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Old 10-11-2017, 11:20 PM
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Running k-jetronic without fuel accumulator?

So after getting the fuel distributor fixed on my 78 280slc, I noticed that starting was getting more and more difficult. I soon identified the fuel accumulator as the culprit. Fuel was coming out of the vent fitting on the back of the accumulator, indicating that the membrane inside was punctured/cracked.
Trouble is: the exact part number is out of stock almost everywhere, an when it is available the price is ludicrous (over U$ 300), because this same part number is also used on a Ferrari of the same vintage. It's pretty obvious that it will be a while before I get my hands on a new one.

So my question is this: It is bad to run without a fuel accumulator? (Besides the obvious starting issues). I'm thinking that removing the accumulator could improve starting slightly, given that I would be eliminating a pressure leak at least, right?

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Old 10-11-2017, 11:32 PM
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Bad? No. As noted, starting, particularly when hot, may be more difficult. The accumulator serves to dampen pressure fluctuations that can be caused by sudden changes in metering plunger position, or by voltage changes that result in pump speed variations.
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Old 10-12-2017, 06:13 PM
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The external vent is a really , really bad design. Even worse on some other brands where the vent is atop the exhaust. I've seen other K jet cars plumb the vent into the tank to pump hose to eliminate the leakage / fire issue.

I'd look around for any accumulator with the same fittings and go from there. Unless someone has an actual pressure / volume chart from Bosch, I'd think the only real difference is a mounting bracket / fitting location.

If you bypass the accumulator, when hot manually running the fuel pump before cranking to build pressure might be helpful.
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Old 10-12-2017, 08:17 PM
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Yeah, currently I am running the pump manually before every start. I simply disconnected the safety switch on the air sensor body so that the pump runs with the key on the ignition on position.
The vent on my accumulator actually connects back to the fuel line, just before the pump inlet.
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Old 10-13-2017, 08:20 AM
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Quote:
Originally Posted by 97 SL320 View Post
The external vent is a really , really bad design. Even worse on some other brands where the vent is atop the exhaust. I've seen other K jet cars plumb the vent into the tank to pump hose to eliminate the leakage / fire issue.


The vent should never be external. The accumulator should only vent back to the tank, and the only time it should actually “vent” is when it is bad and the membrane has failed.

My understanding of the accumulator is NOT to dampen pressure fluctuations. Once the spring is fully compressed, it cannot do anything other than expand to increase (or balance) low pressure. Higher pressures are relieved by the fuel pressure regulator at the fuel distributor. After the fuel pump is turned off, the accumulator should help maintain pressure as small leaks (like a leaky injector or pressure regulator) allow the pressure in the system to decrease. The spring in the accumulator helps maintain pressure and keep the fuel in the lines from evaporating, which might create a vapor lock situation.

If you have no fear of vapor lock or hard hot starts, then you may be just fine without the accumulator in the system.

I believe this is the one I used on my 1978 450slc 5.0:
https://www.ebay.com/i/351736499245




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Past mb: '73 450sl, '81 280slc stick, '71 250, '72 250c, '70 250c, '79 280sl, '73 450sl, parted: '75 240d stick, '69 280s, '73 450slc, '72 450sl,
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  #6  
Old 10-13-2017, 07:58 PM
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Quote:
Originally Posted by fonzi View Post
The vent should never be external. The accumulator should only vent back to the tank, and the only time it should actually “vent” is when it is bad and the membrane has failed.
K jet SAAB's had an external vent, right above the transverse muffler.

I'm, pretty sure same era VW's had an external vent also.

I need to define what is being vented. With this type of accumulator, the "vent" side needs to be open to atmospheric air pressure ( or through the fuel tank ) so the diaphragm can move. If the " vent " is blocked, pressure required to move the diaphragm will be higher. There are accumulators ( in general ) that are unvented with gas pressure as a countering force. ( ABS / hydraulic suspension for example )

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