Author Topic: Fuel accumulator non return hack  (Read 5931 times)

George

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Fuel accumulator non return hack
« on: 13 July 2011, 02:19 AM »
I phoned the Benz distributors and is asking me a whopping R4000 for a part that's roughly $600. It would be cheaper for me to import one myself.

But looking at this system I have tried making sense of the accumulator's function, I can only see that it was meant to smooth out the pump operation or account for hiccups or spikes and to maintain a smooth running pressure. Like a capacitor in a circuit or a mechanical balloon, a 4K balloon.

Both my secondhands does that when the motor runs, they just don't seal properly making fuel return overnight.

My guess is because the res is leaking it causes a vacuum opening the pressure compensation valve on the motor fuel distributor side allowing all the fuel to run back trough the feed line.


Can anyone see any reason why I can't just fit a standard non return valve into the fuel line hose going from the fuel filter to engine right after the fuel filter?

Well at least until I can get a replacement that fits my pocket.
« Last Edit: 13 July 2011, 02:36 AM by George »

George

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Re: Fuel accumulator non return hack
« Reply #1 on: 13 July 2011, 02:31 AM »
I phoned the Benz distributors and is asking me a whopping R4000 for a part that's roughly $600. It would be cheaper for me to import one myself.

But looking at this system I have tried making sense of the accumulator's function, I can only see that it was meant to smooth out the pump operation or account for hiccups or spikes and to maintain a smooth running pressure.

Both my secondhands does that when the motor runs, they just don't seal properly making fuel return overnight.

My guess is because the res is leaking it causes a vacuum opening the pressure compensation valve on the motor fuel distributor side allowing all the fuel to run back trough the feed line.


Can anyone see any reason why I can't just fit a standard non return valve into the fuel line hose going from the fuel filter to engine right after the fuel filter?

The valve should halt fuel excaping the res since it's creating a vacuum seal and won't inhibit the function of the pressure comp on the distributor since it's only function is to keep fuel volume shrinkage/volume from messing arround.

Well at least until I can get a replacement that fits my pocket.


s class

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Re: Fuel accumulator non return hack
« Reply #2 on: 13 July 2011, 02:34 AM »
Is this a K-jet car you are talking about?

Where are you located? 

Here in South Africa I have supplied several club members with accumulators, and for the W116's the total cost is something like ZAR1250 each. 

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

George

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Re: Fuel accumulator non return hack
« Reply #3 on: 13 July 2011, 02:59 AM »
In JHB, this is for a 126

s class

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Re: Fuel accumulator non return hack
« Reply #4 on: 13 July 2011, 03:46 AM »
WHich W126?  Some use the W116 style accumulator for ZAR1250, most use the smaller type and it is a lot more expensive - ZAR2200 or so. 

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

George

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Re: Fuel accumulator non return hack
« Reply #5 on: 13 July 2011, 04:09 AM »
It had the larger one but without the second inlet in front. 1980 280se 126.022

It's still alot of money for a disposable component, in what way will the car suffer without it or if there's a checkvalve on the fuel filter outlet? The leaky components still offer fuel spike damping to the fuel pump. The check valve could extend their usage unless it causes other problems.


s class

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Re: Fuel accumulator non return hack
« Reply #6 on: 13 July 2011, 04:12 AM »
There is already a check valve on the pump outlet (or there should be, at least).  That check valve may have failed.

Question - are you sure the accumulator is the problem?  Fuel pressure can leak away out of :

a) check valve
b) accumulator
c) one or more injectors
d) cold start valve
e) base of fuel distributor. 

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

George

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Re: Fuel accumulator non return hack
« Reply #7 on: 13 July 2011, 04:25 AM »
The pump valve is tested OK and with the disconnect test on the resevoire, it's leaking some fuel when the engine is running  but not squirting.

I have tested the car start problem with clamping it's exit hoze before turning the motor off at night. It started easy the next morning in one go.

The car runs OK with it it just doesn't start that well.

s class

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Re: Fuel accumulator non return hack
« Reply #8 on: 13 July 2011, 05:18 AM »
looks like your car needs A000 476 08 21.

Approx ZAR1550.00 all incl. 

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

koan

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Re: Fuel accumulator non return hack
« Reply #9 on: 13 July 2011, 07:12 AM »
Fuel accumulator holds a volume of fuel under pressure and also maintains pressure in the fuel line running to the fuel distributor. This prevents fuel in the lines vaporising in hot climates, it also means fuel under pressure is available for engine start rather than pointless cranking as fuel pressure comes up.

koan
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Type17

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Re: Fuel accumulator non return hack
« Reply #10 on: 13 July 2011, 07:16 AM »
In answer to your original post, the accumulator does two things - smooths out the pressure flow when the pump is running, and holds the system pressure high for a few hours after the (hot) engine is stopped, preventing fuel vaporisation. Note that the pressure does drop to near zero after that, but by then the engine is cold, so there is no chance of vaporisation. Additionally, a cold start with the system pressure at near zero is not an issue, as no air is admitted to, or fuel lost from, a healthy system, so the system remains full of fuel, and the pump can raise that fuel to the required system pressure level in well under a second.

A faulty accumulator makes warm/hot starting difficult, but a cold start should still be perfect. If it's not, then there is a different problem than the accumulator, or another problem in addition to it. (Had this exact issue with my 116, new accumulator fixed it, with no other changes to the fuel system)

Edit: Just like Koan said!
« Last Edit: 13 July 2011, 07:27 AM by Type17 »

George

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Re: Fuel accumulator non return hack
« Reply #11 on: 13 July 2011, 11:22 AM »
I think you are right, all theory had to fly out the window this afternoon. I tested the second hand replacement I fitted yesterday and it does not even have a drop of fuel coming out the back with the motor and running even after having used the car to drive to work today.

So there's something else that might still be amiss here, it struggled starting this morning and this afternoon on the way back from work.

What bugs me is the old one was leaking and the test I did a few days ago clamping it's leak/atmospheric hose going back to the damper did make the car start off the bat after standing over night. Proving that if I could stop the fuel return through the exit/leak that the car would start easy, and it did.

I guess maybe it was just one time luck. I'll do the test again to try and disprove the accumulator. I did get a second one from the secondhand merc shop again on the way home so now I have a second one for spare.

Paid R100 for each but there's no guarantee on how long they will last.
« Last Edit: 13 July 2011, 11:25 AM by George »

koan

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Re: Fuel accumulator non return hack
« Reply #12 on: 13 July 2011, 02:19 PM »
So there's something else that might still be amiss here, it struggled starting this morning and this afternoon on the way back from work.

This is cold starting is it?

Warm starts are OK?

Does it backfire in the inlet manifold if the throttle blipped after a cold start?

When warmed up how does it run?

koan
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George

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Re: Fuel accumulator non return hack
« Reply #13 on: 13 July 2011, 11:35 PM »
Warm starts are OK.

It has to heat up to about 50degrees celcius before the throttle can be pushed normally but there's no obvious backfiring just a engine dying away effect if you push it while it's cold.

When it runs it's doing OK or have been running OK but this morning it was rather lazy, it's an automatic gearbox, If I push the pedal down to trigger full enrichment it did not respond with it's usual power. Something else must have died in this past week.

atm it's behaviour when warm has been pretty random, one day it was a loose plug wire, the next the plug wires are OK but something is not 100%, then in the afternoon it would be OK again with seemingly normal power at low speeds and good power at full throttle.

The clamp test failed to give me the same results this morning so that means there's more issues or a new issue has presented itself now, the accumulator is no longer leaking so it's time to move on.

I think a full fuel injection inspection and minor service is in order.
« Last Edit: 13 July 2011, 11:40 PM by George »

koan

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Re: Fuel accumulator non return hack
« Reply #14 on: 14 July 2011, 01:06 AM »
Have you checked ignition timing and the vacuum plumbing associated with the distributor?

A compression check would be worth doing.

koan
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