This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
For more than ten years, Porsche used the Bosch K-Jetronic Continuous Injection System (CIS) on all production 911s. The system in general is very reliable, and was only bested in 1984 by the introduction of the Motronic engine management system that controls both ignition and fuel injection together.
The CIS meters the amount of fuel provided to the cylinders as a function of the air drawn into the engine. The CIS air flow sensor plate measures the amount of air that is being drawn into the system. This sensor plate is attached to a fuel distributor that evenly meters the amount of pressurized fuel sent to each injector. The system doesn't differentiate among the six injectors: it sends the same amount of fuel to them at all times. Unlike pulsed-injection systems like the Motronic system or even the earlier Mechanical Fuel Injection system, the CIS injectors distribute fuel to the cylinders at all times. The opening and closing of the intake valves controls the intake of fuel into and out of the combustion chamber.
Through 1979, CIS was a semi-open-looped system, which means that it didn't monitor the output gases from the engine to see if the air/fuel mixture was set at the appropriate levels. In 1980, emissions requirements forced the introduction of an oxygen sensor into the system, which helped the engine to better run at the appropriate mixture level through the regulation of fuel pressure inside the fuel distributor.
MAINTENANCE AND ADJUSTMENT
The two primary adjustments for the CIS are the idle speed and the mixture adjustment. The mixture can be adjusted by turning a small hex screw that is located in-between the fuel distributor and the sensor plate. Use a long hex key, or the special mixture adjustment tool to turn the screw.
The screw is indexed so that it should click when you rotate it. Turn it clockwise to richen up the mixture, or counterclockwise to lean it out. The mixture should only be adjusted when the car is hooked up to a CO meter. Make sure that you rev the engine for a few seconds before you take a reading from the meter, and that you don't leave the tool in the adjustment screw: the screw itself must be free to move up and down without any weight on it.
The idle speed is adjusted by allowing more air to bypass the closed throttle. Using a large flathead screwdriver, simply rotate the screw clockwise to reduce the idle, and counter clockwise to increase it. The idle should be set to be about 950 rpm.
So what can go wrong on with the CIS? Plenty. Although the CIS has a reputation for reliability, it is also known for having a few characteristic problems. Warm and cold start problems are common, as well as backfiring and poor performance at startup. On the early 911s, the CIS didn't even operate properly, until the car was warmed up. A hand throttle was a standard part of the car in order to facilitate easier starting and warm-up. On the later model CIS cars, the addition of the auxiliary air valve eliminated the need for the hand throttle.
One of the most common problems for the older CIS cars is the existence of vacuum leaks. The entire system works almost entirely on vacuum. Any vacuum leaks will cause problems in regulating the mixture, and may also result in an uneven idle. To check for vacuum leaks, spray some carb cleaner or starting fluid around the hoses and injectors of the fuel injection system. If the engine idle increases, then you probably have a vacuum leak that needs to be tracked down. Make sure that you have a fire extinguisher handy: if there is arcing in the spark plug wires, you may ignite the carburetor cleaner.
Another common problem is the warm-up regulator (WUR), or control pressure regulator. Contrary to popular belief, the WUR regulates fuel pressure in the system at all times. Cars that experience difficulty adjusting the mixture on may be suffering from a defective WUR. This failure may also contribute to starting problems.
Since the CIS depends upon correct fuel pressure for proper operation, the fuel pump and fuel accumulator must be in working order. Using a CIS fuel pressure tester, verify the proper operation of the fuel pump by measuring the pressure when the ignition is turned on. The pressure measurements need to be taken at specific times, and at varying engine temperatures. For more details on these specifications, see the Porsche Technical Specification Book for your year 911. Also make sure that your fuel accumulator (Pelican Technical Article: Fuel Filter and Accumulator Replacement) and the pressure check valve in the fuel pump are operating properly.
Sometimes the idle on the CIS cars will oscillate up and down. An incorrectly set mixture will often cause this to occur. Recheck the mixture with an exhaust gas analyzer to make sure you are in the proper operating range.
Hot start problems are common with CIS systems and may be caused by a faulty fuel accumulator (see Pelican Technical Article: Fuel Filter and Accumulator Replacement) or a faulty fuel pressure check valve in the fuel pump (see Pelican Technical Article: Fuel Pump Replacement). Cold start problems can often be attributed to the thermo-time switch or the warm-up regulator.
For more information than you will probably every need on the CIS, take a look at the book, "Bosch Fuel Injection & Engine Management" by Charles O. Probst. It provides an excellent guide for understanding the physics behind the CIS, and also includes an invaluable troubleshooting section.
The CIS idle speed is set in a similar manner to the setting of the speed on carburetors and the MFI system. A throttle bypass system regulates the amount of air that passes into the fuel injection system when the throttle is at rest. This screw is indicated by the white arrow. You should adjust the idle until it reaches about 950 rpm. If you have problems achieving this setting, then you might want to double check the mixture adjustment.
The mixture is adjusted by placing the adjustment tool in-between the fuel distributor and the large rubber boot (shown by arrow). The screw is indexed so that you can feel distinct 'clicks' as you rotate it. Turn it clockwise to richen up the mixture, and counter clockwise to lean it out. Make sure that you have the exhaust gases checked while you are performing the adjustment so that you can verify that you are in the proper operating range.
The intake manifold hoses (shown by arrow) are common sources for vacuum leaks. If the carb cleaner spray test indicates that there is indeed a vacuum leak near the hoses, then they should be replaced. Unfortunately, this requires the removal of the fuel injection from the top of the motor: a task that is not easy considering the tight clearances in the engine compartment. Also a source of vacuum leaks are the injector seals and the plastic inserts that fit into base of the intake manifolds. Check and replace these if you find evidence of vacuum leaks near the base of the manifolds.