BMW Motronic Fuel Injection Problem Solving

Timing Belts

Some engines have timing belts instead of chains. These give quiet operation but must be changed at least every 40,000 miles or five years. Some sources advise a new belt every 30,000 miles. It isn't a huge job to change a belt but the cost of one breaking can be an engine rebuild! Make sure that you have evidence of the timing belt being replaced recently before you buy the car. Models with timing belts include:

  • E30 316i and 318i made after mid-87 (not 318iS)

  • E30 320i, 323i, 325i

  • E34 518i models before mid 1993

  • E28 520i and 525e (not 525i)

  • Early 12 valve E34 520i and 525i

  • Euro spec E36 316i and 318i saloon, convertible and touring before mid-93

  • Ignition Problems

    Check the HT leads connecting the spark plugs to the distributor and coil are not cracked or touching the hot exhaust manifold. A new set can be expensive from BMW but generic leads can be as good and cost only a few pounds each. Most BMWs don't have these if built after the early-90s, they place a voltage amplifier directly above the spark plug.

    You can't really check the spark plugs without removing them, replace them with a new correct set once you buy the car. I once saw a freshly serviced 1982 520i with three types of spark plug fitted, they were all the wrong type! On BMWs with a distributor (generally pre 1991) it's worth checking the rotor arm and distributor cap as worn sets can make a huge difference to power and mpg.

    Fuel Injection Problems

    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 butterfly 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.

    Problem Possible Cause Solution
    Won't start No petrol!

    Flat battery

    No fuel pressure

    Injectors not being fired

    No ignition spark

    Dirty connections

    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

    Dirty wiring

    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

    As above

    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

    Vacuum leak(s)

    Spark plugs

    As above

    Clean wiring connections / replace.

    Adjust valves.

    Check all air hoses for cracks and a good seal

    Check the type of plugs, their condition and the gap. Check all
    cylinders have the same type of plug and HT leads are OK.

    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

    Dirty wiring

    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
    the auxiliary fan only with the A/C switch.

    General overheating Bad water pump. Early M50 units had plastic impeller I'm told.

    Clogged radiator

    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 dipstick marker

    Replace water pump if there is play in the bearing

    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!


    BMW M30 engine diagram

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

    BMW M30 engine diagram

    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

    12 Throttle cable

    13 Brake fluid reservoir

    14 Power steering fluid reservoir

    15 Distributor

    16 Coil

    17 Auxiliary 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