From 1973 into the nineties, a lot of European cars ran on Bosch CIS, K-Jetronic. Porsche 911 and 930, Mercedes, VW Golf and Jetta, Volvo 240, even the Ferrari 308. Same idea in all of them. The original K-Jetronic has no computer and no fault codes. Only air, fuel, springs, pressures and very precise parts. Later versions got an oxygen sensor, and KE-Jetronic, CIS-E, got a small control unit, but the heart stays the same mechanical system.
For me CIS is the finest of the analog era. Like a well tuned manual machine, or a steam engine. When you understand how many things have to be set right and work together so the car starts and runs the same every time, it is mind boggling. When the digital era came, everything became easier. Computer fixes a lot of small mistakes by itself. CIS does exactly what it is set to do.
I had my own moment with this on the Mercedes 450 V8. Early ones have electronic injection, Bosch D-Jetronic, already in the early seventies. Working on them I was amazed, and I always wondered why electronic did not win against CIS. From the design it looked like it must be the better one. What I found is that electronics of that time were the weak point. Control unit was analog, a lot of parts, no microprocessor. Around 1976 Mercedes went from electronic to mechanical, to CIS, on the 450 too. Later electronics got better and took over. Porsche was first and last with K-Jetronic, from the 1973 911T to the 1994 911 Turbo 3.6.
How it works. It starts with the fuel pump, and it is a special one, with a check valve built in. CIS runs around 5 bar, about 70 psi. Carburetor car runs about 3 psi, so CIS needs some 20 times more, and even later electronic injection runs only about half of the CIS pressure. Pump pushes fuel through the accumulator and the filter to the fuel distributor. In the distributor sits the system pressure regulator. It holds the main pressure, the system pressure, and sends extra fuel back to the tank.
Air going into the engine lifts the sensor plate in the air funnel. Plate sits on a lever, and the lever pushes up the control plunger in the fuel distributor. Plunger opens the metering slits, one for each cylinder. More air, plate goes higher, slits open more, more fuel.
On top of the plunger sits the control pressure, and it pushes back against the plate. This pressure comes from the warm-up regulator. Cold engine, warm-up regulator keeps control pressure low, plunger goes up easier, mixture is richer. Bimetal strip inside gets warm from its own heater and from the engine, control pressure goes up to the warm value, mixture comes back to normal.
Behind every slit there is a differential pressure valve. It keeps the same small pressure drop across the slit, so every cylinder gets the same fuel for the same plunger position. From there fuel goes to the injectors. They open at their pressure and spray all the time, no pulse like on modern cars.
Cold start, the cold start valve sprays extra fuel while cranking, and the thermo-time switch decides how long. Auxiliary air valve gives extra air for higher idle until the engine is warm. When you shut it off, the check valve in the pump and the accumulator hold rest pressure in the lines, so fuel does not boil and the car starts hot.
Look how fine this is. Metering slit for each cylinder is about 0.2 mm wide. Differential valve holds only 0.1 bar across it, against about 5 bar of system pressure. Warm-up regulator, with a small heater and a bimetal strip, takes control pressure from about 1.5 bar cold to 3 bar and more when warm. And all cylinders must get the same fuel, Bosch allows only about 10% difference between them.
Every one of these has its number. System pressure, control pressure cold and warm, rest pressure, plate height, injector opening pressure and spray pattern. All of it pre set by hand, and all of it working together in symphony. One wrong and the whole thing moves. Like a steam engine, everything has to be right at the same time.
Now the truth. Most of the old cars I see have at least one thing not right. Tired pump. Accumulator that does not hold pressure. Warm-up regulator out of spec. Air leak after the sensor plate. Cold start valve that leaks. Plate not centered. Injector that does not spray right. Very often somebody turned the mixture screw to cover a different problem, and now two things are wrong. The car still runs, so the owner thinks this is how an old car runs. Hard start when hot, rough idle, stumble when you get on it, fuel smell. Many of these cars run way below what they can.
And every fault shows in a number, if you know where to look. Car starts hard after it sat few hours, then it is fine? Very often it is the cold start valve leaking. It is fed in a way the plunger can not shut off, so it bleeds the rest pressure away. Rest pressure drops fast after shut off? Then it is the accumulator, the check valve in the pump, the O-ring of the pressure regulator, the cold start valve or an injector, and I test them one by one.
Warm control pressure has to climb and level off within few minutes after the warm-up regulator gets power. If it does not move, heater or bimetal is dead. If it is high all the time, passages are plugged or the return line is restricted. Pressures I check first, and later the volume, return line into a container for 30 seconds. Pressure can still look fine with a weak pump, the volume shows it.
Air leak after the plate is the Achilles heel of CIS. Every bit of that air is not measured, so car runs lean and idle hunts. On a 911 SC one backfire can crack the plastic airbox, and the car does not start at all. Sensor plate rests level with the narrowest point of the cone, never above, max 0.5 mm below. Lifted slowly it must feel even, and it must drop back free. Fuel distributor that sat long can stick, plunger does not slide free, and the car does not start or runs bad after storage.
And the mixture screw. Turned to hide a bad warm-up regulator or an air leak. Then you fix the real problem and the car does not even start until the mixture gets set again. This I see often.
How I work. I start from how it was designed, not from guessing. Engine and ignition have to be right first, and no air leaks after the plate. Then I measure. System pressure, control pressure cold and warm, rest pressure and how long it holds, and later the fuel delivery. Then the sensor plate, height, centering and how it moves. Injectors I check for opening pressure and for spray pattern too. Good one makes fine even cone, no streams, and it does not drip. Every number tells me which part is the problem, fuel distributor, warm-up regulator, accumulator or pump. Mixture is the last step, not the first. I have the right tools for this job, and I work on CIS cars as a professional since 2002.
Up here at 7,000 feet there is one more thing. Air is thinner, and plain CIS has no altitude correction, not on the 2.7 or SC 911, not on the 930. In Denver absolute pressure is about 0.84 bar, against about 1 bar at sea level, so 14 to 17% less air, depending how warm it is. CIS does not go rich by all of it. The way the plate is balanced, car set at sea level goes rich only about 8 to 10% in Denver, and up here in Salida, around 0.78 to 0.79 bar, about 10 to 14%, more on a hot summer day. Some CIS cars got an altitude compensator, but very often it does not work anymore, and the car runs like it has none. So the mixture gets set where the car lives. And if you take it down to sea level often, remember it goes the other way. Car set up here runs leaner down there, and that matters at full throttle.
And then the mountain passes. Monarch Pass, just west of Salida, is about 11,000 feet. Absolute pressure up there is about 0.67 bar, about a third less air than at sea level, so a non-turbo engine also has about a third less power. Car set at sea level goes about 20% rich up there, and even a car set here in Salida gets about 9% richer on the way up. You climb about 4,000 feet from Salida, engine working hard up the hill, cold thin air at the top, then down the other side with throttle closed. Stop at the top for a picture and the engine heat soaks into the fuel lines. In thin air fuel boils easier, so only good rest pressure keeps the car starting again. Plain CIS has no sensor to correct any of this. Everything has to be set right before you go.
This is why this mechanical symphony has to be in tip top shape. At sea level a car with one thing a bit off still runs, and nobody notices. Up here there is no margin left. Car that is a bit rich already goes way rich on the pass, fouls plugs and idles bad at the top. Weak rest pressure, and after a stop on the summit it does not want to start hot. Small air leak hides up here behind the rich mixture and shows up only when you drive down to the plains. A well set CIS car goes over the passes like it is nothing. That is the moment the owner feels what this old analog machine can do, and why it was worth to set every number right.
When everything is in sync again, the car starts hot and cold without drama, idles smooth and pulls clean, the way it was designed. This is the work I like. Bringing these cars back to their glory.
Also: Porsche service · Euro car service · Air-cooled 911 PPI