inside the fuel accumulator

[Alan L]

So I had occasion to cut mine open Don't plan on using it again .
You can see how it works. There is a hat that sits on top of the spring and moves slightly in the chamber. That hat rests against the rubber diaphragm which is connected on the other side to a small pin that seats on the housing holding the diaphragm.
When the system is at rest and low pressure the pin is pushed open by the spring on the diaphragm. As the pressure builds when the engine is started fuel will push down past the pin on to the diaphragm until eventually the spring has travelled far enough to close the pin. At that point fuel is simply flowing in one port and out the other to the FD. Normal running conditions. Once the engine shuts down pressure will slowly bleed away - from a number of places. As the pressure drops above the pin, it relaxes the pressure on the diaphragm and the pin can open, allowing what fuel is trapped above the diaphragm to slowly contribute to maintaining a pressure in the FD system, until it is exhausted.
I just haven't quite figured why the FA has specified inlet/outlet ports. Other than the geometry of the fuel lines, it would seem to make no difference. There does not seem to have been any seating on the pin on the center outlet port.


Alan

[brighton911]

Thanks for posting, nice to know how the internals work.

[boyt911sc]

Alan,

What is the Bosch ID # of your dissected fuel accumulator? That is something different from any of the fuel accumulators used in 911 CIS. Here are my FA’s (0 438 170 xyz) and a dissected -009 from a ‘78/‘79 SC.



Parts from top to bottom:


Parts from top to bottom (upside down):

The above dissected 3-port fuel accumulator is a Bosch 0 438 170 009.

Tony

[mike sampsel]

Tony, is the dash 31 used on an 83 SC? Looking at a larger one for my 78 as JW says they hold pressure longer, so as long as I’m going to add a 3 port accumulator, thinking of expanding.

[boyt911sc]

Originally Posted by mike sampsel

Tony, is the dash 31 used on an 83 SC? Looking at a larger one for my 78 as JW says they hold pressure longer, so as long as I’m going to add a 3 port accumulator, thinking of expanding.

Mike,

I got some of them from late CIS units that I purchased many years ago. When you purchase a used fuel accumulator, make sure it is tested and guaranteed to work. Most of the used FA’s I had purchased were defective. Only the two (2) FA shown in the picture with orange sticker are GOOD. The rest are all leaking and defective.

The most common description is “it was working when remove” and don’t fall into this situation unless the seller agrees to refund your money if tested defective. Or have it tested before delivery.

Tony

[mike sampsel]

Originally Posted by boyt911sc

Mike,

I got some of them from late CIS units that I purchased many years ago. When you purchase a used fuel accumulator, make sure it is tested and guaranteed to work. Most of the used FA’s I had purchased were defective. Only the two (2) FA shown in the picture with orange sticker are GOOD. The rest are all leaking and defective.

The most common description is “it was working when remove” and don’t fall into this situation unless the seller agrees to refund your money if tested defective. Or have it tested before delivery.

Tony

Thanks,

I’d buy a new one.

[Alan L]

Originally Posted by boyt911sc

Alan,

What is the Bosch ID # of your dissected fuel accumulator? That is something different from any of the fuel accumulators used in 911 CIS. Here are my FA’s (0 438 170 xyz) and a dissected -009 from a ‘78/‘79 SC.

Tony

Hi Tony - took a while to find a number. When I removed it I could not find one. A bit of solvent and wire brush found one.
It is an 031.
Been on the car for the 20+ yrs I have owned it and some unknown time prior.
Regards
Alan

[boyt911sc]

Alan,

I learned something new today from your work. I decided to cut open one of my FA-031 and compare it to the smaller FA-009.





I was surprised to see how big was the spring for FA-031. It’s humongous! And the presence of a check valve. FA-031 is a better mouse trap than the earlier models.

Tony

[Alan L]

Tony - I think you have a reed valve on the 009?
But yes - I would rate the 031 the better unit. It has lasted a long time on my car. Who knows - it may even be original.
Right now I made the mistake (while waiting for cold/warm final calibration of my WUR) to check my clutch clevis pin. A very difficult job on a RHD car with an aircon unit stuck in the way. Lucky I checked. I had a clevis pin break once and drove several hundred Kms without a clutch. This one was ready to do same.
The work on these old cars never stops, between the SC and the poor old 930 that spends its days on the track.
They keep me out of trouble anyway. Too busy trying to get them running again.
My SC is currently holding 1 bar pressure after last running 5-6 hrs ago. It was an 031 I pinched off the 930. I don't think I will be doing that job again for a while.
Regards
Alan

[khamul02]

Here is one from a Mercedes. IT is insane how big the spring is in these things:

https://www.youtube.com/watch?v=mD2UrQSWsxI

[mike sampsel]

Originally Posted by boyt911sc

Alan,

I learned something new today from your work. I decided to cut open one of my FA-031 and compare it to the smaller FA-009.





I was surprised to see how big was the spring for FA-031. It’s humongous! And the presence of a check valve. FA-031 is a better mouse trap than the earlier models.

Tony

Doh, now I wish I’d have gone the extra mile, found the appropriate line from accum to fuel filter and bought the bigger one. JW said they are better. Still may someday.

Tony, the check valve is the third piece from the left?

[fintstone]

This is an interesting discussion as I have a small one that was stock on my '74 but am running a tweaked 3.0...and really do not have very good luck with holding pressure as long as it should. I am thinking maybe an update t the larger one. Has anyone done this? If so, how well did it work out?

[Jay Laifman]

Alan initially questioned why one is marked in and one is marked out. I note that on my current one, I don't think it's marked. Is there an in and out? Could I have it on backwards?

At the risk of embarrassing myself, mine is currently mounted with the in on the side, and the out from the center. I did mount this one. But I put it back in the same way the last one was in. I did not do that one.

I looked at it this morning. I don't see how it matters. The two inlets go to the same chamber. There is no extra mechanism or valve. Fluid presses equally in all directions. So from my perspective, I now think it doesn't matter. And maybe that's why it isn't labelled.

This picture suggests the center is the inlet:

[Funracer]

These two pics show the hard “S” fuel line between the filter bottom and the center port of the FA. Are the lines on top of the FA in the correct place or are they switched? Note these are 3 port accumulators if that makes any difference.

[Funracer]

NVM. Found it.

[E Sully]

It seems the accumulator has a few different internal designs to smooth the fuel flow through it.
Some cars used a different accumulator style. Instead of flowing through, it was a single connection off to the side. This particular Mercedes is an example, along with a fuel inlet damper. Our Porsche's directed the leaking fuel from a bad one to the return line, this went back to the inlet.

[kupferbraun77]

Hi! This (really retro) video describes the difference between inlet and outlet port really clear. It`s all in the details.

https://youtu.be/a4fJAfXYxWk?si=D7D-IynhbXntqd1-&t=218

Warning.. don`t fall asleep while watching!