Manual - Chapter 5 - Fuel & Oil

6: Fuel tank components: general

Note: Refer to the warnings given in Section 2 of this Chapter before working on any part of the fuel system.

Filler cap

  1. The tank filler cap is removed by unlocking it. raising the flap and by removing the four screws which retain it to the tank; note the sealing ring between the filler cap insert and the tank. Renew the ring if it is damaged or worn in any way, ensuring that the modified type is used.
  2. On refitting, the sealing ring must be positioned so that it does not block the overflow hole; do not over tighten the retaining screws or the tank sealing lip will cut into the ring and damage it. Check the operation of the filler cap lock; if stiff, through water penetration or lack of lubrication, lubricate it with CRC5-56 or WD40.
  3. Note that the presence of fuel around the filler cap recess is usually due to a faulty cap sealing ring but may also be due to a build-up of pressure as a result of blocked or kinked vent and drain hoses; check that these are clear.
  4. The filler cap was modified in late 1984; the original pushbutton release being deleted in favour of a cap which springs up automatically when unlocked. At the same time the sealing lip was raised and a new sealing ring introduced, so that any water that collected in the filler cap recess or was forced into it by a pressure washer, was directed into the drain hole. This component was fitted to all 100 models from that date onwards and to all 75 models. Subsequently an additional gasket was introduced which can be fitted to all later-type caps; the gasket is fitted underneath the cap by removing the three screws which secure the cap lower part to the top. If owners of earlier models encounter persistent problems with water getting into the tank, an authorized BMW dealer will have all the necessary details of the modifications.
  5. Note that new locks and replacement keys are available from authorized BMW dealers, also a blank lock which can be coded to match the key already in use on the machine so that only one key is still needed if the lock has to be renewed.
  6. On US models fitted with the EEC system, the filler cap assembly incorporates a flap which is pushed aside by the pump nozzle on filling the tank but which is deliberately designed to prevent the tank from being filled to the brim. This is to allow room for the expansion of the fuel when warm and to prevent neat fuel from passing into the crankcase via the vent hose; if the engine were started with a quantity of petrol (gasoline) in the crankcase the resulting engine damage
    could be very severe indeed, also the possible personal injury that might result. Do not remove or modify the filler flap in any way.

Internal hoses and pipes

  1. Reference to the text and illustration in this Chapter will show the number and layout of the pipes inside the fuel tank. Blow through all vent pipes, drain pipes etc to check that all are clear whenever the tank is removed.
  2. On very early 100 models low fuel pressure may be due to loose hose connections inside the tank. To check the security of the connections, disconnect the fuel feed hose from the fuel rail and plug it with an 8 mm (0.32 in) rod clamped securely by a hose clamp. Run the starter motor for approximately 15 seconds, by which time the pressure will be at the maximum possible. If the hose connections are intact. they will be serviceable in normal use. If any have given way, renew the hose if split or damaged and secure it with a (smaller) 12 mm clamp. Note that on all connections the hose should be pushed fully on to the metal pipe stub so that the clamps locate securely behind the pipe flared ends. Although the smaller clamps were fitted to all later models, this point should be checked carefully on any machine which loses fuel pressure.

External hose

  1. Note that all fuel feed and return hoses have a petrol (gasoline) resistant inner layer surrounded by a tough outer layer which is designed to give protection against cleaning detergent. salt etc. Since this outer layer is not fuel-proof It the hose clamps are not securely fastened, fuel can work its way between the two layers causing the outer layer to swell and then deteriorate. Always ensure that all hose clamps are securely fastened and that the fuel return valve (where fitted) is securely fastened and that its gasket is not leaking.

Fuel return valve

  1. On the original 100 models a spring-loaded ball valve was fitted in the fuel return hose to seal off the passageway whenever the tank was removed. This was found to be annoyingly noisy in operation however, especially from just above idle speed, and so the valve was modified on 1985 models, The modification, which consists of removing the spring, can be carried out as follows on earlier models.
  2. Remove the tank, drain the fuel into a suitably-marked container and unscrew the valve. Lever up its top rim and prise out the cover disc, withdraw the spring and refit the cover disc, then carefully peen over the top of the valve to secure the disc again. Refit the valve, ensuring that its gasket is not forgotten and that it is securely fastened. Note that fuel may drip from the valve from now on whenever the tank is removed; the valve lower end should be plugged or capped to prevent this.
  3. With the introduction of the K75S models in mid-1986 the return valve was deleted completely and replaced by a long stack pipe which passes up to the top of the tank; be very careful not to tilt the tank on removal or fuel may be spilt.

Tank insulation

  1. Note that under certain weather and traffic conditions it is possible for the temperature in the vicinity of the engine to rise to extremely uncomfortable levels, especially in the case of fully-faired models such as the K100RS, K100RT and K100LT.
  2. When these machines are ridden in hot weather the heat build-up, apart from being very unpleasant. can cause fuel starvation due to evaporation in the fuel feed hose and fuel rail. Paradoxically US models fitted with the EEC system can suffer from excessive mixture richness since the rate of evaporation inside the tank is so high and all the vapours are sucked into the engine via the crankcase and breather.
  3. To effect a cure, BMW lined the underside of all tanks with a layer of reflective insulating material. On the fully-faired 100 models a modification was introduced to improve the flow of air through the fairing. The gaiters are removed and replaced by deflectors which actively scoop cold air into the fairing, the metal fairing inner panels being modified to deflect the hot air away from the rider's legs and outwards through the fairing vents. Also pre-cut knee pads are supplied which are to be glued to the sides of the tank to protect the rider's legs from its heat and a higher output pump is fitted.

Evaporative Emission Control System

  1. The system's components and layout are explained in Section 1 of this Chapter. The system requires no maintenance beyond a quick check that the hoses are clear and the valve is working properly whenever the tank is removed.
  2. To remove the hose and pressure relief valve the tank should first be withdrawn so that the hose lower end can be released from the union on the crankcase next to the coolant pipe stub. On refitting, note that the arrow on the valve body indicates which way up it is to be fitted.
  3. If any part of the system is found to be damaged or faulty, it must be renewed; use only genuine BMW replacement parts.
  4. If the machine seems to be running rich, especially when the weather is hot or if the machine is used extensively in heavy traffic, the EEC system may be the cause of the problem; take the machine to an authorized BMW dealer for careful checking. For machines used outside California it may be possible to modify the system, but this must be done only if necessary and in accordance with EPA regulations.

Fuel tank - general

  1. Whenever the tank is removed, check that there is no dust or water inside and flush it out if any is found; not only is the fuel system as susceptible as any other to the presence of water but the pump (especially later types) can be seriously damaged if dirt is allowed to enter it.
  2. Note that the tank is constructed of aluminium alloy; this makes its repair a task for the expert alone. An authorized BMW dealer should be able to advise you of the nearest person able to undertake such work. Due to the number of internal pipes and passageways, the tank must be flushed out even more carefully than normal before it is taken for repair. Make sure that the repairer is aware of the tank's complex construction so that all breather and vent passages, for example, are kept clear.

6.1 Remove retaining screws to release filler cap assembly - check gaskets carefully

6.9 Check all external hoses regularly for security and condition