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Old 11-08-2015, 10:21 PM
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Location: Southern California
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KE fuel accumulator destructive failure analysis

As promised (sorry for the delay) here is the failure analysis of the fuel accumulator that caused a hard starting problem last spring on my '88 190E 2.6 (only 84K miles), and this part is common to virtually all Mercs with the Bosch KE fuel system. I started a thread on the problem, which included my analysis of why the accumulator was a prime suspect and a simple test to determine whether it is functioning properly. Based on other fuel system problem reports, I think this component is often overlooked. That thread is located here:

M103 starting problem

I had a hard time even finding one, but it looks like they are now readily available. Mercedes probably built about 3 million cars that use this part. The Bosch part has the Bosch logo and part number, and the Mercedes part has the Star and MB number, but they are functionally equivalent and of identical design and quality, both manufactured by Bosch.

The accumulator is designed to hold 20cc of fuel at system pressure, which helps maintain fuel pressure despite normal small bleed down from the pressure regulator and fuel pump check valves, which makes the car easier to start after sitting for some time. The first photo is a thumbnail of a new accumulator from the Pelican parts Web site. The thread at the bottom connects to a manifold that joins the high pressure side of the accumulator to the fuel pump(s) outlet and fuel filter inlet. Despite the large coupling, the inlet has a very small restrictor - probably about .010" diameter that restricts both the inflow and outflow rate.

The nipple at the top of the photo attaches a low pressure hose that tees back into the fuel supply line from the fuel tank, which provides an ambient pressure reference and contains any fuel spillage from an internal accumulator leak by shorting it back to the fuel supply.

Since the accumulator holds up to 20cc of fuel I assumed it must have some kind of moveable elastomeric diaphragm and a spring under high preload, probably something on the order of 200 pounds given the dimensions of the device and KE operating pressure. This called for a carefully thought out disassembly so the thing didn't explode like a bomb!

With help of a friend, we drilled holes in a couple of wood blocks to snuggly contain the outlet thread and vent nipple, then placed the whole contraption between the jaws of a large vice. The plan was to use a sawsall to cut off the crimped section, then slowly unwind the vice to unload the spring. The steel was well work hardened where the crimp is formed, and it took six cuts and a couple of blades to cut away the crimped area that holds the two halves of the housing together, but all went according to plan other than, as usual, it took longer than expected.

Photo number one shows the "exploded" view - the two housing halves, plastic spring retainer, spring, and diaphragm. It's a simple device - only five parts.

The diaphragm appears to be something like fiberglass cloth with an elastomeric coating on both sides and is thinner than I expected.

Photo number two is a closeup of the diaphragm and the awl points to small cracks at the outer edge of the high pressure side. The outer portion of the diaphragm also functions as the seal for the two halves of the housing, but most of this area was cut away in the sawsall operation.

There were no obvious cracks on the concave side (photo 3) of the diaphragm, which is the ambient pressure side, however when I turned the diaphragm inside out I could see small cracks. It's tough to see with the low resolution photos based on the this site's maximum jpg file size, but the discolorations at the tip of the awl are actually impressions made in the plastic spring retainer by the diaphragm mesh, but I don't think this is part of the failure mechanism.

So I didn't find an obvious rupture as I expected, but the series of small cracks probably developed over time and eventually caused enough microscopic leak paths to short the fuel back to the supply line, so the system could not hold or build sufficient operating pressure. Thus the hard starting.

Note the three impressions in the diaphragm that match the contour of the high pressure housing side. When unloaded the spring pushes the diaphragm hard against the high pressure side of the housing leaving the housing geometry impressions.

The diaphragm is most flexible in the center, and it can move about one-half inch. This bends and stresses the fold where the cracks are. It's a combination of stress, and as with any elastomer, time, which eventually leads to cracks, leaks, and ultimately failure of the accumulator to do its job.

No, it's not ethanol in the fuel... just normal wear and tear. Elastomers don't last forever!

So if your KE-equipped car develops a starting problem - long cranking time and then snotty running for several seconds after it finally is barely running, it will probably get worse with time. Don't overlook the accumulator. It is easy to test and easy to change.

Attached Thumbnails
KE fuel accumulator destructive failure analysis-ke-fuel-accum-new.jpg   KE fuel accumulator destructive failure analysis-ke-fuel-accum-1.jpg   KE fuel accumulator destructive failure analysis-ke-fuel-accum-2.jpg   KE fuel accumulator destructive failure analysis-ke-fuel-accum-3.jpg  

Last edited by Duke2.6; 11-08-2015 at 10:45 PM.
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Old 11-09-2015, 02:11 PM
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Great followup !! Thanks
1985 300D 198K sold
1982 300D 202K
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Old 11-10-2015, 03:12 PM
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Trevor Hadlington
Join Date: May 2014
Location: Worcestershire in England
Posts: 1,171
Thumbs up thank you

Duke2.6 thank you i have had a small problem with my 260e Starts up fine but not clean, it is missing but not long enough to be a problem .I just know its not right. Also when i visit the car shows at our local inn thats only 3 miles away .I switch the car off ,by then its up to temp .On restart after 3 hours it starts and stops right away.But then i give it a little gas and it will start .It runs like a dream other than this .So thanks to your post i will go ahead and purchase another FPA . PS this as drove me mad for 2 years and all things i have done never fixed it
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Old 11-10-2015, 03:46 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,072
I don't understand why you say "purchase another FPA", which is the fuel pressure regulator that's located under the air cleaner. The accumulator is under the floor pan next to the fuel pump(s) and filter.

Have you tested the accumulator as I described in the link in post #1? If it shows any leakage it's probably on the out and should be replaced. If no leakage is detected, it's okay.

I've also had the hot start/stall problem, but just learned to goose the throttle after it fires, which avoids the stall. I don't know yet if the new accumulator has mitigated this problem because I put the car in storage just after I changed it last spring and just brought it out of storage last weekend, but only did one hot start.

I only drive the car two or three times a month, so it will take some time to determine if hot starting performance is better.

Also, I want to emphasize that my analysis of the accumulator failure mechanism is that it fails slowly over time, not suddenly. Once leakage starts it will slowly increase with use/time eventually reaching a point where the hard starting is obvious and consistent.

Based on the design of the diaphragm there should be absolutely no internal leakage on a properly functioning accumulator. Any detected leakage indicates that the failure mechanism has started. It could take some time to get bad enough that starting is an iffy propositon, but it's a good idea to replace it before the problem becomes severe and possibly leaves you stranded.

My car has had the hot start/stall problem for some years, yet cold starts were usually normal. Then finally one day on a hot start after an hour or two of hot soak, it was reluctant to start, but finally started firing on two or three cylinders, and I kept goosing the throttle until it began to run smoothly after a few seconds. The same thing happened on the next several cold and hot starts, then it went away, but returned after about a dozen or so starts. That's when I got serious about finding and correcting the problem.

The FPR may be partially to blame as its bleed down rate probably increases with miles/time. The accumulator will replace the bled down fuel from the FPF and maintain pressure until its 20 cc is exhausted, then pressure will drop rapidly. Then when you attempt to start, there will be insufficient fuel pressure until the accumulator refills through that .010" restrictor.

After sitting for six months I expect this was the case as it barely started and ran on only two or three cylinders for several seconds until a smooth cold idle was established. On the hot restart that same day, it fired right up, no stall, and I didn't goose the throttle.


Last edited by Duke2.6; 11-11-2015 at 10:33 AM.
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Old 11-11-2015, 12:22 PM
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Trevor Hadlington
Join Date: May 2014
Location: Worcestershire in England
Posts: 1,171
Duke yes i know the part thats under the car by the filter is Acumulator -FPA .I have fitted one on last month .Also new filter at the same time .I think my problem is nothing to do with this part as it was fitted in and made no change to my starting up . And as i have replaced a few parts that change nothing i will replace the FPR regulator next job. Sorry if i oonfused you .
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