Author Topic: Exact fuel pressure test hookup help needed for a California 6.9  (Read 1083 times)

secondslc

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #30 on: 05 September 2020, 09:00 PM »
After more testing...I put my old WUR In just to see what the difference would be.

System Pressure is within spec.

-Cold control pressure is .8Bar
-Plugging the WUR in, the pressure rises slowly, but only gets to 2 bar in 3 minutes.
-Leaving the valve open and shutting the fuel pump off, the gauge pressure drops from 2 bar to zero in one minute.
-Leaving the valve closed and shutting the fuel pump off, the gauge pressure drops to 3 bar pretty quick, but stays above 2.8 bar for well over a minute.

My question:
-What has my system control pressure so low?
-At what pressure will the injectors fire, cause now the car will not start...harumph.

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 #31 on: 06 September 2020, 03:15 PM »
I may be talking to myself, but at least it'll be documented ;D

Gauged the fuel pump itself by attaching the gauge to the supply line. Cycled fuel pump and wont stay above 4 bar, even then...it drops to zero after 20 seconds or so.

I think I have a bad check valve.

But would the bad check valve cause the control pressure to read low?

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 #32 on: 06 September 2020, 03:34 PM »
How sure are you your pump is good?  If you have not yet replaced the pump (sorry too lazy on a Sunday PM to read the thread from the start 8)) I would for sure buy a new Bosch pump.  They are not too pricey and will last the rest of the life of the car.
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 #33 on: 06 September 2020, 03:57 PM »
Pump is a new Bosch. Which annoys me that the check valve could be bad. But Im going to say grunge got into it and clogged it.

I ordered a check valve, accumulator, and fuel filter. I’m not gonna want to do this again.
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 #34 on: 06 September 2020, 07:16 PM »
I agree it's good to have a solid baseline so you can look at it from a process of elimination approach.
Daniel
1977 450 SEL 6.9 - Astralsilber

raueda1

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #35 on: 07 September 2020, 08:36 AM »
No, not talking to yourself (not yet anyway).  Drove from S. Lake Tahoe to SLC yesterday and managed averaged of about 74mph including (frequent!) gas and rest stops.  These cars like to go fast.  Anyway, now that I'm home I've got the manual handy and can better summarize some of this stuff.  These tests are in the specific order that Bosch specifies and numbered per the manual.  Test 1 is throttle linkage adjustment and 2 is fuel volume.

3. Primary pressure test:
> hook up gauge and unplug electrical connector on WUR.
> CLOSE valve on downstream side of gauge
> turn on ignition (fuel pump will be off), then unplug fuel pump safety switch plug by the air intake (fuel pump should turn on).
> Read pressure.  Should be in the region of 5.2-5.8bar.  Need to look it up for your specific car.
> If you're not getting this then you need to adjust with the washers till you do.
> Now disconnect the fuel return line and plumb the fitting to a container.  Retest pressure.  It should be the same. If it's now lower then there's a restriction in the fuel return line.

4.  Static pressure test
> Open the gauge valve
> turn on ignition, unplug safety plug and run fuel pump for 30 sec to fill the accumulator
> turn off ignition and read static pressure.  It should be around 2.8bar and hold steady.  After 30 minutes it should still be around 2.5bar.
> Causes of pressure loss include faulty primary pressure regulator, cold start valve, fuel pump return valve, accumulator and FD internal leakage.  Can't hurt to replace the stuff you mentioned.

5.  Fuel injector leak test - static pressure needs to be good before doing this
> With ignition OFF and with good static pressure......   Open valve on gauge.
> press air sensor plate down SLOWLY and observe gauge.  It should move momentarily then return and hold steady.  If not then an injector is leaking

6.  cold start valve system test
Let's skip that for now and move on to . . . .

7. Cold control pressure test.  If stuff above is all OK then it's time to try to start the engine.
>  Open valve on gauge and unplug connector on WUR to keep it from heating up.
>  Start engine and note system pressure.  Pressure will depend on the WUR temp.  I'll post a table, but at 77deg F it should be about 1.3-1.7bar.
> Shut it off and do NOT warm up the engine.  Next test is aux air valve.  If car is hot then you can't test it. 

7.  Aux air valve test
Let's also skip for now and return to the WUR......

8.  Testing warm control pressure
> Open valve on gauge.  PLug in WUR connector.  Start engine and watch the gauge.  Pressure should start going up as WUR warms up.
> Pressure should get to 3.4-3.8bar.  This is with engine vacuum of 40mm Hg.  If engine vac is lower 0.1bar and vice versa.

So, that's most of Bosch's process up to the point you're at.  I suggest repeating it to see where you're at as each item relies on the previous item being correct.  Hope this helps!  Cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

raueda1

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #36 on: 10 September 2020, 08:29 AM »
Any progress?
-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 #37 on: 11 September 2020, 06:14 PM »
Of all those tests, I do know that when I shut the system off, and keep the gauge connected, I drop to zero within a minute. There is no residual pressure.

I just got the accumulator, check valve, and fuel filter in the mail last night. It rained today. So I'll be swapping tomorrow and going through your list carefully after that.

Thanks for the help! and I'm glad you enjoyed your trip  :D
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

Randys01

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #38 on: 11 September 2020, 09:22 PM »
The O ring in the piccie of the FPRV seems damaged.

secondslc

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #39 on: 12 September 2020, 06:51 AM »
Yep, I replaced it.
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 #40 on: 12 September 2020, 06:09 PM »
Alright, first things first. The Accumulator was original... or just old. It leaked fuel out the end, and looked grungy. I replaced it, along with the fuel filter. But Id didn't replace the check valve because the one that was sent to me had the wrong threads.... However, the Bosch fuel pump is new (was replaced before I got the car), and so I THINK the check valve is ok.

3. Primary pressure test was OK. In fact, with the accumulator replaced, the pressure went up slightly. I'll take it.

4. Static pressure test... well.. it never reaches anywhere near 2.8 bar with the valve open. This is where I'm stuck. However, I have a couple videos below.

As an aside, the system will bleed from ~3 bar to 2 bar over 30 minutes. This is improved from bleeding to zero within 30 seconds. So I did something right.

It seems with the valve open, I can't get to any sort of control pressure above .8bar.

Video one shows the valve closed and just system pressure. When I shut the fuel pump off, you hear two thunks, and the fuel pressure drops accordingly. It sounds like its the system pressure regulator bouncing. Could it be that I used the wrong o rings? I matched them up at Napa, but have the 100 dollar o ring kit from AutohausAZ I can throw in.
https://www.youtube.com/embed/p9i0Jg4tKDk

Video two shows the cold control pressure (gauge open). Pressure drops to zero pretty quickly after I shut the fuel pump off.
https://www.youtube.com/embed/Fd5c3pn0tTg

Video 3 shows the WUR doing it's thing
https://www.youtube.com/embed/Ew1jM09Naac
« Last Edit: 12 September 2020, 06:27 PM by secondslc »
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

John Moore Sr

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #41 on: 14 September 2020, 04:25 AM »
Hi SecondSLC. I notice in your photo of fuel pump assembly, it has the somewhat elusive and mysterious plumbing that Benz identifies as "Pressure Compensator". This is the small line that taps off the inlet connection of the filter, that connects to the "Compensator" doodad that is under the center of the mounting bracket, which then connects into the leak off line of the accumulator. The Mysterious part about is that there is no mention in the Benz service manual, at least none that I have found. It is my assumption that it is a type of Pressure Regulation Valve that prevents over pressure in the system, that dumps back into the suction side of the pump assembly. You related that your pressure does not achieve specification. It may be that this "Compensator" is compromised, and is recirculating partial pressure back to the pump inlet. This "Compensator" circuit was eliminated in later versions of fuel pump assemblies.  It's just a thought.  Hope this helps.  JM Sr.
1972 350SLC Euro Parts car, 1974 350SL Euro 3.5 V8 4 speed, 1978 450SLC Euro. 1979 6.9 #4630 1979 6.9 #7003 Parts Car, 1980 450SLC Parts car, 1985 380SL...and once had 1975 6.9 #142 until JuNIOR commandeered it.

secondslc

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #42 on: 14 September 2020, 04:40 AM »
Oh, I was assuming that system was a "don't touch it". But now wonder if it can somehow be removed?

As an aside, the car still wont start. But I fear I have wet spark plugs, or something else is going on. I need to re-visit the check valves in the vacuum system, as well as the lines themselves. I replaced all of them, and verified their accuracy, but I can still be wrong.
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

John Moore Sr

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #43 on: 14 September 2020, 04:53 AM »
I don't know if it can be eliminated without changing other components. The accumulator and possibly the pressure regulator at fuel distributor could be calibrated for it. Another part of its mystery, owing to not having a definite explanation of it's purpose other than one's own conjecture. Benz spent the funds to include it at first, but obviously eliminated it later in production.  Another part of the fun of older cars !   JM Sr
1972 350SLC Euro Parts car, 1974 350SL Euro 3.5 V8 4 speed, 1978 450SLC Euro. 1979 6.9 #4630 1979 6.9 #7003 Parts Car, 1980 450SLC Parts car, 1985 380SL...and once had 1975 6.9 #142 until JuNIOR commandeered it.

ptashek

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Re: Exact fuel pressure test hookup help needed for a California 6.9
« Reply #44 on: 14 September 2020, 08:51 AM »
Not sure if any of this is helpful, but this is how the two different assemblies are shown in Xentry.
The one with the two output-port accumulator is shown as being used up-to chassis #5693, the other one after #5693.
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