CIS Fuel Injection Operation

by Bob Tucker

There is so much mystery surrounding the CIS (continuous injection system) fuel system Bosch has dubbed K Jetronic, I wanted to try and make some sense of it for all Porsche owners. The bits of the system include the fuel pump, pre-pump on some models, fuel filter, accumulator, fuel distributor, warm-up regulator, injectors, auxiliary air valve, air sensor plate, cold start valve, thermo-time sensor and on newer models an oxygen sensor, frequency valve and mixture control unit were added.

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Fuel Pump

If you think you're fuel pump is not working, here's how to check it out:

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Bosch K Jetronic

The Bosch K Jetronic, or continuous injection system (CIS), is a mechanically-based fuel injection system that was used on the 911, 911 Turbo, 924, 924 Turbo, and 928 models during the 1970's and into the early 1990's. As these systems age, there are several components you can change to tune up the system, which is what we'll cover, using a 911SC as the example. The components we'll look at are the in-tank fuel filter, the regular fuel filter, and the fuel injectors. Note that CIS fuel injectors have a non-replaceable filter inside the fuel injector.

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Suspension and Tires

Before looking at any add-on improvements to your suspension or tires, you should perform a detailed maintenance check on what you have today. 

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Tire Pressures

We all know how important maintaining proper tire pressure is. Tire pressures affect safety, grip, and tire wear. Tire pressures are affected by temperature, both the ambient temperature, and the actual tire temperature. Normal highway driving causes tires to heat up, and causes pressures to increase. Track days and autocross events produce a lot more heat so what can we do the minimize the increase in pressure

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Seats and harnesses

When you're starting out in drivers education, your stock seats and 3-way seat belts are fine. As skills grow, many will find themselves fighting to stay in their stock seats. So you might consider switching to another seat, and there's a gaggle of them available:  With reclining or fixed backs, and steel, fiberglass, carbon fiber/kevlar, or aluminum construction. 

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