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E21 New Owner FAQ

Page 1: Safety Checklist
Page 2: Bosch K-jetronic
Page 3: Engine
In Running Order
Now that the car is road worthy we can concentrate on running issues. Most of the questions from new owners orient around the engine. The M10 (assuming the most usual E21) is a very simple engine. Learning the "ins" and "outs" of it won't be too difficult. Many of the terms we throw around may be confusing for some people. So here is a crash course in BMW acronyms:

CIS -- Continuous Injection System... AKA K-Jet WUR -- Warm Up Regulator
T-Stat -- Thermostat CSV -- Cold Start Valve
LSD -- Limited Slip Differential P/N -- Part Number
TB -- Throttle BodyAF Ratio -- Air to Fuel Ratio (14.7 = lambda)
ECU -- Engine Control UnitCO -- Think back to chemistry... Carbon Monoxide

Bosch K-Jet
The fuel injection system on these elder E21's baffles many people. The term "Continuous Injection System" doesn't really sit too well when all your buddies are talking about injector duration and performance chips. There are no chips available for K-jet E21's. I won't offer horsepower tips here, but I will try and explain the basic function of the injection system found under your hood.

First off, I'm not the expert on the subject of bosch K-Jetronic. In fact I would recommend that you pick up a fresh copy of "Bosch Fuel Injection and Engine Management" (20 USD from amazon.com). The book goes into great detail about K-Jet (CIS) and L-Jet (EFI) engine systems, which is handy for those that may have a newer E30 BMW or other marque automobile. Charles Pobst (the author) is probably one of the foremost experts on Bosch control systems. With a shop manual and the Bosch book, there is little that you will be incappable of doing. Reading the CIS section will familiarize yourself with all the components that go into making the M10 or M20 tick. Learning the function of all the pieces will help you diagnose problems and avoid "shotgunning" the fuel system, which is very common.

Common K-Jet Problems and Recommended Solutions
Rule #1 -- If the car is running right and passes emissions, don't play with the fuel system. Keep the fuel circuit clean with a regular dose of fuel injector cleaner and drive the car. K-Jet when running properly is a very stable and reliable fuel injection system. The only problem is that it can be very sensative to subtle changes done by the tinkering type (yours truly).
  • Trouble Starting -- Although sometimes a rather vague question, this one seems to find its way into the forum quite a bit. There are many kinds of "starting trouble" and I'll try and break down a few of them here that we know the cause of.
    • Hot Starting -- "My car doesn't start when it sits for more than 30 minutes but less than 2 hours" i.e., the engine is still warm. There are two camps on this one.
      • Camp One: says the Cold Start Valve is malfunctioning. The CSV richens the mixture when the engine is cold, thus the name. If the engine is warm the CSV isn't really needed. In the event that it does function with the engine warm it can flood the motor.
      • Camp Two: says the Fuel Pressure Accumulator is bad. The fuel pressure accumulator is a diaphram at the rear of the car (near the fuel pump) that shuts off the fuel system when the engine stops. This blocking of the fuel line keeps pressure in the hardlines and all the other fuel lines in the car. If this unit fails the fuel slowly trickles back into the tank, leaving the fuel system dry. If the motor is still warm, the ECU assumes that there is still pressure in the fuel lines and does not turn the pump on immediately. Therefore you car tries to run on air (not going to do much).
    • Cold Starting -- There can be a few cold starting symptoms that are common.
      • "It just cranks and slowly catches" -- Here it most likely is the CSV. There isn't any other part of fuel system that IMMEDIATELY affects the way the car starts when cold.
      • "The car fires a few times and then slowly dies off" -- This is the WUR (warm up regulator). The warm up regulator has a bimetallic spring (much like a thermostat) that richens the mixture when cold. It is located on the drivers side of the motor below the starter. A very exspencive piece new, but some people have extras they are willing to part with or junkyard guys usually don't know how exspencive it is.
Rule #2 -- Junkyards can be your best friend.
  • Loopy Idle -- A "loopy idle" is when the cars motor revs up and down without input from the driver. Also called a "searching idle" because the car seems to be searching for the right rpm. Like most things there are a few areas to check here as well. Remember... this section is for fuel. Loopy idles can be caused by other factors (see below).
    • O2 Sensor -- The 02 or oxygen sensor on an E21 is a important part of the cars ability to monitor what is going on. The 02 reading from this sensor tells the ECU what the mixture of the combustion chamber is. From this reading the ECU can control certain fasets of the K-Jet system to correct a lean or rich mixture. A malfunctioning 02 sensor can give the ECU a false reading (reach or lean) which can cause the ECU to react by leaning out the fuel or allowing more into the combustion chamber. Leaning out the mixture will cause the cars Revolutions Per Minute to increase. While a richer mixture will bog the motor down making the RPMs decrease. You see where I'm going with this? The 02 sensor can cause a loopy idle. Not to mention having a correct reading from the 02 sensor, regardless of idle control, can increase fuel economy. A single wire 02 sensor for an E21 can be had for below 40 dollars.
    • Lambda Injector -- That seems like a weird name right? Well Lambda is a greek letter, we all know that. What most don't know is that the letter "lambda" also stands for something called stochiomic mixture. Stochiomic mixture is the perfect balance of fuel to air for a complete burn in the combustion chamber. Ok so the Lambda Injector (sometimes called the Frequency Valve) has something to do with the air to fuel ratio in the cylinder. The Lambda Injector is the way in which the ECU controls the mixture in the combustion chamber. The reading from the 02 sensor is turned into an injector duration for the Lambda Injector (it is basically an EFI injector). This injector richens or leans out the mixture, either by increasing or decreasing duration (the injector is always moving fuel). Like the 02 sensor (sometimes called a Lambda Sensor for obvious reasons) the Lambda Injector, if malfunctioning, can cause an incorrect AR ratio which can inturn cause a weird idle.
Rule #3 -- Tapping on frequency valves and spraying carb cleaner in a 02 sensor won't fix anything. 02 sensors should be replaced annually for best gasmilage.
  • Other Fuel Related Problems -- Not all fuel problems are apparent at idle or start up. Many more can exist and differing stages of throttle or conditions (coolant temperature / alltitude). Each one of these issues should be addressed by using the Bosch book, a trusted mechanic, or a trusted forum member. We all have experiences with K-Jet, good or bad. Have an experience with the fuel system you remedied? Can you explain the problem and solution clearly and concisely? IMS me and we'll add your expertise to this page.
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