Author Topic: Exact fuel pressure test hookup help needed for a California 6.9  (Read 1012 times)

secondslc

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Alright guys.

After weeks of delays, I got a fuel pressure test kit from Napa. It has a shutoff and a bleed valve, and a gauge that reads to 7 bar.

I was able to put a switch on the fuel pump too.

I'm having trouble however and hope I can ask the hive mind for help.

The way it's hooked up is that I have the tester between the large WUR fitting and the top of the fuel distributor. Picture attached.

With the valve open, I cycled the pump and bled the air out. I then shut the valve off, and the fuel pressure gauge shoots past 7 bar really quickly before I shut the fuel pump off. Am I doing something wrong or is something wrong with my 6.9 pressure regulator?


1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

secondslc

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #1 on: 30 August 2020, 08:14 PM »
I have a feeling I have a return line issue
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

rumb

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #2 on: 31 August 2020, 06:27 AM »
I have never done this before, but isnt the test with the engine running?
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

daantjie

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #3 on: 31 August 2020, 09:01 AM »
I did mine with motor running.
Daniel
1977 450 SEL 6.9 - Astralsilber

secondslc

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #4 on: 31 August 2020, 10:01 AM »
motor wont start... otherwise I’d do that.

System static pressure is way too high.
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

daantjie

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #5 on: 31 August 2020, 10:30 AM »
Not sure if you have the specs for US 6.9, but here it is.  This is for WUR model 0 438 140 010 with full load enrichment but no altitude compensation:

System pressure: 5.0  - 5.6 bar
Control pressure engine fully warmed up: 3.4 - 3.8 bar
Full load enrichment (vacuum hose removed) at idle: 3.0 - 3.4 bar
Control pressure, cold engine with 20C ambient: 1.1 - 1.5 bar
Stabilization time @ 20C: 3 - 6 minutes

Hope this helps :)
Daniel
1977 450 SEL 6.9 - Astralsilber

rumb

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #6 on: 31 August 2020, 11:23 AM »
I have a feeling I have a return line issue

Using some rubber hose, connect the fuel in and fuel out lines together and run the pump for a while.

Or use compressed air to blow thru the return line.
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

rumb

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #7 on: 31 August 2020, 11:35 AM »
OK, let back up here for a moment.

Looking at prev posts your car was running.
When did it stop working?

I see you said "The two adjustments we were trying to dial in are the idle air screw and the fuel enrichment 3mm screw"  The 3mm screw is the absolute last thing that should be touched and then very carefully.

If you have turned that screw a lot it may be very difficult to get back in range.

I would probably suggest sending the fuel distributor and WUR to CIS Flowtech for rebuild and setup.  Once returned and following their instruction the car should run great.  - You will still need to dial in the ignition timing.
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

raueda1

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #8 on: 31 August 2020, 08:39 PM »
This whole thing is a process.  See if you can get ahold of the Bosch K-jetronic diagnostic manual.  It lays out the whole process for this stuff from start to finish.  There's a .pdf on the M100 site but I'm not sure if that's accessable.  I'd email it but file is too big.  It really ought to get added to the tech info here. 

Anyway, let's start with basics.  Before proceeding I suggest testing fuel delivery and return.  You don't need to start engine.  Disconnect the fuel delivery line and put into a collection vessel, about 1/2 gallon.  Turn on ignition and unplug the blue socket and fuel pump will activate.  Run the fuel pump for 30 seconds.  If I remember right you should collect about 1 liter of fuel.  If not then you need to fix that before doing anything else (clogged line, clogged filter, etc etc). 

Next reconnect fuel line and disconnect the fuel return line and take gas cap off.  Using compressed air check to be sure the return line is clear and unoccluded.  You should hear bubbles in the fuel tank.  You can be deceived by using much pressure which can blow air past a bottleneck in the return line. 

If all is ok THEN test the primary pressure per Daniel's specs and your gauge hookup.  It might be necessary to build WUR and FD, or it may not.  Too early to say until you work through the process, so hold off for the moment.  I'm traveling now, will provide more specifics on return early next week if you're still stuck.  Cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

secondslc

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #9 on: 31 August 2020, 08:46 PM »
Ill start from scratch and go
From there.

The WUR is from kjet, the FD is rebuilt, the fuel pump is new, as is the filter, injectors, damper and the tank is cleaned out.

Also, i can blow through the return line.

1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

raueda1

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #10 on: 01 September 2020, 09:21 AM »
Ill start from scratch and go
From there.

The WUR is from kjet, the FD is rebuilt, the fuel pump is new, as is the filter, injectors, damper and the tank is cleaned out.

Also, i can blow through the return line.
If all that stuff was done then you're probably not t0o far from OK.  If you're shooting straight to 7 bar then yes, system pressure is obviously wrong, pointing towards the pressure regulator.  However, if the FD was rebuilt then I'd assume was checked at the same time.  But maybe not? 

Though I can't be 100% certain, I suspect that the return line thing is actually very important.  The line carries a lot of volume and if there's too much backpressure it would presumably increase the measured fuel pressure.  Hence my emphasis on that.  Others may chime in with better info.

Regardless, assuming fuel flow is OK (which it almost certainly is), sounds like time to check the pressure regulator.  Unscrew it and inspect the o-rings, plunger and washers.  Pressure is set by adding/removing washers and noting the effect.  Rebuild kits are widely available, cheap and include washers.  Tedious process - measure pressure, remove regulator, add/subtract washer(s), remeasure pressure, repeat.  But if o-rings are NG then washers won't help.  I'd just replace o-rings on principle.  I have another thread here someplace on the pressure regulator, worth a search.  If the skinny spring-loaded shaft isn't exactly right it will never be right.  I got a new regulator and the shaft was about 1mm different from the original and it didn't work!  Probably best not to disassemble the regulator body itself.  I'll redouble my efforts to get the Bosch manual posted in the tech section.  Stick with it and keep posting....    Cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

secondslc

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #11 on: 01 September 2020, 03:12 PM »
To be honest, the FD was pulled out of a box "as rebuilt" 20 years ago. It was sealed, but who knows WHO rebuilt it"

I fear I may have clogged the regulator by swapping lots of parts and maybe something going through at some point.

I pulled it, it had some stuff in it. But I also noticed the little o-ring isn't as new as it should be. Does anyone know the diameter for that? Is it "standard"? Its a cast iron FD

Are there more than one O ring?...I only see one?
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

secondslc

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #12 on: 01 September 2020, 07:48 PM »
Tomorrow I will run the fuel pump for 30 seconds and measure how much fuel gets pumped into a gas tank.

If its well below 1 liter, I have to start at the tank.

If its about 1 liter... then I move forward and find out why I have a return blockage.

Also, is my pressure regulator missing parts?

Images attached.

1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

daantjie

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #13 on: 01 September 2020, 07:56 PM »
Make sure you keep a fire extinguisher handy ;)
Daniel
1977 450 SEL 6.9 - Astralsilber

secondslc

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #14 on: 01 September 2020, 08:02 PM »
Oh...and push the car outside too
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8