Fuel accumulator test

Wouldn't you want to verify that the old one is bad before putting in (or buying) a new one?

The bottom line tests for your type accumulator are:

1. Is the fuel system pressure after shutoff about ~3 bar and doesn't fall below ~2.6 bar after 20 minutes? (although failing this test can be caused by a leaking check valve, bad accumulator, or several other causes -- see page D43 in the 281/83 Mondial8/QV WSM for description)

2. No fuel flow out of the breather fitting (or in the return line to the tank, if so equipped).

3. No external fuel leakage (up to system pressure of ~5.5 bar).

Any "test" system to verify those conditions would be as complicated as the actual fuel system so don't see how that would save work.

 

I'd put a much higher probability on the fuel pump check valve for this symptom, but IMO you should:

A. first verify that the root cause of the warm restart problem is that the supply fuel pressure after shutoff is falling below spec, and

B. if that's verified, the tests described on page D43 describe how to isolate whether the primary side of things (leaky check valve, leaky pressure regulator, leaky cold start injector) or the control pressure side of things (WUR) is the culprit.

Does your model leave the breather port of the accumulator open to atmosphere or do you have a fuel line connecting the breather port of the accumulator back to the top of the fuel tank? The main failure mode of the accumulator would be a leak in the diaphragm -- and that would have fuel coming out the breather port.

 

There is. Diagnose first, then replace parts. The Bosch CIS problems/symptoms are exceptionally well known and documented thoroughly. Steve has done a phenomenal job in walking you through the steps. Even the factory manual excels here. With a pressure gauge and appropriate valve, your hot/warm start problem could be diagnosed efficiently. (I taught Bosch CIS diagnostics to repair centers about 20+ years ago. This is how we would troubleshoot any problem; control and system pressures known first, then replace parts.)

No need for this fear, see my first response.

Your car does give you subtle warning signs. However, what you propose is a truly great idea. However, keeping in mind that the car is 30 years old and subject to past care and fuel quality, I would say that, currently, all components are subject to eminent failure.

Best,
David

 

Robert I believe the check valve on your 85 QV is external. The cross reference part number is E3030-27424 and is the same for the 328. I replaced mine not too long ago as I found a small drip coming from one of the copper washers.

Here is a link. Put the part # in and your ready to go. A bargain at $17.88.

http://www.importeccatalog.com/

 

Mods

This info needs to go upstairs in the parts xref sticky.

Eurospares sells them, the check valve that goes on the FD, dirt cheap too.

 

Robert,

I replaced EVERYTHING as I have Bosch just right down the street. WUR & FD were rebuilt. As has been said it is all 30+ years old and you have NO idea how those parts were cared for during that time.

Wasn't cheap but it wasn't that expensive either. Should be good until long after I meet Enzo in person.

 

Could someone please tell me if the external fuel pump check-valve is model specific, or universal, as I suspect the same problem with my 1980 400i(every part is X2 of coarse). Thank you very much,indeed.

 

Yes, it is model specific, but some models are a more difficult conversion than others because of the plumbing/fitting changes needed -- the good news is that your model (400i) is the easy one because the line leaving the fuel pump outlet is already a banjo fitting so all you need (2X) is the later fuel pump, an external check valve, and the female cap nut (which is also readily available as a Mercedes part) plus the various copper sealing washers.

Just went thru this with FChat member jeffsjag90 offline so you might send him a private message and ask for the details. If you can't reach him, send me an email, and I'll forward the email trail to you.

 

If you have the line connecting the accumulator breather port to the top fuel tank, you wouldn't get a fuel smell even if fuel was (wrongly) leaking out the accumulator breather port. Unfortunately, the '84-'85 US 308QV SPC isn't available anywhere online (that I know of) so I couldn't check first if you have the line or not. If you don't have that line, your nasal observation is a good sign that it is not leaking.

 

Thank you, Steve so very much for your wonderful reply(#13) regarding my 400i. I will check everything out within the next few day. I am continually amazed at your seemingly boundless capacity for helping your fellow travelers..Thank you to the rest of you, also, for this wonderful forum. Jacques

 

I believe it is. My '81 is like the model depicted.

Had the hot start issues so I knew the accumulator was likely at fault.

 

Yes, and would make sense for a US '84-'85 308QV to have it too (since the EPA frowns on liquid fuel leaking out ) so please let us know if you've got the same line (the euro 308QV pictures show that that line is not used on that version).

 

Then the "accumulator diaphragm test" is to disconnect that line (at one end or the other), unplug the safety switch, and turn the key "on" (so the fuel pump runs) -- if fuel comes out = bad accumulator for sure.

 

Don't quite follow this as, if the "pressure tests are good", you wouldn't need a new check valve

If your accumulator is leak free, then I'd still put the highest probabilty for the cause of a warm restart problem as the check valve, but a problem (leak) in the regulator section of the FD (either the main shut-off seal or the WUR return seal) could still also cause the same symptom (if your root problem is losing the residual fuel pressure after warm shut off).