The Vixen Vacuum System A series of plastic and rubber tubes and hoses provide vacuum service to various components of the Vixen TD 21. Can you name them? Rephrased, this distribution net receives no/none/very little preventive maintenance. The primary reason is that most of the tubing is buried out of sight in the walls, floor and other dark and dirty places - including the engine room wherein resides the BMW 524 TD engine and a heck of a lot of heat! The second excuse for not paying much attention to the system is that it isn't very well understood and doesn't usually leave you stranded half way across the state of Nebraska in midwinter! To better understand the circuitry, let's start at the usage points for the vacuum and trace the lines back to the source of negative power - vacuum is nothing more than pressure in reverse! First we have the control valve(s) for the hot water heater coil as located under the front "hood". Said action is manually initiated by the driver from a hand lever on his instrument panel - a vacuum line provides the muscle to position the valve open or close, heater "on" or "off". The second usage is the positioning of air flow controlling doors located in secret and mysterious places such that I have never seen them - but I have been assured that they exist and are hale and hearty. (But badly in need of evaluation.) The third vacuum usage is that of controlling air pollution. Didn't know that one, eh? This is accomplished by controlling fuel feed rates from the fuel injection pump to the individual infection nozzle by changing the stroke of the distributor pump plunger. The details of this slick little deal are forthcoming in another article which Ye Editor has promised to publish shortly. We are all aware that when the engine is operating at very low RPMs our turbo-charger pressure gauge stands at zero - meaning that there isn't sufficient expanding hot exhaust gases flowing through the turbo's turbine wheel cavity to cause the compressor wheel to develop enough air pressure to achieve the needed power. Read: the driver's foot is down hard on the accelerator pedal and the fuel is flowing into the engine - but combustion is substandard because of insufficient air flow. There goes the old air pollution standard and engine efficiency! This "response - time lag" is something we are all aware of - it shortly corrects itself and things are again rosy. Meanwhile back at the ranch, we also face variations of barometric pressure due to weather and elevation changes. The engine was designed to produce desired results when a predetermined number of pounds of ambient air per minute were provided. The lower the barometer the fewer pounds of air are contained within every cubic foot of air. We may be receiving the designed feet cubed of air but possibly not getting the amount of oxygen needed. Ergo - again, substandard combustion and pollution raising its ugly head! A simple compensatory instrument control setup solved both problems at one whack! It involved comparing turbo discharge pressure with a known level of vacuum and modifying the result with an input from an aneroid barometer. This signal increased or decreased the amount of diesel fuel discharged from the injection pump into each injector nozzle line. Sounds complicated but 'taint. Look for the soon-to-be published article entitled "Boost Compensation/Aneroid Control." With pictures 'n stuff. But, you have to have your vacuum system operating as designed. A ready source of vacuum such as found on a gasoline engine (simply tapping into the intake manifold) is not possible on a diesel engine. Instead it was necessary for a very ingenious vacuum pump to be built into the front end of the BMW engine's aluminum valve cover and to have a lobe on the camshaft provide the necessary operating power. Simple and effective. To determine whether or not there is a vacuum level problem connect a vacuum gauge to the line coming from the valve cover: if you read 20 inches of water gauge with the engine running you are doing fine. Then, take another reading while your co-pilot shuts off the engine - the vacuum level should not drop to lower than 19 inches within 1.5 seconds. More than that - you have leakage somewhere. You can take a series of readings (with the engine operating) adjacent to the components receiving vacuum service and narrow down your search. Remember, if you had been dwelling in the BMW engine room's heat and dirt for the last 15 years, you would not be faulted for being a little cracked and leaking! And so to bed. By: Bill Wilcox Last Update: May 11, 2007