Chapter 8 of D-Jetronic article series by Dr-DJet describes fuel-pump and pressure regulator
8.1 Overview of fuel supply
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. 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|
- 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.
- 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.
- 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|
|Fuel pump with sticking rollers|
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:
- Bosch 0 580 464 021 (3,0 Bar L-Jetronic für Porsche 944) plus MB mounting parts A 210 990 03 88, A 123 990 00 53, A 124 478 03 53 plus 2 copper rings 12x1, Kennliste Bosch Benzinpumpen
- or Pierburg 7.21287.53.0, Data Pierburg fuel pumps
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|
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)