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"I have no idea what to check next. I do not even know where to start."
Since the K-Jet/CIS system operates on hydraulic principles, the place to start (IMO) on K-jet problems is by measuring the fuel pressures (there are two). Most people don't want to spend $100± on the special gauge/valve setup required, but without knowing whether those pressures are correct or not, all that's left is mostly guesswork and trial & error parts swapping. And parts swapping could quickly exceed the cost of the needed pressure gauge.
So much for my K-Jet philosophy. What follows is an attempt to outline the basic K-Jet components and how they work—as briefly as possible. I hope it helps you understand why there are few quick fixes.
Just rambling here, hoping to pass on some info and maybe learn something if anyone spots a mistake. This is as brief as I can be in outlining the basics of (and interaction between) the following K-jet elements:
• Fuel Distributor
• Air Flow Meter
• Control Plunger
• Air Flow Plate
• Control Pressure
• Control Pressure Regulator
• ECU
• Frequency Valve
• Oxygen Sensor
The Fuel Distributor (FD) is the heart of the K-jet system — the black lump with all the hoses, sitting on top of the Air Flow Meter (AFM). Inside the center of the FD is a Control Plunger that moves up and down. The higher it moves, the more fuel is sent to the Injectors by the hydraulic action inside the FD.
What makes the Control Plunger move? Answer: The upward movement of the Air Flow Plate below it in the AFM. As the engine turns and sucks in air, the air flow lifts the plate. The more air flow, the higher the plate lifts the plunger, and the more fuel the FD sends to the Injectors. The amount of fuel is proportional to the engine's need, based on the air flow plate's position, temperature, and O2 sensor input (post '79 cars). If the plate doesn't rise, no fuel leaves the FD.
There's also another force acting on the Control Plunger: the "Control Pressure", which is a special fuel pressure derived from the basic System pressure inside the FD, and regulated by the Control Pressure Regulator (CPR). Bosch also calls this device the Warm Up Regulator (more on that later).The control pressure acts directly on the top of the plunger, opposing the air flow plate's upward movement — but only enought to damp out pulsations and keep the air flow plate stable. The plunger's height relative to the airflow plate is adjustable (a 3mm set screw in the plate). This provides the FIRST of three types of mixture control in the K-jet/Lambda system.
The SECOND type of mixture control is the "Control" pressure itself. The amount of control pressure is controlled by the CPR/WUR. So by varying the control pressure, the the amount of plate/plunger lift can also be varied for a given air flow. For example, by lowering the control pressure on a winter morning, the plunger and air flow plate can rise higher than they would otherwise (for a given airflow rate), and more fuel is injected. The amount of air to the engine, being controlled by the throttle, does not increase significantly. The result is a richer mixture for cold starts. Of course the Cold Start injector helps too, but that's only while cranking.
With the engine started, and in a "cold running" state, the CPR/WUR gradually raises the control pressure acting on the plunger. It does this over a period of 2 or 3 minutes by means of a heated bi-metallic arm acting on a pressure relief valve, and aided by heat from the warming engine. This rising control pressure gradually increases the downward pressure on the plunger, causing the rich "cold starting" mixture to lean out toward a "warm running" mixture. That's probably why Bosch calls the CPR a "Warm Up Regulator" (WUR).
[If that's not too much, there's a little more on how the O2 sensor, Frequency Valve, and ECU fit in. Let me know if you want it here (within the 2-hour edit window) or as a separate post. Or was this too much.]
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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