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.

Duke

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