fuel accumulator

[davidc89]

I noticed on my fuel accumulator that I only have two lines going to it (towards the front of the car) and it doesn't have a port coming out on the motor side. Anybody else seen this before? Every one I have seen has had three ports.

[Chris 16409]

Are you positive? Was it ever replaced? That seems odd. Does it look something like this?:

33395

[davidc89]

Yes it looks exactly like that.

[DMCMW Dave]

Volvo part.

[Chris 16409]

Volvo part.

Does the car run correctly with that type of accumulator. Doesn't the fuel coming out of the small barb go back to the tank?

[davidc89]

My car runs fine. Someone had plugged the line coming off the T fitting. I had a fuel leak and I noticed a fuel line plugged with a bolt and a hose clamp. I did some research and figured out it goes to the accumulator, but I dont have a port to put the line on.

[DMC5180]

It just means that if the diaphragm leaks in that unit the back side chamber will eventually pee fuel out the vent bleed port. The positive of that is that you immediately know if the accumulator has gone bad.

[Bitsyncmaster]

If that port is plugged air tight, that would reduce the accumulator capacity a little since it would build up pressure and limit the spring movement inside the accumulator. I don't think you would notice the difference but these smaller accumulators would have less capacity anyway.

I've found with my car, you need to hold rest pressure (above 5 PSI) for almost 4 hours. If you don't you will have a "warm start" problem on a hot summer day trying to start it at 3.5 hours after shutdown. That is why I designed my RPM relay to extend the rest pressure another 3 hours.

[Jonathan]

If that port is plugged air tight...

If I understand how the accumulator works inside, you have fuel on one side of a diaphragm and air (and a spring) on the other. Fuel pressure pushes the diaphragm back, increasing the volume of fuel within the accumulator. If the engine requires more fuel than what the fuel pump can give (if you really push the accelerator to the floor for an extended period of time) that extra volume satisfies the engine's temporary need for extra fuel.

When you get your foot off the accelerator, the fuel pump has a chance to catch up and refill that small extra volume. On the non-fuel side of that diaphragm, you have air and the spring. In a car with the return port hooked up to a hose going back to the fuel tank, I would expect that as the diaphragm goes in and out, it pushes out air or pulls in air depending on which way it is moving.

When an accumulator fails, I believe it is the diaphragm seal that stops sealing and it allows fuel to go back to the fuel tank instead of developing this extra volume on the fuel side of the diaphragm. When you turn the car off and the fuel pump stops the pressurized fuel feed, there is no more fuel to replace what is being allowed to pass by the diaphragm seal. This might not be a big problem in the accumulator itself or the feed or return hoses, but above the engine where it is still hot, the lack of pressure lowers the flash point of the fuel and you get the vapor lock preventing you from restarting the car while it is "hot."

If you have that return port plugged air tight and no return hose connected at all, you may be right, as mentioned, that you'll know quickly if your accumulator fails. What I'm wondering, however, is if your accumulator would already be full of fuel on both sides going all the way back to the first time you ever ran the car and shut it off with the plugged port accumulator modification?

Reason being, if on a known good accumulator car without a hot start issue, we always say you should be able to press down on the metal air intake plate and feel resistance for at least two or three hours after shutting off the car. A hot start troubled car will lose that pressure resistance soon after shutting the car off. So even on a good car with the design as it is, where we know the fuel pressure eventually dissipates to zero, does that mean it slowly leaks past the diaphragm in the accumulator and back to the engine intentionally? The fuel system has to have an outlet somewhere to let the pressure go down once the pump is off. Is there another pathway that it would slowly decrease by?

I was thinking that if our cars normally use that pathway to relieve fuel pressure when turned off, if this car without that hose and the port plugged air tight, wouldn't it be relieving itself of fuel pressure wherever it could, and so that small volume of air on the backside of the diaphragm (that has no connection to be refilled with air) would have been long ago filled with fuel?

If that was the case, would you be able to see performance suffer when you press hard on the accelerator? Would the accumulator still push with its spring if there was fuel flooded on both sides of the diaphragm and no ability on the "emptying" side to replace the fuel leaving with air (developing a little vacuum even perhaps)?

[DMCMW Dave]

If I understand how the accumulator works inside, you have fuel on one side of a diaphragm and air (and a spring) on the other. Fuel pressure pushes the diaphragm back, increasing the volume of fuel within the accumulator. If the engine requires more fuel than what the fuel pump can give (if you really push the accelerator to the floor for an extended period of time) that extra volume satisfies the engine's temporary need for extra fuel.

When you get your foot off the accelerator, the fuel pump has a chance to catch up and refill that small extra volume. On the non-fuel side of that diaphragm, you have air and the spring. In a car with the return port hooked up to a hose going back to the fuel tank, I would expect that as the diaphragm goes in and out, it pushes out air or pulls in air depending on which way it is moving.

When an accumulator fails, I believe it is the diaphragm seal that stops sealing and it allows fuel to go back to the fuel tank instead of developing this extra volume on the fuel side of the diaphragm. When you turn the car off and the fuel pump stops the pressurized fuel feed, there is no more fuel to replace what is being allowed to pass by the diaphragm seal. This might not be a big problem in the accumulator itself or the feed or return hoses, but above the engine where it is still hot, the lack of pressure lowers the flash point of the fuel and you get the vapor lock preventing you from restarting the car while it is "hot."

If you have that return port plugged air tight and no return hose connected at all, you may be right, as mentioned, that you'll know quickly if your accumulator fails. What I'm wondering, however, is if your accumulator would already be full of fuel on both sides going all the way back to the first time you ever ran the car and shut it off with the plugged port accumulator modification?

Reason being, if on a known good accumulator car without a hot start issue, we always say you should be able to press down on the metal air intake plate and feel resistance for at least two or three hours after shutting off the car. A hot start troubled car will lose that pressure resistance soon after shutting the car off. So even on a good car with the design as it is, where we know the fuel pressure eventually dissipates to zero, does that mean it slowly leaks past the diaphragm in the accumulator and back to the engine intentionally? The fuel system has to have an outlet somewhere to let the pressure go down once the pump is off. Is there another pathway that it would slowly decrease by?

I was thinking that if our cars normally use that pathway to relieve fuel pressure when turned off, if this car without that hose and the port plugged air tight, wouldn't it be relieving itself of fuel pressure wherever it could, and so that small volume of air on the backside of the diaphragm (that has no connection to be refilled with air) would have been long ago filled with fuel?

If that was the case, would you be able to see performance suffer when you press hard on the accelerator? Would the accumulator still push with its spring if there was fuel flooded on both sides of the diaphragm and no ability on the "emptying" side to replace the fuel leaving with air (developing a little vacuum even perhaps)?

The accumulator does NOT provide the "capacitor" effect you describe. The fuel pump is capable of more volume than the engine will ever take. It's sole purpose is to maintain pressure in the injectors so that the fuel does not boil in the injectors/engine lines and create vapor lock i.e. the hot start problem we all know and love. When the fuel vaporizes in the injectors, the vapor is compressible and won't therefore develop enough pressure at cranking speed to pop the injectors. [This is not necessary on an EFI system as the injectors are actually electrical valves rather than a spring-loaded pressure valve. ]

Fuel should not leak past the accumulator diaphragm at all. If the "sealed bottom" version fails the bottom will eventually fill with gas and it will no longer "accumulate".

The version we are using has a spring under the diaphragm and is meant to have fuel in the back side, that can displace as the diaphragm moves as it pressurizes. Same idea but the chamber is filled with fuel rather than air. the only advantage that I can see to this is that trace amounts of fuel leakage (internal) would not be a failure, so maybe it will last longer in normal use.

In the event on a catastrophic failure of the diaphragm, the design we use could impact performance if excess fuel bleeds back to the tank and drops system pressure. I've never seen one that bad.

The reason pressure bleeds off over time (i.e. the 1/2 to several hours observed) is the fact that neither the accumulator, fuel distributor, nor injectors are perfect.

[Bitsyncmaster]

The reason pressure bleeds off over time (i.e. the 1/2 to several hours observed) is the fact that neither the accumulator, fuel distributor, nor injectors are perfect.

+1
I'm guessing the injectors are the most leak prone parts. We also have all the hose and pipe fittings that are not perfect seals. And don't forget that check valve on the pump.

[PJ Grady Inc.]

Actually no because the dist. pres. regulator cuts off the pressure before the injectors with the engine off and even if they were to leak they would not drop enough fuel presure to affect starting. They could cause a little hiccup or two on startup with the ounce or or so of fuel they could drop in the cylinder. It would clear up quickly as the lightly fouled cylinder(s)cleared. The reserve pressure would be unaffected at rest.
Rob



+1
I'm guessing the injectors are the most leak prone parts. We also have all the hose and pipe fittings that are not perfect seals. And don't forget that check valve on the pump.

[Jonathan]

The accumulator does NOT provide the "capacitor" effect you describe. The fuel pump is capable of more volume than the engine will ever take. It's sole purpose is to maintain pressure in the injectors so that the fuel does not boil in the injectors/engine lines and create vapor lock i.e. the hot start problem we all know and love. When the fuel vaporizes in the injectors, the vapor is compressible and won't therefore develop enough pressure at cranking speed to pop the injectors. [This is not necessary on an EFI system as the injectors are actually electrical valves rather than a spring-loaded pressure valve. ]

Fuel should not leak past the accumulator diaphragm at all. If the "sealed bottom" version fails the bottom will eventually fill with gas and it will no longer "accumulate".

The version we are using has a spring under the diaphragm and is meant to have fuel in the back side, that can displace as the diaphragm moves as it pressurizes. Same idea but the chamber is filled with fuel rather than air. the only advantage that I can see to this is that trace amounts of fuel leakage (internal) would not be a failure, so maybe it will last longer in normal use.

In the event on a catastrophic failure of the diaphragm, the design we use could impact performance if excess fuel bleeds back to the tank and drops system pressure. I've never seen one that bad.

The reason pressure bleeds off over time (i.e. the 1/2 to several hours observed) is the fact that neither the accumulator, fuel distributor, nor injectors are perfect.

Thanks Dave. I'm glad I asked and glad you took the time to explain. All along I had thought of it as being there for that extra "capacity" idea (more of a volume consideration) and not in terms of maintaining pressure.

I'm trying to think of a water treatment or process piping analogy, and what comes to mind is either pulsation dampener or surge arrestor/water hammer arrestor.

On some of the bigger pumping (water forwarding) systems, shortly after the pump discharge header, we would have one or more of these surge arrestors. It is basically a cylindrical tank connected to the piping on a tee where inside the tank is a bladder. On the piping side of the bladder is the water coming off the pump discharge and on the other side of the bladder is typically a nitrogen charge. As you mentioned with air or nitrogen being compressible, this surge (or water hammer) arrestor tries to protect downstream piping or equipment from shock waves of water pressure when an additional pump kicks on by absorbing the brunt of the pressure spike.

With a pulsation dampener, you tend to find them on the discharge end of pumps that don't give you a constant push of water, but instead come in strokes, or pulses. It doesn't regulate much, but will knock down the peak of that pressure spike to protect downstream equipment.

So is the accumulator on our cars more or less helping to smooth out the supply pressure so that the downstream pieces (like the PPR and CPR) are getting a consistent feed pressure to work with and then are able to better control their own discharge pressures into the engine?

[Bitsyncmaster]

The books say the accumulator also smooths the pressure pulses that the fuel pump produces.

[DMCMW Dave]

So is the accumulator on our cars more or less helping to smooth out the supply pressure so that the downstream pieces (like the PPR and CPR) are getting a consistent feed pressure to work with and then are able to better control their own discharge pressures into the engine?

Not really. There really is not any pulse effect with an electric pump, and since the injectors are more of a constant flow they aren't pulsing any backpressure either.

[DMCMW Dave]

Actually no because the dist. pres. regulator cuts off the pressure before the injectors with the engine off and even if they were to leak they would not drop enough fuel presure to affect starting. They could cause a little hiccup or two on startup with the ounce or or so of fuel they could drop in the cylinder. It would clear up quickly as the lightly fouled cylinder(s)cleared. The reserve pressure would be unaffected at rest.
Rob

As long as everything is perfect. . .

I've seen cars hold pressure everywhere from 30 seconds to over night!

[PJ Grady Inc.]

Yes when everything is in good shape the car should hold it's pressure overnight. I'm curious about how long the tiny 4 cylinder accumulators will hold it and I'll probably hook up a gauge to the next one that comes in to do some testing for my curiosities sake. I'd rather not use them however unless the 6 cylinder sized unit is phased out even if the smaller one works well enough in hot restart situations which is their main function. Testing to follow!!!
Rob


As long as everything is perfect. . .

I've seen cars hold pressure everywhere from 30 seconds to over night!

[David T]

The accumulator does NOT provide the "capacitor" effect you describe. The fuel pump is capable of more volume than the engine will ever take. It's sole purpose is to maintain pressure in the injectors so that the fuel does not boil in the injectors/engine lines and create vapor lock i.e. the hot start problem we all know and love. When the fuel vaporizes in the injectors, the vapor is compressible and won't therefore develop enough pressure at cranking speed to pop the injectors. [This is not necessary on an EFI system as the injectors are actually electrical valves rather than a spring-loaded pressure valve. ]

Fuel should not leak past the accumulator diaphragm at all. If the "sealed bottom" version fails the bottom will eventually fill with gas and it will no longer "accumulate".

The version we are using has a spring under the diaphragm and is meant to have fuel in the back side, that can displace as the diaphragm moves as it pressurizes. Same idea but the chamber is filled with fuel rather than air. the only advantage that I can see to this is that trace amounts of fuel leakage (internal) would not be a failure, so maybe it will last longer in normal use.

In the event on a catastrophic failure of the diaphragm, the design we use could impact performance if excess fuel bleeds back to the tank and drops system pressure. I've never seen one that bad.

The reason pressure bleeds off over time (i.e. the 1/2 to several hours observed) is the fact that neither the accumulator, fuel distributor, nor injectors are perfect.

The port for the diaphragm return to the fuel tank is sized with a very small ID. I believe it was done so even if the diaphragm ruptures and you leak back to the tank the pump can still can provide enough pressure and flow to run the motor once you get it started. When the diaphragm ruptures it is usually not a pinhole. On the ones I replaced we were able to shake chunks of the rubber diaphragm out of the ports after we removed it so there was no diaphragm for all intents and purposes. I also read that the accumulator is supposed to smooth out pulsations but I doubt that is necessary. At least with the fuel pump we use. Maybe in another car with a different set-up, not a Delorean. On a car with a really tight system even the small one would work but if it is leaky the larger one will hold rest pressure up longer (more capacity).