Fuel Injection

BMWs use a variety of excellent Bosch fuel injection systems, these are extremely durable and reliable if looked after.

As a rough guide pre 1982 models use Jetronic K fuel injection. This is mechanically more complex than later systems but can still run very well, the Porsche 928 used this system as well.

Jetronic L and LE came next followed shortly afterwards by many versions of Motronic. These used more electronic sensors to monitor the engine and had progressively more complex electronic brains to control it.

The first thing to check is that the air filter is clean. A dirty one will stop the engine running efficiently and revving smoothly. Air filters are cheap and simple to replace. A friend's car was running very poorly and wouldn't rev over 3500rpm. Fuel economy was dreadful. The air filter was quite unbelievable, a new one totally transformed the car!

Next thing in line is the air flow meter. This is a large moving flap which monitors the volume of air flow and intake temperature. The flap should move smoothly. Some systems have an idle speed adjust screw built into the air flow meter, this controls the amount of unmetered air allowed to pass through at idle.

Beside and behind the air flow meter you may find an idle speed control valve. This is a simple air flow switch which allows some extra air into the intake manifold to stabilise engine idle. There are two types. Older ones have an internal adjustment screw while later models (post 83?) are fixed. These can wear out but more likely is a build up of soot causing bad operation. Cleaning with a toothbrush and carb cleaner every 30,000 miles should keep everything in good order.

Behind these we have the throttle butterly itself. The throttle plate should move freely and spring back. Cleaning the valve seat is a good idea. There is also a throttle position sensor switch, two types were used. These indicate to the engine management system when the throttle is fully closed or fully open.

Next we have a cold start injector in many 1980s models. This is a simple valve which feeds extra fuel in when the engine is first started under the control of the engine management system. At high mileages the valve can dribble causing a rich mixture. It isn't expensive to replace but can be awkward to get at.

The fuel injectors themselves tend to be very reliable. A does of Techron (USA) or Redex (UK) in the fuel tank just before every oil change will help keep them unclogged.

There are several other sensors such as the thermo-time switch and crankshaft position and speed pickups which the computer uses to manage the engine. Failure of any one can result in poor operation, stalling and bad idling. If in doubt consult the Bosch Fuel Injection Handbook.

As a rule you should avoid BMWs without fuel injection. I had an '87 316 and the carb caused me nothing but hassle. BMW mechanics are not used to dealing with carbs these days!

Problem Solving

If you don't have a mechanical or electronics background then don't read any further, leave it to the experts.

If you experience engine problems in any BMW over six or seven years old the first thing to do is clean all the fuses with abrasive paper to remove oxidation. You should also ensure that all connections to engine sensors are clean and watertight. Cleaning the Motronic speed and TDC sensors totally solved the stalling problem on my 635CSi.

Next thing is to start replacing relays. Relays don't last forever and have a hard live in a demanding environment. The simple kind most commonly found in BMWs cost <$10 each and are simple to change. The main and fuel pump relays only tend to last ten years and cause cause starting and stalling problems. Unloader and power distribution relays can also cause problems. Always use genuine Bosch relays.

The table below refers mainly to 1977-1994 Jetronic-L and early Motronic systems but is applicable in part to both older and newer systems.

Warning! Cranking an engine which won't start may be putting unburnt fuel into your catalytic converter which can permanently damage it, be careful.

A clean well adjusted BMW engine is 100% reliable and a joy to behold. Keep yours in good order for maximum enjoyment!

Component locations guide. This is my 1985 635CSiA euro.

1 Airbox 2 Air Flow Meter (AFM) 3 Idle control valve 4 Viscous fan clutch 5 Thermostat housing 6 Fuel pressure regulator (FPR)     attached to fuel rail 7 Throttle position switch 8 Engine oil dipstick 9 Cooling system pressure cap 10 Coolant warning level sensor 11 TDC & speed sensor wiring       connections 12 Throttle cable 13 Brake fluid reservoir 14 Power steering fluid reservoir 15 Distributor 16 Coil 17 Auxilliary fan control sensors       (not used in UK) 18 Fuse box 19 Brake "Bomb" 20 Oil filler cap 21 Washer fluid level sensor 22 ABS electronics pod 23 Heater fan cover 24 Battery

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