-flow sensor.

Air Flow sensor:

The airflow sensor plate moves in relation to the air drawn into the engine, and is connected via a lever arm to a piston in the fuel distributor. This piston regulates the fuel flow to the injectors.

Failure mode: Misalignment of the plate causes sticking.

Air Restrictor:

The air restrictor is a small opening placed in the breather line from the rubber boot which connects the air flow sensor to the throttle body. This breather line connects to the oil tank. The purpose of the restrictor is to reduce the influx of unmetered air to the engine when the oil tank cap is removed while the engine is running.

Auxiliary Air Regulator :

Provides extra starting air (bypassing air around the throttle body) with a cold engine. The valve slowly closes with heat, provided by an internal resistor, and to a lesser degree, from heat from the engine itself. For part number 0 280 140 200 (from a 1976), the cold resistance of the heater is about 17 ohms. This device was added in '76.

Failure mode: May remain partially open in cold ambient conditions due to open resistance element. Idle speed stays high after warm up. If it stays closed due to mechanical malfunction, the car may be hard to start.

Auxiliary Air Valve:

Provides bypass air around the throttle when the engine is first started. When the intake manifold vacuum reaches 5 - 6 inches, the valve closes. The closing is independent of engine heat. Part number 0 280 160 400 pulls in at ~8" vacuum. This regulator supplies air to the intake manifold during warm and hot starts when the auxiliary air regulator is closed. This was added in '76.

Failure mode: When not closing properly, can cause high idle speed , or idle speed to oscillate.

Cold start valve:

Similar to a fuel injector, this device dispenses supplemental fuel into the air box during cold engine start. It is controlled by starter voltage and thermotime switch.

Failure mode: An open circuit in the solenoid valve circuit prevents operation, causing hard starts. The valve can also leak during time it is not energized, causing an overly rich mixture and excessive fuel consumption.

Control pressure regulator (also called Warm Up Regulator):

Drops control pressure when engine is cold, resulting in a richer mixture during warm up. . The term warm-up regulator is a bit of a misnomer in that this component actually regulates control pressure the entire time the engine is running. It changes the pressure during the warm-up period. Power is applied to bimetal strip in the regulator when fuel pump is on. As the internal resistor heats up the strip, the control pressure supplied to the fuel distributor increases, leaning the fuel mixture. On cars with vacuum control, the WUR enriches mixture under load determined by lower intake manifold vacuum. It is unclear when the the Vacuum controlled units were phased in to the CIS. The Porsche factory workshop manual shows a page for 1974/1975 models with WUR specifications that include both vacuum controlled versions and non-vacuum versions. US and ROW model differences may account for this. Some versions also modify pressure with altitude. Lambda sensor systems don't use the vacuum chamber because of this adjustment being automatically accomplished by the lambda system electronics box. '81 saw a change in the internal resistor configuration for the WUR. A second resistor was added to heat the bimetallic spring, controlled by a bimetallic switch. This allows the mixture to lean out more quickly after start-up,about 2 minutes compared to the former 3 minutes.

Failure mode: Typically, the cold control pressure gets out of adjustment. It is mechanically possible for the warm pressure to be out of specification limit, but usual failure mode is cold pressure. If the internal heating element opens up, car will run rich until heated by conduction from engine. If the filter in the regulator is plugged, control pressure can rise resulting in too lean a mixture at the fuel distributor, likewise if the return line to gas tank is restricted. Some adjustment is possible with the cold pressure regulation. One method was described in Oct 84 Pano in an article by Bruce Anderson. A November 85 article showed how to make one adjustable so the plug can be "un-knocked" if it is driven in too far (also see Upfixin der Porsche Vol. VII). Adjustment requires a pressure gauge. Old style regulators can be replaced with newer ones, placing a piece of tubing over the vacuum connection and leaving it open, pointing down to avoid trash. The internal vacuum diaphragm(s) can leak; check this with a vacuum pump. The diaphragms shouldn't leak over 2 " vacuum in 15 seconds.

Deceleration valve:

(Also called Auxiliary Air device, or vacuum control.) When decelerating from high RPM with a closed throttle, this valve prevents high vacuum in the intake manifold by providing a path for air bypassing the closed throttle. This provides sufficient air in the air distribution box for a combustible mixture. It is one of three devices that bypasses air around the throttle plate. It opens with about 15 to 18 inches of intake manifold vacuum. Deceleration valve 0 280 160 111 pulls in at ~14" Vacuum. This component was supplied in two configurations on the CIS cars. The first type was on the early cars through the 79 models. It was mounted on the throttle body on the "firewall" side, pretty much out of sight. The second appeared on the '80 through '83 and can be seen on the right hand side of the engine, behind the fuel distributor.

The function of this device is the same for both configurations. For conditions of high intake manifold vacuum, such as when the throttle is abruptly closed, the valve opens, bypassing air around the throttle plate. This adds extra air to the mixture preventing a temporarily over-rich mixture. It opens at about 17 inches of vacuum, and closes again as the vacuum falls. In normal driving, (acceleration, and cruising) the valve remains closed.

In working an cars with the later model valve, I have frequently found the actuation line (small line) intentionally plugged by the owner or mechanic. The reasons for this escape me. I suppose that some drivers don't care for the slow fall in RPM caused by the operation of the valve when the accelerator is lifted at high RPM.

Failure mode: The diaphragm can leak, causing a too-high idle.

Frequency Valve:

This component is used only on the engines with Lambda control. The duty cycle of pulses sent to the device by the Lambda control box varies the pressure in the lower chambers of the fuel distributor, which controls the fuel mixture.

Fuel filter:

This element is pretty much a no-brainer. It helps keep contamination out of fuel distributor and injectors.

Failure mode: When clogged, can reduce fuel flow to point of fuel starvation.

Fuel Pump relay:

Operation of the '76 and later relay:

The relay has two sets of contacts - Normally Open (NO) and Normally Closed (NC). In electrical lingo, "Normal" infers no power on the relay coil. On the basic Porsche round relay, pin 87 is NO, pin 87a is NC and pin 30 is the "arm" of the contact that switches between the two.

Relay Connections:
The fuel pump is connected to pin 30 of the relay socket. (Note that you can't jumper power to the fuel pump by jumpering +12 Volts to a fuse on the fuse panel. The fuse which protects the fuel pump is in the wiring that goes from the NC contact to the ignition switch). +12 Volts from the starter terminal of the ignition switch goes to the NO contact and +12 volts from the "Run" terminal goes to the "hot" end of the relay coil (pin 86) and to the NC contact. The "cold" end of the relay coil (pin 85) goes to the Safety Switch on the air flow sensor housing, and the other contact of the Safety Switch goes to ground. The switch is closed when the airflow sensor plate is at rest (engine not running).

Operational Sequence:
Ignition switch turned to "Run" (Not Start) - Relay energized. Relay contact pulled to the NO position.

Ignition switch turned to "Start" - Starter voltage goes to fuel pump through the NO contact. Fuel pump runs while engine is cranked.

Engine starts and ignition switch released from "Start" position - The Safety Switch opens as the air flow sensor plate rises off its stop. This de-energizes the relay coil, and the relay arm flips back to NC, sending voltage from the "Run" terminal of the ignition switch though the fuse and through the NC contact to the fuel pump. Engine is running, fuel pump is running.

Engine dies due to accident or other malfunction - Airflow sensor plate goes to its stop, closing the Safety Switch, and energizing the fuel pump relay (remember the ignition is still on). Relay pulls the contact back to the NO position. But since there is no voltage on this contact from the starter, fuel pump is disabled.

Key is turned off - +12 volts is removed from relay, fuel pump is now off for good.

Another function of the same circuit is overrev protection (unless overrev comes from downshifting problem). At the rev limit, speed sensor closes an internal switch, wired to the same terminal as the Safety Switch, energizing the fuel pump relay, shutting down the fuel pump.

Fuel pump & check valve:

The fuel pump provides fuel at the proper pressure and flow rate to the accumulator and fuel filter. At the output of the pump is a check valve, which prevents fuel flow back to the fuel tank when the engie is shut off. This helps keep fuel pressure in system for restarting.

Failure modes: Inadequate flow and pressure. The pump may freeze up, drawing excess current, pumping no fuel and blowing fuses. The check valve may leak, making for hard starts, particularly when the engine is hot.

Fuel Distributor:

The fuel distributor has the functions of regulating the fuel pressure, adjusting the fuel flow to the injectors based on the position of the air flow sensor and the control pressure from the WUR.

Failure mode: On relatively rare occasions, an internal problem may occur which can result in faulty metering of fuel to one or more cylinders. The fuel distributor is typically not repairable by the DIY'er as the reapir parts are not made available by Bosch. An item in the June issue of the Porsche Panorama magazine (1977) suggests that the units can be rebuilt with a lot of care, but ackowledges that Bosch will not sell internal parts to the public.


The injector opens at ~3.0 bar pressure. The volume of fuel injected is determined by fuel distributor, and the proper pattern is a conical spray.

Failure mode: Filter screen can clog up causing a lean condition, sometimes can be back flushed. The injector can be tested for opening pressure on a pressure pump. Deposits on the injector pintle can alter proper spray pattern, dribble out fuel causing rich running. No fuel should leak out at pressures .5 bar under opening pressure.

Injector lines:

The lines were hard plastic through the '79 model year. In '80 the lines were changed to pre-formed steel lines.

Intake Runners:

The runners provide a path for the air/fuel mixture from the intake manifold to the intake port of each cylinder head. The late 1973 runners were made of formed bent tubing; from 1974 models on, they were of cast aluminum.

Oil Breather Restrictor:

On year models where the oil tank is vented into the air boot connecting the airflow sensor and the throttle body, this restrictor prevents the engine from stalling when the oil tank cap is removed. Removing the cap without the restrictor would allow enough "false air" to enter the system to stall the engine while checking the oil level.

In 1980, the US CIS system was modified to include an O2 sensor system, also called the Lambda system. It consists of the sensor, the control box, the frequency valve which adjusts the mixture at the fuel distributor, and several input devices to the control box. The input devices include an engine temperature sensor, a throttle switch, and the O2 sensor itself, mounted in the exhaust flow. The basic function of the Lambda system is to adjust the fuel mixture to stoichiometric or 14.7:1 air to fuel ratio. This particular air/fuel ratio enhances the operation of the Catalytic convertor, with which the Lambda system was designed to operate. The Cat operates more efficiently in reducing emissions when the fuel mixture is at stoichiometric.

The Lambda system does its job by adjusting the duty cycle of the frequency valve, which is plumbed into the lower portion of the fuel distributor control pressure circuit. If the O2 sensor determines the mixture is too rich, it signals the frequency valve to reduce the duty cycle which leans the mixture determined by the fuel distributor. If it is too lean, it increases the duty cycle. If the mixture is spot on (14.7:1 air-to-fuel), the duty cycle to the frequency valve is 50%. Note that the default mode for a disconnected O2 and warm engine is also 50% duty cycle.

The O2 electronics box also has an enrichment function controlled through the throttle switch. This enrichment task is taken over from the vacuum operated WUR of the earlier CIS engines. The O2 sensor was first installed with the '80 cars, and the vacuum operated WUR was discontinued. In '81, the O2 electronics were slightly changed from '80 but still provided the enrichment function. The electronics box provides cold starting warm up enrichment in addition to the cold engine enrichment provided by the WUR. On a warm engine, this is a closed loop system, continually adjusting the mixture. The system reverts to open loop control in both the start-up and WOT modes.

The cold start enrichment by the Lambda system sets the frequency valve Duty Cycle at 65% , overriding the input from the O2 sensor, until the engine temperature reaches 15 degrees C. At temperatures in excess of 15 degrees C, determined by a thermo-switch, a warmed-up oxygen sensor takes over providing a variable duty cycle. The throttle valve switch also provides enrichment at WOT. There is an idle switch, operated by the throttle valve, which dampens the response of the O2 electronics for a smoother idle.

In '81, the O2 system was modified. A different switch on the throttle valve was added. This modified the system operation in the following way: There are now a total of three switch contacts. The first switch had contacts at 15 degrees and at 75 to 80 degrees. When the second idle switch , at about 1 degree, and the 15 degree switch are closed, acceleration enrichment occurs when the engine is cold (determined by the thermo-switch). The small relay beside the Lambda electronics box was added in '81, which then increased the duty cycle to the frequency valve from 50% to 75% providing acceleration enrichment when the engine is cold, dropping out after about 4 seconds.

Malfunction: Usually the O2 sensor itself. A good one produces about 500 millivolts at 14.7 air/fuel, The sensor actually detects free oxygen in the exhaust gas.

On Disabling O2 sensors:

Disconnecting the O2 sensor electronics box (and not just the O2 Sensor) on CIS engines designed to use them will reduce the cold start enrichment, and the full throttle enrichment. The engine may also probably lose some gas mileage.

Failure mode: The O2 sensor itself usually suffers from a low output over time, and may result in a constant 50% duty cycle during closed loop operation regardless of the actual mixture.

Safety switch on air flow sensor:

This switch prevents fuel pump from running when ignition key is on and the engine is not running.
Failure mode: If the switch fails open or is disconnected, fuel pump runs as soon as key is turned to "Run". If doesn't open, fuel pump won't run.

Thermotime switch:

This switch allows operation of the cold start valve at engine temperatures below 45 Degrees C (113 degrees F) and then only for a few seconds during starting. Power is applied to the switchs internal resistor from the starter. The internal resistor heats a bimetallic strip which interrupts the current flow on the ground side of the cold start connection. After engine warmup, engine heat is responsible for holding open the bimetallic strip, so that the cold start valve will not energize when the starter is operated on a warm engine. Also, after several unsuccessful starting tries on a cold engine, the internal resistor will heat the thermotime switch and cold start valve operation will be inhibited until the switch cools.

Failure modes: An open circuit at the internal resistor will allow the cold start valve to operate an excessive amount of time if the engine does not start soon on a cold engine. An open circuit at the bimetallic strip in the switch will prevent operation of the cold start valve.

Thermal valve:

Holds off vacuum from the warm-up regulator when engine is started cold ('77 - '79 engines). Power is applied to the thermal valve when engine is started, and approximately 30 seconds later, the valve opens. When the valve is open, vacuum to the WUR is then determined by the intake manifold. --The vacuum holdoff results in a mixture richening at warmup. The inner hose connects to the intake vacuum source and the outer hose to the WUR.

Failure mode: An open circuit in the internal heater will keep the mixture from properly leaning out as the engine wams.

Throttle body:

The throttle valve controls the air allowed into manifold, based on accelerator position. The idle air screw adjusts the amount of air flow around (bypassing) the throttle plate.

Failure mode: Usually not the cause of problems.

Throttle operated control pressure regulator:

Early CIS only ('73 - '75) Operated by a cam driven off the throttle shaft. Enriches the mixture at full throttle and at idle by decreasing the control pressure.

Failure mode: Leaking diaphragm keeps fuel mixture too rich at part throttle.

Throttle switch 1:

Closes one set of contacts at idle to just off idle, closes another at Wide Open Throttle.

Throttle switch 2:

Closes at idle.

Vacuum switch (see Thermal Valve)

Warm Up Regulator (see Control Pressure Regulator):

WUR Relay:

For the '73 CIS cars, the WUR got power directly from a fuse in the engine bay powered from the ignition switch. On the '74 and '75 the wiring diagrams show that the power for this relay is derived from the three diodes in the alternator that go to the charge indicator light. Starting in '76, the same relay that powers the fuel pump powers the warmup regulator. So only whenever the engine is running on the earlier cars, or when the fuel pump is actually running on the later cars, should you have voltage on the WUR (except for the '73 as above).

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