Anatomy of a 328 accumulator

Thanks for posting the details. Intel is always important at some point in the future. Rgds Wayne

 

quite interesting as i've just replaced mine.
seems the useful life of this part is around 20-25 years.
hopefully i won't have to replace it again.
that's a big ole spring.
i was expecting something a bit smaller in gauge.

 

I replaced mine in my 87 328 last year as well. Did it due to difficult hot starts. Really don't understand what it does. I think it maintains pressure in the fuel system.. Steve??

 

That is a big spring and it maintains the fuel pressure in the fuel distribution system when the motor is not running. Suggests that the fuel pump is a very high pressure pump, too.

The Bosch continuous flow fuel injection system is definitely not a low pressure system like a carburetor system.

It helps when starting the motor so you don't have to wait for the fuel pump to build up pressure in the system.

As far as I know, all Bosch constant flow fuel injection systems use an accumulator. The accumulators probably have the same pressure specs, just different plumbing attachments.

I think a Mercedes Benz is a direct replacement/substitute for the Ferrari, just costs less.

Jerry
DRG

 

Look forward to seeing it all back together and working

 

The accumulator prevents vapor lock. Very simple physics. Without it you would never get a hot restart. It has nothing to do with not waiting for the fuel pump. The pump creates full system pressure in about 2 seconds.


If they do not leak they are good. Since 1976 or so working on these cars every single day I have only replaced 2.

Lots of reasons for a loss of residual pressure, it is almost never the accumulator.

 

It does provide a couple more functions aside from hot start pressure in the system. reduction of fuel pump harmonics to the fuel distributor, noise reduction via the dampening of the pressure cavity.

Also it should be noted that the spring pressure is just below the opening pressure of the injectors, that varies between mfg's and models in some cases so it's not the best idea to swap parts from other German marques even though they also used CIS. The CIS fuel system is unique in that it is a hydrolic pressure differential system for metering fuel to air based on calculated air mass pressure to the requirements of the engine. The fuel pressure and flow is part of and critical to the proper operation of the engine, from tank to injector.

 

Guys, what sort of pressure does the system run on?

 

Five bar, says the shop manual. That's about 70 psi, which counts as high pressure.
And 70 psi over two or three square inches of diaphragm is a lot of force - not surprising that the spring won't compress from stepping on it.

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Unfortunately (for me), my experience echoes Brian’s post. Last fall, I replaced my accumulator based on what I read here but didn’t really see any improvement in hot/warm start behavior. It continues to be “not impossible to start, but requires more cranking and throttle when hot or warm”.

At least the part was cheap and it was relatively easy to install. I didn't check for a leak (I had already ordered the part when I heard about that trick) so can't share that piece of info.

 

I replaced my accumulator last year also. It holds pressure in the injector system so that you hart right up. ave start fuel in the intake before the fuel pump starts. The fuel pump does not start until air flows through the throttle plate and pulls the disc down in the injector box to start the pump switch. With out it there is no fuel to start the engine on. It stores a very small amount but enough to keep the pump from having to come on which is a good thing if you stall and the metering system flowed and flooded out. The only way that I could get the engine started was to push down on the disc to add fuel and it would start right up.A few tow trips back to the garage got me testing fuel pressure and BINGO! A bad accumulator.

 

Oh I forgot to add that the residual pressure leaked through the diaphram into the return line to the fuel fill neck connection. You could hear it trickle into the tank until the pressure was Zero.

 

Carburetor gasoline fuel systems that operate with low fuel pressure systems, typically with a mechanical fuel pump driven off of the vehicle engine, were very susceptible to vapor lock during the 50's - 60's.

We had to deal with vapor lock regularly during that period of time, not so much with now.

The first continuous flow fuel injection units I worked on in the early 60's were the Rochester fuel injection units on the early Corvettes. They had a relative high pressure cable driven pump, driven off of the distributor. They did not have an accumulator, and I don't ever remember a vapor lock problem. There was apparently sufficient pressure on the fuel system to prevent the problem.

I remember the really old mechanics would put heat sinks on the fuel lines to try to prevent vapor lock. Some were truly ingenious devices.

High pressure fuel injection systems with high pressure fuel pumps are much less prone to vapor lock, even though many times that is the diagnosis when the engine fails to start.

The fuel accumulator is to maintain nominal pressure in the fuel injection system, especially on start up. What ever other benefits are derived from the proper function of accumulator are conveniently serendipitous.

Jerry
DRG

 

I love it when people are curious and do this kind of thing, many thanks Craig for posting, fascinating, all the pics are in my archive.
And yes that spring was way bigger than I would have expected too.
Regards, Bell

 

That is not how the system works. Get a book and read up on it. The fuel pump runs with the starter and the accumulator only holds pressure for an hour or two. Under your scenario it would never start cold.

 

Mechanical fuel injection systems are hydraulic systems in the same way brake systems are hydraulic. They work by displacing a given amount of fluid. If there is air in the system the fluid cannot be displaced, the air just compresses so no fuel delivery takes place. When the car is running the system is under rather constant high pressure. As any graduate of 8th grade science will remember boiling points of most fluids are higher under pressure, gas is no different. While running the gas is under high enough pressure it cannot vaporize. When the car is shut off (if there was no accumulator) the pressure would drop to zero and with the ambient temperture in the engine bay the fuel would vaporize in the lines. With air (vapor) there is no fuel delivery and the motor will not start. The accumulator maintains pressure for a long enough period of time for the engine bay to cool to the point the fuel vaporization can no longer take place.


The two classic failures causing a hot restart problem in the K Jet system are bad injector nozzles and bad fuel pump check valves. Combined they seal off both ends of the system so that the pressure reserve created by the accumulator doesn't just bleed of into either the fuel tank or cylinders with leaky injectors.

Bosch K Jetronic is a very simple system yet this thread shows how misunderstood it is by many who are working on them here and many others that feel qualified to advise others. There are so many books covering the principals of operation and diagnosis printed by Bosch there is no reason for so much misunderstanding.

 

It is one of the simplest fuel systems ever made however most here have no idea how it works. Bosch has printed so many books on it, just get one of those so you'll get correct information. A set of the fuel pressure gauges to diagnose it are not expensive.

2 things I will suggest though. Injectors wear out. Bosch meant for them to be regular serice items much like spark plugs. Replace yours. Also get a fuel pump check valve. Between the two if the system is all adjusted correctly will correct most hot start problems.

 

Just a quick one save me hunting for it, where is the fuel pump check valve?
Many thanks, Bell.

 

should be right on the end of the fuel pump, going off memory the original pumps had them internally. Mercedes used CIS right up to 1991 so they had an external check valve 'kit' if you will for replacement pumps that no longer have the internal check valve. so a new pump could be used by using the mercedes check valve.

Keep in mind that the CIS system uses very different pressures and flow rates from current EFI so the pumps are not interchangeable.

 

Ah ha, that makes sense, many thanks
Regards, Bell.

 

The 2 valve cars had a check valve integral to the pump. Most if not all QV cars had an external check valve as did all 3.2 cars. It is not an expensive part.

 

So the check valve is set slightly higher than the system control pressure valve in the fuel metering unit or are they set roughly the same?

Regarding the Warm Up Regulator it enriches the mixture, by varying the control pressure ie low pressure=Rich/Cold Start and High Pressure=Normal Running/Warmed
What are those pressures? I assume there are just 2 readings...
Many thanks, Bell