Author Topic: WUR return blockage in fuel distributor?  (Read 129 times)

raueda1

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WUR return blockage in fuel distributor?
« on: 10 July 2018, 05:42 PM »
Now I return to my insoluble WUR problem, which is apparently not a WUR problem.  I've progressed a bit so I thought it better to start a new thread than adding onto the older WUR thread.  So, to recap, the problem is that [control pressure] = [system pressure]  no matter what.  It could be a WUR issue, a defective FD or just a blockage in the fuel return lines.  Earlier I checked and unclogged the tank fuel return.  That didn't help.  Fuel volume is fine.  I therefore surmised that it was a WUR issue.

Now it gets more interesting.  On the advice of many here, and others too, I decided to get the WUR rebuilt by CIS Flowtech.  I installed it today.  Unfortunately that didn't help - new WUR, same old problem.  @#?!@!$?!?$!@$!!!    However, I did make an interesting discovery: 

The fuel flow through the WUR is substantial, and is supposed to return to the fuel tank return via the WUR return line to the primary pressure valve system in the FD.  I knew a  blocked fuel tank return line is pretty common and will cause my problem and so focused on that.  I also tested the return line from the WUR to the FD, and that was fine too.   Then I did something different:  tested control pressure with the WUR return line disconnected from the FD (and sent back to the gas tank by a long hose).  Eureka!  When the WUR return is disconnected the control pressure is very low, ~0.5 bar.  Cold WUR control pressure ought to be about 1.8 bar at 90deg F (yeah it's hot here). 

To me that suggests that the WUR IS working, more or less, and able to reduce the primary pressure when there's no backpressure on the return.  Presumably some backpressure on the return would increase the cold control pressure.   I hadn't tested it this way before, just didn't occur to me.  So, this leads me to thinking that there's a FD blockage in that pressure valve assembly someplace between the WUR return line and the tank return line.  FWIW, the pressure regulator valve is new along with o-rings etc on the internal plunger etc.

Is my reasoning sound on this?  And, if so, does anybody know what might be going on with that whole FD assembly that could create the blockage?  It's like the plunger or needle valve is stuck wrong or something.  I was able to blow carb cleaner into the WUR return fitting and it blew freely out the valve opening, so blockage must be downstream of that.  Anything other orifices to blow out or check?  I'll be asking CIS Flowtech similar questions in the morning, just thought I'd post first and see if anybody has any thoughts.  I'll report back with what they say.  Regardless, I have a feeling that a rebuilt FD is in my future.  Thanks and cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

raueda1

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Re: WUR return blockage in fuel distributor?
« Reply #1 on: 11 July 2018, 05:39 PM »
Problem solved!!  ;D   ;D   ;D

Solving it took a kind of detective-mode conversation with Larry of CIS Flowtech, whom I sincerely and graciously thank for the help.  It turned out not to be a WUR problem at all.  Rather, the primary pressure valve assembly was screwed up.

To recap, both the control pressure and system pressure were the same.  The car was able to run (surprisingly well in fact) using a very low system pressure by removing the adjustment shims.  The WUR wasn't doing anything.  This can be caused by a blockage in the fuel return lines that creates enough backpressure on the system.  I scoured all the MB and K-jet forums I could find, and everybody always looks for clogged return lines, blocked strainers, etc.  But I didn't have any of those, everythinhg was checked, rechecked, hoses replaced etc.  So where's the backpressure coming from? 

The problem turned out to be the pressure regulator assembly.   Larry did say that a blocked/nonfunctioning pressure regulator could be the cause, though it's a very reliable part and fails very rarely.  Yet I had replaced mine very early on without effect.  Could the original and its replacement both be faulty?  The answer turned out to be "yes," though for reasons I'd never have guessed.   On closer inspection I discovered that the central valve pin on the replacement regulator was about 3 mm shorter than the original.  I suspect that this difference was enough to allow the main return plunger to open enough for fuel to return but NOT open the valve in the regulator for the WUR return, hence the backpressure.  After replacing the O-rings on the original everything was good!  Presumably the original didn't work because the o-rings were all shot.  Never again will I assume that a replacement is really a replacement - measure everything!  Per Larry's advice I also reset the resting position of the air flow sensor plate, which was about 2mm too high.

Now all the CIS pressures are set right and I'm in business.  What's the result?  Really, really good!!  First of all, the idle is immeasurably improved.  It was pretty rough before and would stall from time to time, especially if hot and the AC was on.  Subjectively the car also feels substantially faster.  I'll measure at some point, but it's very obvious that it pulls harder.   The car also seems to just run smoother.  At speed the engine felt smooth enough before and certainly wouldn't strike anybody as rough or just crude (like, for example, 4 cylinder engines in general).  But now it has that kind of electric locomotive smoothness that just pulls and pulls and pulls.  Reminds me of my old BMW 328, one of the smoothest engines ever made, and also had that electric engine feel.

And so, this CIS horror story now comes to an end.   I was saved $US 500-600 for a fuel distributor rebuild and I'm back on the road.  Now I'll be able to drive to Lodefest with perfect confidence.  So I'm psyched!  Now back to the cruise control . . . . . ;D !!

 

-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0