Hot starting - check valve

[adamb]

I am renewing my knowledge of how to fix hot starting problems for k-jet. It's been many years and I've forgotten. I am about to commence analysis as per https://handbook.w116.org/cd/Engine/107/M117_45/073-120.pdf. Point 18 says that if control pressure drops to 0 immediately upon shutting off the engine, renew check valve on fuel pump or subsequently install.

I don't think I am aware of such a check valve. Would anybody care to share a link or image of one?

[raueda1]

First check the CIS manual on the tech section.  It's the Bible for this stuff.  That said, my first guess is that your fuel accumulator is shot. If I recall the return check valve is on the fuel pump but accumulator is more probable IMO. Cheers,

[revilla]

That manual is so confusing in that regard. It requires knowledge and logic for interpretation.

But it's so simple in reality.
There are only 3 things holding pressure after pump stops running:
-Accumulator
-Pressure regulator
-Pump check valve

Of course, injectors are also holding those 3 bars of pressure, but they will not allow the pressure to go back to zero suddenly, if they leak, pressure drop is gradually. So that's ruled out.

So those 3 things are the sole responsible for keeping static pressure. In order of probability of occurrence in my experience:

-Orings in pressure regulator (all 3 of them). Very easy to check visually once removed from FD. Please allow me to insist, all 3 of them hold fuel from going back the return line path.
-Accumulator shot, very easy to check once removed, just put some compressed air pressure (gradually and no more than 6 bars) through the inlet nipple and your thumb on the opposite side. No air should pass through, zero.
-Pump check valve, I've never seen one fail. It doesn't mean they are eternal. To test, remove pump outlet fitting. Compressed air. Zero return is good.

Good luck

[raueda1]

That manual is so confusing in that regard. It requires knowledge and logic for interpretation.

But it's so simple in reality.
There are only 3 things holding pressure after pump stops running:
-Accumulator
-Pressure regulator
-Pump check valve

Of course, injectors are also holding those 3 bars of pressure, but they will not allow the pressure to go back to zero suddenly, if they leak, pressure drop is gradually. So that's ruled out.

So those 3 things are the sole responsible for keeping static pressure. In order of probability of occurrence in my experience:

-Orings in pressure regulator (all 3 of them). Very easy to check visually once removed from FD. Please allow me to insist, all 3 of them hold fuel from going back the return line path.
-Accumulator shot, very easy to check once removed, just put some compressed air pressure (gradually and no more than 6 bars) through the inlet nipple and your thumb on the opposite side. No air should pass through, zero.
-Pump check valve, I've never seen one fail. It doesn't mean they are eternal. To test, remove pump outlet fitting. Compressed air. Zero return is good.

Good luck

Yes!  I forgot that I had the same problem.    This may help.  Cheers!

[adamb]

https://youtube.com/shorts/bzWMfPdEJ8k?si=CLc7-RCd41RnSuIp

Here's a video of my current pressure drop. Does this look like a fast drop? If yes then instructions suggest issues with the check valve.

Some history: the problem started when I replaced the fuel pump and filter. I thought that during that time the accumulator failed so I proceeded to replace the accumulator too. No change. To me it seems that the check valve is inoperative. If it's part of the fuel pump then I may need yet another new pump.

[Max-NL]

That is indeed a quick pressure drop. It should at least stay around 2.8 bar after shutting off.

If the problems started after you replaced those three components, then I would indeed suspect on of those components.

You could check the accumulator to see if it's leaking. You just need to unscrew the leak line and check to see if it's leaking fuel.

The check valve can be screwed loose from the pump and be replaced. MB part # is A0000900110, Bosch part #1587010532. Or you could screw in the check valve from the old pump (assuming that one worked correctly), just replace the copper crush washers.

[adamb]

Recently I posted a topic on the fuel reservoir/accumulator but there were no responses/follow ups. Please take a look at
https://forum.w116.org/mechanicals/fuel-accumulator-leaks/msg158564/#msg158564

I have the old pump and it was working fine but beginning to get quite noisy on hot days. Stupid me replaced items preventitavely. My replacement was from MB directly. Is it possible it came without a check valve?

[Max-NL]

With a pressure loss that rapid I'd expect to see a massive flow out of the leak line with some pressure. But seeing your statement that it doesn't leak with the pump off, and only a small leak when running (manual states "a pressureless quantity of leaking fuel is allowed") I am inclined to say that the accumulator is ok. But perhaps a more informed person could give some more information about it.

Regarding the check valve, you shouldn't be able to hook up the fuel pump without it, as the fuel line to the filter is attached to it (see attached image). Perhaps you could test it by hooking a pressure gauge to it with a banjo union to see if it holds pressure.

Edit:
Just checked 07.3-280 regarding replacing the fuel pump. It seems that up till april 1980 the check valve was internally in the pump, and after april 1980 the valve is external. The internal system used a hollow bolt to attach the hose towards the filter onto the pump, while the external version uses a cap nut to attach the hose. See https://handbook.w116.org/cd/Engine/107/M117_45/073-280.pdf

Do you know how your hose from pump to filter is attached (or perhaps share a picture)?

[raueda1]

Do I have the chronology right -
   1. FP was noisy-yet-working
   2. You replace FP, filter and accumulator and hot start problem occurs
   3. You test accumulator and find the leakage seems small, not a full flow
   4. Pressure test shows normal control pressure and rapid depressurization.

So, as Max points out, accumulator is probably OK.  This again points to pressure regulator.  I'd check that before going further.  As a bonus, it's a lot easier to wrench around than the fuel delivery system.  Specifically, just replace the o-rings.  Then at least you'll know that the regulator is not the problem.  It might seem like an unlikely coincidence, but odd things can happen.  Who knows, maybe something got loose when you changed the FP etc. Regardless, when I had a similar problem it occurred abruptly. I was convinced that it was the accumulator but it turned out to be the tiny o-ring inside the pressure regulator. A small chunk of it had broken off and it was hard as a rock. Cheers,



It's easier to replace gaskets

[ramiro]

You can test the fuel return valve in the fuel distributor very easy , just remove the return line after you ran the fuel pump there shouldn't any fuel come out of the fuel distributor when the fuel pump is not running.

[UTn_boy]

You can test the fuel return valve in the fuel distributor very easy , just remove the return line after you ran the fuel pump there shouldn't any fuel come out of the fuel distributor when the fuel pump is not running.

The fuel return/check valve mentioned above is in the high pressure supply side of the fuel system, not the low/return side, and is mounted in the fuel pump itself.  See Max-VL's picture above for a picture reference of location. 

[ramiro]

The fuel return/check valve mentioned above is in the high pressure supply side of the fuel system, not the low/return side, and is mounted in the fuel pump itself.  See Max-VL's picture above for a picture reference of location. 

I know that , but nobody said that it is 100% the valve at the fuel pump , and i once saw the same pressure drop when the valve in the fuel distributor was leaking.

[raueda1]

The fuel return/check valve mentioned above is in the high pressure supply side of the fuel system, not the low/return side, and is mounted in the fuel pump itself.  See Max-VL's picture above for a picture reference of location. 

I know that , but nobody said that it is 100% the valve at the fuel pump , and i once saw the same pressure drop when the valve in the fuel distributor was leaking.

^^^This. I did as well.  Focus on FP is misplaced IMO until it has been ruled out.