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Fuel System

Posted: Thu Aug 23, 2012 11:31 am
by Jeroen
Basic info for starters:

Carburetor engines
The available E21 models were equipped with either a Solex Pierburg carburettor (on models 315-316-318-320-320/4-320/6) or with a Bosch K-Jet mechanical fuel injection system (318i-320i-323i). For details on what carb is fitted in your car please refer to the specs pages in the pre-facelift and facelift specs pages.

Fuel leaks are common with 30+ year old rubber hoses and more info on replacing fuel hoses can be found in the post below.

Coming up soon:
- Info on maintenance and adjustment for the Solex 4A1 carb for the 320 6 cilinder
In the meantime, there is a lot of info on this German website:
Please note that this carb is one of the toughest carbs known to man to adjust. Make sure to absolutely follow the adjustment instructions in the link step by step for correct adjustment!
Common replacement: Weber 32/34 DMTL see tuning Weber 32/34 DMTL

K-Jetronic fuel injection (318i-320i-323i)

Video that explains all the basics and components of our fuel injection system:

- AutoData K-Jet troubleshooting
- Bosch manual on K-Jet basics, adjustment and troubleshooting (1.2 Mb .pdf)
- Info on the K-Jet warm up regulator on this Google site
- Interesting article on the Thermal Time Switch at
- Below: detailed info on fuel lines and hoses, and how to fix leaks

Re: Fuel System

Posted: Mon May 18, 2015 12:23 am
by Jeroen
E21 fuel system, lines, hoses, leaks

Several messages have been posted regarding the fuel system and this can be considered as a collection and summary of all the postings. It can be used as a lead to find different causes of leakages and fixes. It is not necessarily about weak points since it is not more than logical that rubber parts become weak after 30 years. Dried out rubber parts is the largest cause of problems. Leakages are annoying, but think as well of the effects of a petrol leakage near the exhaust or when it is sprayed around in a hot motor compartment.

Thanks all for the posts, but most of all to Michel, aka Wondermike, for figuring out all the part numbers and to Jan Willem, user Wilmo, for translating section A for us.

Below you will find:
A. The fuel system of the E21
B. Replacing fuel hoses (tank, pump), based on K-Jet version 318i-320i-323i
C. Description of the work to be performed (to follow shortly)
D. Fuel lines in the engine bay (to follow shortly)

A. The fuel system of the E21

To get started, a very basic description of the system. This description is based on the injection E21’s since the system is here most complex. These models have a pump that is mounted over the right side drive shaft. Models with a carburetor don’t have this pump and have only a fuel line to the mechanical pump that is mounted on the engine. Both are fitted with the same system for filling and air venting. Because of the age of the rubber, it is not smart to cut off a cracked part of a fuel line, for your safety it is absolutely best to change the whole line. The description is most based on the later E21’s with two separate tank halves, many early E21’s (‘til ’77) have only one tank that has to be removed at once.

Fuel lines
Most lines are to be found under an oval hatch under the backseat, on top of the tank. Two lines run backwards over the right tank halve. The thick one (12mm) is the feed for the pump, the thin one (8mm) runs from the pump to the fuel distributor in the engine compartment and is fitted to the fixed line under the same hatch (between the pump and the fixed line), so it is easy to replace. Only a return line from the front is left, this is a short 8mm hose, this one is easy to replace as well.

In the engine compartment, at the fuel distributor, are of course as well some fuel lines. Not many problems are known with these, but it is worth to inspect these for cracks or traces of leakage.

Tank air vent
For the tank air vent are there two smaller hoses of 12mm from both tank halves that come together on a Y-piece and continue in a thicker 14mm hose. This Y-piece is to be found under that hatch as well. The air vent runs to a expansion tank on top of the right wheel arch, which can be reached from the trunk. The thicker hose can be replaced without any problem, however, for the left line is a removal of the left tank halve needed.

To replace the thicker venting hose, it is needed to disconnect the hose at the Y-piece and then open the trunk. Grab the expansion tank on top of the right wheel arch and pull it, with the just disconnected hose, gently towards you. A new hose can be fitted and when put back, the hose should find its way to the Y-piece by itself.


Hose number 4 (see picture) could be made a bit shorter, the left one should be replaced. Number 5 is the thicker hose between the Y-piece and the reservoir on the wheel arch.

Other risks
Except the earlier mentioned hoses and lines are there some more suspects:
- Rubber piece of the filler pipe
- Rubber piece between both tank halves
- Rubber gasket for the sending unit
- Rusted welds of the tanks itself
- Leakages in the engine compartment, for instance the rubber mounting of the carburetor, rubber rings of the injectors and other injection parts.

B. Replacing fuel hoses, based on K-Jet version 318i-320i-323i

Dutch member Wondermike composed a nice grocery list a while ago for the entire operation. All hoses can be replaced rather easily using the cover under the rear seat and a garage jack. There's only one breather hose on top of the left side tank half that requires that half to be removed. Piece of cake though, only three bolts and the large connecting hose. Make sure it's empty though 8) and disconnect it on top of the other tank half before you start.

Removed left hand side fuel tank


View from left hand side showing the right hand side fuel tank that is still in place, and the pipe that connects the two tank halfs


Parts list
Tank sender/pick up rubber O-ring 16111744369
Hose between filler and tank 16121176793
Connecting hose between tank halfs 16121176795 **
Breather hose to breather tank 134cm 14mm 16121177553
Breather hose left side tank 1 meter 12mm 16121176440 *
Y-connector breather hoses 16121115537
Hose pick up to fuel pump 1 meter 12mm 16121176440*
Fuel hose 8mm 1 meter 16121180409

* 16121176440 used on different places, a total of 2 meters is sufficient.
** only applicable for models produced after Aug 77, earlier models have one combined fuel tank

Hose between breather tank and blue overflow hose 6mm 13311272750
(also use abt 7cm between plastic line and steel line above right tank.
I personally left the breather hose from the breather tank to the filler to retain original looks in the trunk, that hose is also less prone to wear anyway.

Hose clamps:
18-24mm (4 X) 07129952109
15-19mm (8 X) 07129952107
09-12mm (5 X) 07129952102
42-48mm (2 X) 07129952119
37-43mm (2 X) 07129952117
12-15mm (6 X) 07129952104

And last one for now are the rubber bushes used for the fuel pump (etc) brackets: (3 X) 16121177891
Check lines between the pump, pressure regulator and fuel filter as well. How old is your fuel filter anyway? :wink:

For model specific details, part nos and pricing you can always check out BMW Classic or BMWFans.

This post is part of an extensive thread in the Dutch section. Perhaps a translation tool can be helpful until I find time to translate the whole story that is located here: viewtopic.php?f=60&t=8559

Re: Fuel System

Posted: Fri Jan 12, 2018 8:31 am
by Jeroen
Copying some text from a recent thread on K-Jet running, adjustment and maintenance issues:

The mechanical K-Jet fuel injection system (continuous injection system or CIS) has these two regulators working together:

Warm Up Regulator regulates the control pressure, ie the counter pressure on the fuel distributor plunger, and in that way it can make the mixture richer by allowing more fuel flow. This is being done by a bimetal strip (approx. 26 Ohm resistance and running on 12V) that operates a membrane inside the WUR and in this way regulates counter pressure. The membrane can get stuck and if you adjust your fuel mixture on the fuel dizzy in this condition, the engine will run lean when cold. Restarting after a WUR overhaul, the fuel mixture may be way off now.

Aux Air Valve aka slider aka throttle bypass valve is either bimetal operated (electrical version) or by coolant temp (wax element mechanical version) and allows more air flow through the intake thus 'asking' more air, further opening the air lever connected to the fuel distributor and in that way raising idle speed. These can also be defective or stuck of course with the same possible maladjustment issues as a result. However, apart from vacuum leaks I don't see many defects on this side, usually it is the WUR.

Temp Time Switch typically switches on for 8 seconds if the coolant temp is low. During these 8 seconds this operates the cold start valve, an extra fuel injector that is located centrally in the intake manifold to further richen the mixture. After these 8 seconds this switches off. If this section fails it will become slightly harder to start a cold engine (requiring longer cranking) but if the rest is set up fine it will start after a few secs. Once running for over 8 secs this section has no function or effect anymore.