Fuel Accumulators - How Do They Go Wrong?

Sunroof · 12-19-2005, 01:16 PM

All the literature tells me that if I have a starting issue, check the accumulator! Okay, but before I put the fuel pressure testers on my primitive 1973.5T CIS system, I have to wonder just what can go wrong with an inline accumulator? Or, better yet, if one was to slice an accumulator in half just what would they see? Springs, seals, bladders, chambers? I had a bunch of junk come thru my filter (fine sediment) from a rusty tank for awhile, so can sediment mess these little devices up? If on cold start it takes a while for the fuel to get to my injectors is that the accumulator or infamous check valve that I personnally think does not really exist in those made for 1973.5T fuel pumps!!

Thanks
Bob
73.5T

Superman · 12-19-2005, 01:18 PM

The bladders tear. The bladders are supposed to absorb fuel pressure fluctuations, and to hold fuel pressure when off.

Sunroof · 12-19-2005, 02:22 PM

Thanks, but how do you know they are not functioning? And what causes the bladders to tear?

Inquisitive about the 911's you know.....

Bob

MotoSook · 12-19-2005, 02:39 PM

Do you have a 76 and earlier accumulator or a 77 and newer accumulator? The 77+ accum's have a third connection on the bottom to allow fuel to leak into the fuel return loop in the case that the diaphragm leaks.

The diaphragm is a flexible material, probably buna, with a spring behind it. It can develop a puncture or tear over time. A good shock (pressure spike) to it can expediate the failure, although that's not likely with a stock pump. If it leaks, it will bleed off your residual fuel pressure. It can fool you into thinking you have a bad check valve at the pump.

RarlyL8 · RarlyL8's Garage

I had a bunch of junk come thru my filter (fine sediment) from a rusty tank for awhile, so can sediment mess these little devices up?

This is the kiss of death for CIS. There are several extremely close tolerance areas in the fuel head. Injectors can plug as well.

larry@oma · 12-19-2005, 09:18 PM

the cis fuel filter will filter fine sediment and plug up and restrict fuel flow,before passing fine sediment to the other parts of the cis fuel system? a plugged fuel filter will probably shut the engine down.anybody agree or disagree?

Joe Bob · Joe Bob's Garage

BTW..If it's a hot start problem....look at the check valve in the neck of the fuel pump.

Sunroof · 12-20-2005, 04:38 AM

The 1973.5T CIS fuel pump is unique in that it was designed with an internal check valve. I had a rusty tank and was changing the fuel filter several times a year. Before getting the tank restored, replacing all the fuel lines and adding a new fuel pump, I could see fine sediment coming from the filter after removing it. All the injectors have been replaced recently, so I am suspecting the accumulator, if indeed the bladder can be impacted by sediment. But, their is always the fuel distributor, which is the original unit!! My cold start seems to work with just fine 90 out of 100 times with the hand throttle, but its more of a warm up start issue. It takes time for the fuel to repressurize in the system after letting it sit for awhile turned off. Like as if everything is flowing back in line. If it is the fuel pump check valve, they are worthless and I can see why Porsche when engineering the '74, placed the check valve on the outside of the pump, which can easily be replaced.

From the pump check valve, to its WUR, to the way the injectors are set into the cylinder heads and the hand throttle, the first CIS car (73.5T) was certainly unique.

Appreciate the feedback

Bob

Elombard · 12-20-2005, 04:43 AM

Its easy to check with gauges, hook up, start and shut off, see how long the pressure holds. Obviously there is several places the pressure could be leaking off. In my case it was the accumulator...un plug the 3rd line and it pissed all over. Actually ran good once the pressure was up.

Elombard · 12-20-2005, 04:44 AM

You can come and use my gauges if you dont have a set?

DohertyCM · DohertyCM's Garage

Had the same problem on my 74 911. I did the pressure check thing and found out that it was the check valve at the fuel pump.

Sunroof · 12-20-2005, 07:23 AM

Thanks E.
Rattlesnake (Marc) is giving me a hand this week with the new front Koni inserts (need air impact to loosen the top nut). I know Marc has guages so we will see whats going on. I suspect its the accumulator too, but with this CIS you never know!! It was a rebuilt fuel pump, so I hope the check valve is not too screwed up.

What third line are you talking about? Accumulator on my car has the two fittings.

Appreciate it.

Bob

Elombard · 12-20-2005, 07:45 AM

No problem, let me know if I can help. Marc is the man. Some one mentioned above on the later cars there was an "emergency relief" line from the accumulator back to the tank. I guess Porsche decided that it was not a good idea to have fuel leaking all over the engine compartment when the rubber diaphragm finally let go. Kinda like accelerator pumps on carbs. When the rubber gets a hole some carbs will leak gas.

bigchillcar · bigchillcar's Garage

if the fuel pump has indeed been rebuilt then i would think that the check valve would have been replaced 'while in there as well'. my vote is for the accumulator..a bad one would certainly cause warm start issues. do you know anyone with an extra lying around that you could pop in and try? good luck!
ryan

Sunroof · 12-20-2005, 09:11 AM

No third line or relief valve on this puppy! Simple line in and line out set up. The one I have now is about 5 years old and I know has felt a lot of sediment that got thru the filter over those years.
That had to knaw away at that bladder over that time.

We will see what the outcome is this week with the guages.

Thanks
Bob