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Chapter 8 of D-Jetronic article series by Dr-DJet describes fuel-pump and pressure regulator



8.1 Overview of fuel supply

Fuel circulation
Fuel circulation
The Bosch D-Jetronic fuel injection system injects intermittently and determines amount of fuel by opening times of injectors. Nearly all D-Jetronic systems run at 2.0 + 0.1 bar, meaning between 2.0 and 2.1 bar (exception: Volvo). It is eminently important that this value stays constant at all speeds and loads of engine. Otherwise ECU cannot control the amount of fuel injected. In order to achieve this, fuel pump must always supply more fuel than the engine might consume ain any state, no matter whether you accelerate or run at full-load on a German highway. Otherwise pressure would decrease and amount of fuel injected would be indeterminable. That is why fuel pump always supplies much more fuel (0.6 to 0.8 liter in 30 seconds, in some cars even more than a liter). 0.8 l/30s would mean a fuel consumption of roughly 100 l/100km if running at 200 km/h. If your car consumes more than that ;-), this cannot be supplied by your original fuel pump. This increased fuel supply is also needed for cooling and to reduce fuel steam bubbles in engine compartment. Still it might happen that your car suffers from steam bubbles when doing a hot start. This problem could not be completely solved in D-Jetronic, one of the reasons why L- and K-Jetronic use higher fuel pressures to overcome steam bubble problems.

Excess fuel is sent back to tank by a pressure regulator, so that the flow in and out from common fuel line over injectors and cold start valve is always regulated and resulting pressure there is kept constant. You should NEVER follow so called specialists who recommend you to increase or decrease fuel pressure because the real mistake in your ignition and injection system cannot be found. Often car runs too rich in idle-run and they try to reduce fuel pressure below default values to compensate. This is dangerous and you should be aware that playing with fuel pressure will change air / Fuel ration across all engine speeds and load states. it can be that your engine will thus run too lean in part-load and you risk your whole engine because it overheats and finally creates a hole in your piston!

8.2 Fuel pump

Your fuel pump is a wet roller cell pump type, that is bad in sucking in fuel and good in creating pressure. Therefore it is normally mounted close and best below your tank. Fuel flows from tank through a mesh in tank into your fuel pump. In some cases (MB) there is a damper in between. Some early cars also use a pulsation damper with a diaphragm between suction and pressure line. You better replace such solutions. If that diaphragm gets torn, no fuel will be pumped to your engine any more. The normal paper based fuel filter is mounted only after fuel pump so that it does not unnecessarily decrease the flow into the pump. Fuel pump consists of a normal permanent magnet motor with carbon brushes with a power consumption of 30 to 80 W. The motor is flown through by fuel, that's why it is called a wet pump. However it will not burn or explode as we only apply 12V DC. The motor is driven by the ECUECUDefinition: ECU - short for engine control unit. An electronic device that controls engine operation and reads all sensors. Both in D-Jetronic and KE-Jetronic. In German it is called Steuergerät....
via a relay and a 2.5 mm² wire. Its second wire goes to ground. After switching ON ignition you must be able to hear the fuel pump run for 2 seconds. Without starting the engine ECU will then stop it. Be careful when you run it permanently via separate wires. If this motor runs dry, it can overheat.

Late fuel pump: Suction flange in front, pressure flange on back side
Late fuel pump: Suction flange in front, pressure flange on back side
Roller cell type means that when you open the front pump part, you will find 5 freely moving rollers. By centrifugal forces they are pressed against the wall and thus seal and push fuel forward. In order to avoid pressure peaks they have an over-pressure release that will open when forward pressure reaches 3 bar. Additionally you have a non-return valve in pressure line that keeps the pressure in engine compartment up when pump and engine stop. If a car stands still for a long time, usually following will happen:

  1. Empty tank will catch rust that will collect in front of mesh which will block fuel from flowing into fuel pump. As fuel pump is weak in sucking in fuel, mesh must be cleaned.
  2. Roles in the pump can resinify and stick. After that they will not seal any more and thus cannot convey fuel. When this happens, fuel pump must be opened, cleaned and re-sealed.
  3. Non-return valve on newer types is a ball valve. This ball can also resinify or worse it often gets lost completely. Unfortunately Bosch does not supply spares any more. But there are non-return valves that you can insert into fuel line instead.

Early fuel pump: 3 flanges and combined non-return and over-pressure relief valve
Early fuel pump: 3 flanges and combined non-return and over-pressure relief valve
Fuel pump with sticking rollers
Fuel pump with sticking rollers
There are two basic kinds of fuel pumps, either with 3 or with 2 hose connectors. Early pumps have 3 connectors in front for sucking in fuel, pumping out fuel and an over-pressure return line to tank. In case of a fuel pressure above 3 bar, the combined over-pressure and non-return valve opens and lets fuel return to tank. When pump stops, this valve also closes the middle pressure line, thus acting as non-return valve. Later pumps only have a suction connection in front and a pressure connection in back with integrated non-return valve. Non-return valve can be dismounted and cleaned. However often enough the ball is gone and then you have no chance. Without this non.return valve fuel pressure in fuel line at engine cannot be maintained after pump stops. So if you have a quickly fuel pressure after pump stops, check this non-return valve, fuel injectors and pressure regulator as well as cold start valve and fuel hoses. Later pump also has an over-pressure relief in its front flange and short cuts the pump if necessary.

Often it can happen that the pump leaks and will not convey fuel. Then replace both O-rings (33x2mm, fuel resistant) and clean the pump inner (with petroleum, not with water) without opening the motor. When rollers are still clean, pump will afterwards start to work again. But if the rollers or the walls are rusty, there is no way of repair.

Specific fuel pumps do not only differ in 2 or 3 hose connectors but also in motor power and size, thus determining flow rate. Fuel supply to engine must always be higher than the maximum that could be consumed. This serves to cool fuel pump and to prevent fuel steam bubbles. Since late 60s it is compulsory that a fuel pump must stop to pump if the engine stalls. This prevents leaking fuel after an accident and is controlled by ECU.

8.2.1 Why do modern K-Jetronic fuel pumps not fit?

D-Jetronic fuel pumps were dimensioned to supply 650 to 850 cm³ in 30 seconds at a pressure of 2 bar. When pressure increases due to a blocking fuel hose or regulator, this supply falls drastically. At 3 Bar this pump even stops supplying to engine. K-Jetronic has a much higher system pressure of around 5-6 Bar. It must therefore supply more fuel and higher pressures and this does not fit D-Jetronic. It will work when you try, but please remember that it will create much higher pressure peaks and thus create wear on pressure regulator and injectors. And they also cost a lot of money. I had to replace one injector after a return fuel hose was blocked. I recommend to invest money in a proper pump before you have failing injectors and pressure regulators later.

Bosch uses an universal pump with a special connector 0 580 464 999 as replacement. After learning that they are made for 4 Bar system pressure and that hey have no over-pressure protection, I am not so fond of them any more. That is why I recommend instead:

8.2.2 Changing from early to late pump model

You can change from old 0 580 970 xxx pump with 3 flanges to the new 0 580 464 xxx with 2 flanges as also the new pump has an overflow valve inside. However you will have to remove the return hose to tank from pump. Pressure hose is then connected to the backward flange and you are ready. Bosch offers a replacement pump 0 580 464 999 (price 2015: 316€) but I am not fond of it. I rather repair my old pumps instead or recommend to use pumps for 3 Bar system pressure as used on L-Jetronic systems. In principal all pumps 0 580 464 004 to 0 580 464 010 are exchangeable, their flow rate varies a little, but electrical connectors are quite different from screws to receptacles to special VW connectors.

8.2.3 Repairing a fuel pump

There are easy and difficult repairs on these pumps. Inside the housing there is an electrical permanent magnet motor that is cooled by fuel with 2 bearings and coals. Unfortunately this motor is constructed in such a way that you cannot easily open and reseal it. Ifixit would give 3 out of 10 points. So I do not recommend opening the motor housing. What can be easily fixed are punps leaking from the pump part in front. If it leaks there or if roller cells are stuck, remember position of flanges and plates, remove 4 screws and disassemble everything. Then clean well and replace 2 O-rings with fuel-resistant O-rings 33x2 mm. The real trouble starts when the pump leaks from the housing, when it runs noisily or when coals are worn. Then you have to force open the housing carefully and remove the front part without bending the housing too much. Otherwise it will later not seal again, even if you replace the O-ring inside.

Another usual fault is the non-return valve on the back of the newer pumps. It would be an easy fix if Bosch would still supply part 1 583 386 008 which is the complete flange including non-return valve. Somehow this flange often looses the ball inside. After that fuel pressure in fuel line over engine drops immediately when pump stops and thus engine starts worse. As a replacement I propose Pierburg 7.20234.50.0 external non-return valve that you just insert into the fuel line towards engine. In 2015 it cost about 20€ and it fits well to D-Jetronic system pressure.

8.3 Pressure regulator

Spring-diaphragm pressure regulator
Spring-diaphragm pressure regulator
Pressure regulator is nothing else than a diaphragm valve, working against a spring. Spring tension can be adjusted by a screw. Pressure regulator is inserted into end of fuel line over engine and controls the back-flow of fuel to tank. Fuel is inserted into fuel line directly from fuel pump. This regulator at the end of fuel line opens when pressure is above the pre-adjusted maximum which is normally 2.0 to 2.1 Bar. It lets than flow fuel back to tank until pressure is again below this maximum. This is a continuous process and fuel pressure is thus regulated mechanically. If you screw adjustment screw in, you will have a higher pressure in fuel line and vice versa. If you have a fuel pump that delivers too much fuel or is designed for higher fuel pressures, the diaphragm inside pressure regulator and fuel injectors will suffer from much higher pressure peaks and movements and finally wear off.

Warning: Do not adjust fuel pressure outside designed pressure range if your engine runs too lean or rich in idle. Especially turning down fuel pressure (I have seen 1.4 Bar once) will let your engine run so lean in part-load that you will risk overheating and complete engine failure up to holes in pistons.

8.4 Pressure damper

Every car has one or more dampers in its fuel line. Very early versions had a ring pressure damper sitting in both suction and pressure line of fuel pump with a diaphragm in between. The problem with that solution is that a torn diaphragm will also short cut the fuel pressure. Later cars only have a pressure damper behind pressure regulator and some an additional in suction line. Dampers have the task to reduce noise and dampen fuel pressure peaks. Why some also have a screw for adjusting it like we have it on pressure regulators is a miracle that I cannot explain.

Your Dr-DJet (Volker)

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