The Unix Nerd's Domain
Motronic Fuel Injection Problem Solving
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!
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. A height adjustable desk and a good light source will really come in handy, as the fuses are small and will disappear if you drop one on the garage floor. 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.
|Won't start||No petrol!
No fuel pressure
Injectors not being fired
No ignition spark
|Never overlook the obvious.
Could be caused by faulty alarm, courtesy light switches, etc.
Bad fuel pump relay. Can you hear the pump running?
Bad TDC or speed sensors. Power supply or control connections to DME.
Bad coil (late models have one per cylinder). DME problem. Bad rotor arm.
Clean everything in sight!
|Stalls at any speed or load||Bad main or fuel pump relay
Bad unloader relays
|Will eventually turn into a "no start" problem.
Replace with reference to wiring diagram.
Cleanliness is next to godliness!
|Stalls at idle||As above
Worn air flow meter carbon track
Bad idle control valve
Jamming idle control valve
Bad / mal-adjusted throttle position switch
Hard to fix, get a replacement.
Worn carbon track, time for a new one.
Clean with a toothbrush and carb cleaner. Should move freely.
Clean and adjust
|Erratic idle||As above
Bad oxygen sensor
Badly adjusted valves
Clean wiring connections / replace.
Check all air hoses for cracks and a good seal
Check the type of plugs, their condition and the gap.
|Idle too fast||Floor mat stuck against pedal||Really, it did happen to a friend!|
|Runs very rich||Leaky cold start injector
Bad thermo-time switch
Clogged air filter
|Clean or replace, awkward to get at
Clean wiring / replace sensor
New air filter
|Stalls only when cold||Bad thermo time switch or coolant temp sensor||Clean wiring / replace sensor|
|Won't start when warm or is hard to start||Leaking fuel injector
Bad fuel pump check valve
Bad fuel pressure regulator
|Probable cause is fuel pressure bleeding away. The FPR
holds pressure at
one end of the fuel rail and the fuel pump non return valve the other. Clamp
each fuel line in turn after you stop the engine and leave for an 30-60 minutes.
If it starts you may have found the problem.
If it's a leaky injector the car may give off a fair bit of black smoke when it starts.
|Fuel smell||Old fuel lines cracked inside or loose jubilee clips.||Should be replaced every 10-12 years with BMW fuel line.
Other makes may
not stand the pressure and heat.
|Lack of power under load||Bad fuel pressure regulator
Badly adjusted throttle position switch
Clogged fuel filter
Worn rotor arm
Check with voltmeter and adjust
Replace fuel filter
Replace it, it's causing an ignition problem
|Overheating in traffic only||Bad fan viscous coupling
Bad auxiliary fan, ballast resistor or temperature sensors.
|Should turn easily when cold and be stiff when warm.
Replace if bad (32mm
nut with reverse thread!). May also be badly clogged radiator fins.
Test and replace as required. Models in colder regions
(like Scotland!) control
|General overheating||Bad water pump. Early M50 units had plastic impeller
Slack / broken water pump drive belt
Major coolant leak.
Faulty coolant temp sensor.
Have it cleaned out.
Adjust or replace
Get roadside assistance, especially M42 1.8 engine! Don't drive the car!
Clean wiring / replace.
|Grinding noise at idle||Oil low
Bad water pump bearing
Worn chain tensioner
Worn camshaft lobes
Poor oil pressure
|Some engines need to be kept near the upper edge of the
Replace water pump if there is play in the beaaring
Should last at least 80,000 miles
Caused by infrequent oil changes or low pressure
Worn oil pump or clogged oil passages
A clean well adjusted BMW engine is 100% reliable and a joy to behold. Keep yours in good order for maximum enjoyment!
2 Air Flow Meter (AFM)
3 Idle control valve
4 Viscous fan clutch
5 Thermostat housing
6 Fuel pressure regulator (FPR)
7 Throttle position switch
8 Engine oil dipstick
9 Cooling system pressure cap
10 Coolant warning level sensor
11 TDC & speed sensor wiring
12 Throttle cable
13 Brake fluid reservoir
14 Power steering fluid reservoir
17 Auxilliary fan control sensors
18 Fuse box
19 Brake "Bomb"
20 Oil filler cap
21 Washer fluid level sensor
22 ABS electronics pod
23 Heater fan cover