Fuel pressure regulator Vs accumulator - Mercedes-Benz Forum
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post #1 of 3 (permalink) Old 11-14-2004, 12:52 PM Thread Starter
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Fuel pressure regulator Vs accumulator

Attached is an article I found on line. If I understand it right, the pressure regulator works on the pressure while the engine is on and the accumulator maintain "rest" pressure after the engine is shut off. Am I right? My 1991 will not start w/o extra crank after parking for 30 minutes. I am planning to replace the accumulator, pump check valve and fuel filter.

K-Series Fuel Injection Systems
Copyright Jim Newkirk, Autoinc Magazine, 1997 - All rights reserved
[Jim's original article contained details of a system not used in the UK. It also used limited American English terminology and SAE units. Minor editorial amendments have been made to cater for these issues. The 'K' in K-Jetronic is from the German word 'kontinuierlich' - continuous.]
The K-Series fuel injection systems are continuous mechanical fuel injection systems used on a wide variety of European vehicles. The system is one of the most common fuel injection systems on the market, but also is one of the least understood.
The K-Jetronic System constantly injects fuel into the engine as long as the car is running and air flow is present to move the sensor plate in the airflow sensor. The sensor plate is connected to an arm that pushes up on a plunger located in the fuel distributor. As airflow changes, the movement of the sensor plate and the plunger increase and decrease the volume of fuel injected into the engine. Since fuel is being injected constantly, fuel pressure will have a direct affect on driveability. As a matter of fact, fuel pressure is the single most critical element when diagnosing driveability problems in the K-Series fuel injection systems. An accurate fuel pressure gauge must be used when testing these systems, with a range of 0 to more than 6 bar.
You will work with three types of pressures when diagnosing these systems: system pressure, control pressure and residual pressure.
System pressure is the total fuel pressure produced by the fuel pump on a constant basis. The fuel pump must be able to maintain this pressure during all driving conditions from idle to wide open throttle. As a rule, system pressure will run about 5 to 5.5 bar pressure and the pump should be able to produce a minimum volume of 900ml in 30 seconds. When deadheaded, the K-Series fuel pump will produce about 1.5 times the system pressure. System pressure is a function of volume of fluid moved against a restriction, so to maintain system pressure at the desired level, there must be some type of restriction built into the fuel system. This restriction is more commonly known as the fuel pressure regulator. The fuel pressure regulator restricts the return of fuel to the tank by a calibrated amount, maintaining system pressure at the desired level. On early K-Jet systems, this regulator was a slide valve (also known as a push valve) internal to the fuel distributor. Fuel pressure could be adjusted by adding or removing shims from the valve - the kit is Audi part 035 198 685. On later K-Jet systems, the regulator is the conventional diaphragm type - Audi part 034 133 534L for the ur-quattro's MB engine.
Control pressure (or counter pressure) is the pressure that is metered to the top of the fuel plunger on a K-Jet system. By changing the counter pressure, the resistance to plunger movement is changed, allowing enrichment and enleanment of the fuel mixture to the engine. On a car equipped with K-Jet, this pressure is controlled by the warm-up regulator.
The warm-up regulator only compensates for engine temperature and is therefore a rather coarse control of fuel mixture. (Some K-Jet warm-up regulators also have a vacuum port to help with the acceleration enrichment and deceleration enleanment function - they are properly referred to as 'control pressure regulators' since their function is not limited to warm-up enrichment.) Typical control pressures on a K-Jet warm-up regulator are 3.4 bar with the engine at full operating temperature and 1.2 bar on a cold engine. (The colder the engine, the lower the pressure.) There is an electric heater in the warm-up regulator, driven by the fuel pump supply circuit.
Residual pressure is the fuel pressure maintained in the system by the fuel accumulator after engine shutdown. The fuel accumulator is a large spring-loaded diaphragm that maintains a pressure of about 2.8 bar for 30 minutes or more after engine shutdown. This residual pressure provides for fast restart and prevents fuel percolation or boiling (vapour lock). Always check the service manual for the car line you are working on for proper residual pressures and times. Typical symptoms caused by accumulator problems are extended crank time and hard hot starting.
With an understanding of the system and the proper tools, K-Jetronic fuel system service is a straightforward procedure that can keep your service bays full all year long. Give me a call if you have any questions!
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post #2 of 3 (permalink) Old 11-14-2004, 03:16 PM
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RE: Fuel pressure regulator Vs accumulator

The accumulator maintains constant pressure when the engine is running, needless to say variations in fuel pressure while the engine is running will effect performance adversely. When the engine is off the accumulator keeps the fuel pressure constant with the engine hot.

See item five in the article below:


If your accumulator has failed and you are loosing pressure while the engine is hot and cooling after 30 minutes, it could cause hard starting.

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post #3 of 3 (permalink) Old 03-22-2005, 01:03 AM
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RE: Fuel pressure regulator Vs accumulator

You are correct,NewMBie.Accumulator stores that pressure built by pump and regulated by Reg at engine.
For quick,easy,starts.I'm new to MB's myself,but judging by the whirring noise around the back of the vehicle when running,it uses an electric fuelpump.I have a little experience with these in carburated hotrods and found that turning key to run for a few seconds before engaging starter fills system back up to pressure, and allows for instant starts rather than cranking.If u listen u can here the pump sound change when it's built back system pressure. Should find any leaks and repair though, and only do this after car has sat for some time, to save wear and tear on start system.
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