This is the third in a series of "HOW DOES IT WORK" articles describing the various systems of the TD Vixens. The first two described the engine cooling and coach heating systems. This article will describe the engine fuel system.

By Tom Picking
ENGINE FUEL & ELECTRICAL

When I first got my Vixen it was hard to start. The problem got worse until I was afraid to go anywhere. I spent several weeks studying the system and talking to other owners about what might be wrong. I tried many "fixes" hoping they would solve the problem but nothing helped. I learned a lot about the system over those weeks with the help of several people and only after fully understanding "HOW IT WORKS" was I able to find and correct the problem. VIN 0050 is stock with 110,000 miles and starts easily even after being idle for weeks. Yours should, too!

The BMW engine's Bosch fuel injection pump, in my opinion, is a work of art! It is completely mechanical using no electricity (except for the fuel shut off valve) or electronics. It is a very complex system and adjustment or repair of the fuel injection pump should be left to professionals. But if we understand howthe total system works, we should be able to determine if the problem is in the pump or some other component of the system that we can fix ourselves.

In order for a diesel engine to start and run it needs three things: Air, fuel, and heat. It is also very important to make sure that no air can mix with the fuel until the fuel gets into the combustion chamber.
AIR AND HEAT

When the starter cranks the engine, air is sucked in and compressed by the pistons. When air is compressed, it gets very hot, hot enough to ignite fuel if it is present. On very cold days the engine and the air being drawn in are much colder making it difficult for the air to reach the temperature required for ignition. This is where glow plugs come in. They are actually small electrical heaters inside each combustion chamber. They look something like a spark "plug" and when turned on they get so hot they actually "glow". The glow plugs only come on during the initial start up sequence and the amount of time you should wait before trying to start the engine is determined by the engine electronics based on the temperature of the engine. Maximum ON time is about 15 seconds. The "WAIT TO START" light on the dash indicates the glow plugs are being commanded on and the engine should not be cranked till they have had a chance to do their job. If the glow plugs are not working, the engine will still start (except on very cold days) but only with increased cranking times.
AIR AND FUEL

Air in the fuel before it is injected into the engine is a bad thing. The injector pump must be able to develop very high pressure in order to force the fuel into the combustion chamber and if air is mixed with the fuel, this high pressure cannot be achieved.

The injector pump is actually two pumps in one. The first is a low pressure priming pump that floods the primarypump chamber. Any air in this chamber is forced out of a drain line at the top of the chamber along with excess fuel flow from the priming pump. This drainline is common with the drain lines connected to the injectors in each cylinder. If any of these lines leak while the engine is off air will enter the system and fuel will drain backward through the fuel line and empty the primary chamber of the injector pump. The result is hard starting (delayed while the priming pump is filling the primary chamber) but otherwise a normal-running engine.

If there is a leak in the fuel line or the fuel filter between the injector pump and the fuel tank, air will be drawn in by the suction of the injector pump and, depending on the amount of air, the engine may start but will not run correctly under load or high speed. As reported by Charles Rausch (TD 0106) in December's issue of Fox Prints, this same symptom can be experienced if there is a severe restriction in the fuel line such as a clogged fuel filter or a clogged fuel tank filler cap air breather vent.

You can inspect the fuel entering the pump by checking the clear (after 12 years mine is yellow) plastic line connecting the fuel filter and the pump. If you see bubbles in this line while the engine is running or cranking there is a fuel line problem. If you see a large bubble in the line after the engine has been off for several minutes, this is an indication of a drain line problem and that fuel is draining backwards through the fuel line to the tank. If you tap the line, with an indication of air in it, you can watch which side the air bubbles up. That will cut the investigation in half.

Between the tank and the injector pump is a fuel filter. The stock filter has a special manual priming pump built into the head of the filter. Turning the selector head of the filter pump to "RUN" bypasses the internal check valve and makes it easier for the engine injector pump to draw fuel. Turning the head of the filter pump to "PUMP" allows manual operation. The engine will run just fine with the filter pump selected to either position if all the rest of the system is in good condition. Diesel fuel is notorious for dirt and water. The fuel filter is your protection against dirty fuel.

After I learned all of the above, my Vixen still would not start! The problem was that it was not getting fuel because of an electrical problem!

The ignition switch mounted on the steering column (a standard GM part) has several electrical contacts that perform various functions depending on the position of the key in the ignition. One of the functions is to energize the fuel shutoff solenoid valve that allows the flow of fuel to the injector pump. One of the first things I did was to check the voltage to the solenoid valve with the key in the ON position, it was OK. What I didn't realize until several weeks later is that a different set of contacts energize this valve when the key is in the START position. These contacts were defective and when the key was in START and the engine was cranking, the solenoid valve was off and no fuel was available!!!!! This condition can be measured with a voltmeter which eliminates the guess work. A new ignition switch solved the problem.