News:

The ORG
No back-slapping boys club!

Main Menu

Abrupt K-jet wierdness and what should k-jet fuel pump pressure be?

Started by raueda1, 24 June 2022, 12:33 PM

raueda1

My car rather abruptly started running oddly.  No power at all, idle very rough, won't rev beyond about 3k rpm.  I immediately checked system and control pressure.  Both are insanely high, spinning the gauge completely around, off the scale and to the stop.  It's as if the regulator completely and abruptly stopped working.  I'm looking at that now, going through the CIS troubleshooting process.  Things seem to point to something wrong with the fuel regulator.  If anybody has ideas on known problems that appear from one day to the next it would be great.

In the meantime I'm wondering - what's the pressure that the fuel pump puts out without any regulation at all?  I can't find a spec on that, just control and system pressures.  Thanks and cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

Dave I am pretty sure system pressure is what the pump puts out as a baseline, should be 5.5 - 6.0 bar.  Fully warmed up you should see (from memory) between 3.0 and 3.4 bar measured between the FD and the WUR, depending on ambient temps.  I am guessing the exhaust gasses must be eye wateringly rich at the moment?
Daniel
1977 450 SEL 6.9 - Astralsilber

Jan S

Agree with daantjie's figures. And fuel flow at return to tank (after main pressure regulator) should be 1-1.5 liter per minute if I remember the spec correctly.

The pump curve - pressure as a function of flow shows diminishing flow as the pressure rises. I'm not sure what the "dead-head" pressure of the pump is, i.e. when the flow equals zero and the pressure is at max.

If the pressure is significantly higher than 5,5-6 bar the return to tank must be somehow restricted, maybe due to a pressure regulator that is not opening correctly ... or other things. And I'm not sure how the injectors' spray pattern are behaving at too high pressures.

I would test return to tank flow, and check if the return pipe is "open" with the help of compressed air.

1975-mod W116 450 SE with 6.9 engine

raueda1

Thanks gents, helpful indeed.  Oddly, the car actually seemed to be running lean, i.e., rpms go up when metering plate is depressed.  I think that means too lean. :o   But pressure is so high I'm not really sure what rules even apply.  I'm going back through all the manuals, diagrams, postings etc - it's unnerving, cause I've done it all before, but over time I've forgotten details and there's a lot to forget!

Anyway, as for fuel return blockage, that occurred to me too.  I tried the compressed air trick and fuel line does seem to be open.  But maybe just barely.  I'm not sure that it's entirely clear though, just cause I can hear some bubbling.  It took a long time and a lot of pressure to clear the fuel from the line before bubbling was audible. I wonder if the return can be open enough to let some air through but still be occluded enough to create sufficient back-pressure to cause problems?

So - I'm going to put a container on the return outlet so there's no back-pressure at all and see what that does to the pressures.  Will report back in due course....   Thanks again and cheers.
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Jan S

Good idea to let return go to container. And it should be 1-1.5 liter per minute ... but please check the exact specs.

And, you can remove the two short hoses between pipes and tank and blow compressed air ... the remaining fuel in the pipe will shoot out if open.

1975-mod W116 450 SE with 6.9 engine

daantjie

Please remember to keep fire extinguisher handy and to disconnect the battery, safety first ;)
Daniel
1977 450 SEL 6.9 - Astralsilber

raueda1

Progress??  A couple years ago I rebuilt the fuel distributor.  Reassembly had catastrophic results as many here will recall.  Apparently I made another mistake as well by swapping the FD fuel line and return line fittings.  Unknown to me, and not referenced anywhere I can recall, there's a tiny screen filter inside the fuel supply line fitting. The return is clear.  Mine were reversed so fuel return flow had to push past the filter on its way to the tank.  I'm sure somebody will point out that this is well documented, so this is just my most recent stupid mistake from not digging deep enough. 

Anyway, I compared flow with another fitting from a clean FD.  The blow-through-the-filter test showed that the spare fitting filter had much, much freer flow than the original, which was substantially occluded.  But this doesn't entirely explain why the pressures would suddenly go crazy after running great for 2 years.  My theory is that  the return filter collapsed slowly at first and then suddenly.  I had noticed a gradual performance decline over the last few weeks, but car still ran.  Then, abruptly, it scarcely ran at all.  I think all this isn't totally crazy, cause the filter element is sock-shaped.  When installed in the fuel return circuit the fuel flow would be reversed, compressing the filter sock against itself.  Close examination of the respective filters supports this idea.  The filter sock in the bad one was indeed collapsed on itself.  So, now to install the fittings correctly and retest.......  >>drum roll<<

That was it!  Pressures back to normal!  Impeded return fuel flow was indeed the culprit.  So we now know for sure that filtering the fuel return is a bad idea.  I'll never do that again. :o  And neither should you.  Case closed.  Thanks as always and cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

raueda1

Uh oh, I spoke too soon.  Pressures seem to be OK, car starts, but after about 30 seconds she starts stumbling and eventually stalls.  So, some improvement, but problems remain.  Back to the drawing board.... >:(
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

Maybe something to do with tank venting system?  Not sure really just thinking out loud.
Daniel
1977 450 SEL 6.9 - Astralsilber

Jan S

Quote from: raueda1 on 26 June 2022, 12:54 PMUh oh, I spoke too soon.  Pressures seem to be OK, car starts, but after about 30 seconds she starts stumbling and eventually stalls.  So, some improvement, but problems remain.  Back to the drawing board.... >:(

- Return flow ok? Approx 1-1.5 liter/min?
- System pressure approx 5.5-6 bar?
- Control pressure approx 1 bar cold and 3 bar warm?

(these are not exact specs, but in the ballpark)

These three things need to be ok. 
1975-mod W116 450 SE with 6.9 engine

s class

Sky high pressures suggest a blockage in the return, or issues with the pressure regulator, causing the pump to basically stall.

You are correct checking the return line with compressed air.  If that checks OK then the primary pressure regulator may have jammed.  Unscrew the regulator with 16mm across the flats towards the rear.  Retrieve the shims and pressure spring.  Then see if the piston will come out.  It has a rubber seal at the front end.  I've seen a couple of occurrences where this seal has split and gone down the return line allowing the piston the jam.


[color=blue]'76 6.9 Euro[/color], [color=red]'78 6.9 AMG[/color], '80 280SE, [color=brown]'74 350SE[/color], [color=black]'82 500SEL euro full hydro, '83 500SEL euro full hydro [/color], '81 500SL

raueda1

Looks the problem was identified, or at least some of the problem.  Leaking fuel line!!   :o  :o

With fuel pump on, the short line between the filter and pump was spraying out a few fine jets of fuel.  While getting back to basics by testing primary system pressure I noticed that initially system pressure would go up ok, but after a few moments would start slowly falling at an irregular rate. I heard a slight hissing from the rear of the car and thought "ah ha!  Fuel pump noises!"  The noise was the spray of fuel out of the hose - not a flood b y any means, but a few tiny jets.  It seems like maybe a hairline crack.  A couple times I'd noticed a small spot under the car but thought is was just a suspension valve drip. 

New hose ordered, just $16 + shipping (AutohausAZ).  My god! I'm relieved to have found it, this could have easily been a disaster.  It's worth noting that I completely rebuilt the whole fuel delivery module a few years ago.  The hose was replaced at the time.  It's disturbing that it should fail so fast.  Another case of "change all your rubber parts," and even new ones can go bad.  Makes me wonder if was really new or 250 year old NOS laying on the shelf and deteriorating all the while. Anyway, I'll report back on the results.  Cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

Indeed looks like you have avoided a Car-B-Q ;D

As one of my Benz buddies would say:"the entertainment value of an engine fire should not be dismissed out of hand"...
Daniel
1977 450 SEL 6.9 - Astralsilber

Jan S

The short MB fuel hose between pump and filter has a bend, at least the version I installed a while back.

I remember I had to "soften" and work the hose to avoid a sharp bend. Maybe the rubber hose is exposed to some sort of stress due to this bend? Could that have caused the thin crack?

Slightly worrying that a relatively new fuel hose cracks.
1975-mod W116 450 SE with 6.9 engine

raueda1

Quote from: Jan S on 10 July 2022, 03:39 PMThe short MB fuel hose between pump and filter has a bend, at least the version I installed a while back.

I remember I had to "soften" and work the hose to avoid a sharp bend. Maybe the rubber hose is exposed to some sort of stress due to this bend? Could that have caused the thin crack?

Slightly worrying that a relatively new fuel hose cracks.
Yes, worrying indeed.  First thing is to get the old hose off.  I can't do that for a couple weeks, traveling, and in the meantime it will preoccupy me.  :o I'm not 1000% sure that I replaced the hose, though washers are clearly new.  I would have been a fool not to replace it at the time.  Hose is available and very cheap, but I've done dumber things. ::) Anyway, the leak was very close to the banjo fitting on the filter.  My fear is the hose is NOS that's been on a shelf for 25 years.   There's not enough antioxidant in the world to keep a fuel hose going for that long.  I'll post my findings.
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0