How To Fix Your Porsche C.I.S Fuel Injection System.

The fuel distributor is the heart of your CIS fuel injection system

In this lesson, we find the problems with this constant injection system, or C.I.S, and solve them.

The injection system can also be called K-Jetronic and was used in Porsche 2.7-liter engines from 1973.5 to 1983 and in the Turbo from 1975 to the early ’90s. The major components are a fuel distributor, a warmup regulator, a throttle body, injectors, in conjunction with the correct fuel pump. Fuel pressures are an important factor when setting up a C.I.S system. We go through the correct pressures, system, and control pressures, to find the running issues we have with this engine.

Problems on this C.I.S engine

High idle

Rough idle/partial miss-fire

Popping in Exhaust

Lean out fuel = engine running worse

When we started the engine, we could tell it had a misfire and was not firing on all cylinders. To find the cylinder that was not firing correctly we started at the fuel distributor. At the fuel head we cracked each cylinder, the engine will stumble when you drop the pressure. We did not stumble on cylinder two. So, at first cylinder two was the one we checked.

The first check we did on cylinder two, was a leak-down test. Even though all the work is new, you still need to eliminate the possibility of a bad cylinder. Following this, we check continuity on the spark plugs set. The engine passes these two tests, so we move on to fuel pressure.

C.I.S fuel injection relies on the correct fuel pressures
C.I.S fuel injection relies on the correct fuel pressures

How to Check C.I.S Fuel Pressures

To check fuel pressure, you will need to install a fuel pressure gauge between the fuel distributor head and the warm-up regulator. Make sure you have a closable valve on the fuel line and make sure you put the valve on the warm-up side. If you have the tap in the wrong place you will not be able to read system pressures, only control pressures. You want to take note of control pressure and system pressure. During testing, you will be reading cold pressures, hot pressures, and pressures under a vacuum.

Before we start our pressure readings, we need to unplug the electrical connections on the warm-up regulator and auxiliary air slide. These components need to be unplugged so they do not heat up during the cold pressure test.

Testing the Electrical Connections

While you have them unplugged it is a perfect time to test them. We use a small Noid light to test them. We are looking for power and ground. You can use an Ohms meter to do the same thing, the light just makes it more visual!

We had a perfect connection to the warm-up reg, but nothing at the air slide. A little bit of investigation showed the wires were there, just pulled out. This one fix has gone a long way to solving our fast idle problem!

Table could not be displayed.

C.I.S Control Pressure

The warmup reg gives us our control pressure because it controls the pressure to the fuel head. You will need to look up the number on the warm-up regulator, and confirm it is the right one for your year. Each warm-up regulator has a different fuel curve, and you cannot just mix them willy nilly.

We have an 0438 140 033 on this 1976 2.7L engine. This is a supersession from an earlier Porsche number. Take the pressure reading and compare it to your chart. Remember the graph is temperature-dependent, so measure your temperature accurately.

C.I.S System Pressure

The system pressure is the reading when you eliminate the warm-up circuit. The control pressure also has a set range for your engine. Our system pressure is a little too high. High pressure will lead to a lean miss-fire. Whereas low pressure will lead to a richer engine.

On the fuel head, the pressure control valve regulates system pressure. This is an area where we can affect the pressure. Using shims to adjust the pressure on this spring will lower or raise the system pressure.

Testing the Fuel Pressures Warm

To test our system and control pressure warm, we will need to plug in the warm-up reg and the air slide again. The time to get to full temp is about 3 to 4 minutes and the pressure will climb in this time and settle during this cycle.

Rest Pressures

When you shut off the power to the system, you can take note of rest pressure. The fuel pressure needs to hit certain pressures after a set time. In this case, we need to be 1.3 at 10 mins and 1.1 bar at 20 minutes.

This residual pressure comes from a few places in the system. The pressure control valve in the fuel distributor, the warm-up check valve, the fuel accumulator, and the check valve in the fuel pump. Because we do not have a fuel accumulator, we are testing the fuel head, fuel pump, and warm-up reg.

If your engine loses pressure in this test, you will have a hard time starting your engine hot and have long crank times to start.

Testing Fuel Pressure Under Vacuum

On the warm-up regulator, there are two vacuum ports. One is for leaning out under a high manifold vacuum. And the other is for enriching. Ported manifold vacuum on top will increase control pressure to approx. to 3.4 to 3.8 bar to lean out the engine at high-speed running but not under load.

The second port is for enriching and will decrease the control pressure to richen up the fuel.

This warm-up regulator is not correct in all areas of function. We will send it out for repair. This is a specialized repair, and we like to send these parts to a specialized shop.

Comparing fuel volumes in a C.I.S system
Comparing fuel volumes in a C.I.S system

Flow Testing the C.I.S Injectors

To test the flow from our injector we will need to perform a flow test. We are not testing the volume over time; we are just comparing each injector to each other. This will show us if we have a fuel flow out of balance.

We had initially thought that cylinder two was a problem, but when we did the flow test, it looks like cylinder one is the problem. Then by moving the injector from position one to position three we illuminated the injector and narrowed it down to a problem with the fuel head.

Just to be sure again, we did a leak-down and ignition check on cylinder one!

Test Results

We found out that both the warm-up regulator and the fuel head were not within specifications. As I said before, we send these parts out for specialized repair. This is very important for the fuel head as they are sealed in a particular way and pressure tested at the facility. This is where a D.I.Y is strongly not recommended.

Six weeks Later

We would normally send the C.I.S parts out for repair while we were rebuilding the engine, but we wanted to show you the differences. Now that the fuel head and warm-up reg are re-calibrated we can get to the final setup parts of the C.I.S system.

In the meantime, we performed the first service on the new engine and changed out the valve covers. In a first service, we go over the valve clearance, change the oil, and re-torque the heads.

Whenever you have any major components taken off the fuel system, the first start can be harder, as you have to refuel the lines.

Setting Fuel Mixtures on a C.I.S

With all our fuel pressure problems worked out, we can fine-tune the running on this engine. To set the fuel mixture you are going to need a gas analyzer. You cannot do this job without one. There is a big difference between a running engine and one running well!

The major gasses you are looking at are CO% and Hydrocarbons. CO% is the amount of fuel we are putting into the engine. We could not get this number where we wanted it before the repairs. After the repairs, the engine was much more responsive, and we could lower the CO% to the correct place and lower the idle too.

Hydrocarbons are a measure of un-burnt fuel. Before the repairs, our HCs were very high, from a miss-fire. After the repairs the HCs were great.

On a C.I.S fuel injection system you need a gas analyzer to set it up correctly
On a C.I.S fuel injection system you need a gas analyzer to set it up correctly

Hot Start and Cold Start

The last tests, before we take this engine off the test stand and back into the car, are a hot start and a cold start.

A hot start is when the engine has been sitting for at least 20 minutes or so and is a good real-life test. The one thing we did adjust after this test was the de-cell. The deceleration valve helps soften the dip to idle when coming off the gas rapidly.

The hot start went great, and our cold start was also a winner.

What’s up next for this engine

The engine and car are going to get back together. This will be an opportunity to really run down any issues with the engine. No load is no test. Most of the work has already been done but setting up this C.I.S system right now. We will bed the engine in the right way, work on some suspension set-ups and return this old car to its happy owner.

Finished C.I.S engine waiting for the car
Finished C.I.S engine waiting for the car

Want More?

Get the coolest tips and tricks today

Subscribe to our email list and get regular notifications of new videos and blog articles on how to repair your own car.