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Registered
1990 560SEC 334K miles as of 5/19, 1990 500SL (Euro Build) 67K miles as of 5/19
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126 Posts
Discussion Starter #1 (Edited)
Hello all, I have a '90 560SEC, testing with a cold engine. New pumps. Rebuilt fuel distributor Pressure gauge in cold start port on FD and service port. jumped FPR to active pumps. I get 92psi/6.4 bar. Then it drops to 30psi pressure, but I can hear what i thought was fuel leaking, only to hear the sound is coming from the FPR. The "vacuum" line on the back side of the FPR doesn't leak fuel and I can feel a diaphram when I pull and puff on it. Are these signs of it being kaputt? Total failure? Next steps? Tests? Otherwise it can't be revived to my knowledge.....new or used? Does this part fail from gas being in contact with rubber for many years? Its got a spring mechansim in it, but the vacuum diaphram seemed intact unless there is anotrher main one. So buying a low mileage used one is not any better? So go new?
Thanks!
 

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Registered
1990 420 SEC
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101 Posts
Could also be fuel accumulator. Some people suggest pinching return line to see if that helps maintaining pressure. If pinching helps pressure is leaking trough regulator. Maybe there is something on this on the manual in stickies.
 

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Registered
1990 560SEC 334K miles as of 5/19, 1990 500SL (Euro Build) 67K miles as of 5/19
Joined
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126 Posts
Discussion Starter #3 (Edited)
Good Idea! I origianlly thought you mentioned the FPR return line, not the accumulator, Nobody seems to know much abiut this aspect of FD, FPR, EHA. They know how to replace them, sure. but when it comes to diagnosing and adjusting them and using gauges you don't hear much. Even the talented Diy'ers say they send their stuff to a CIS guy in alabama to be calibrted and rebuilt. I just did put a clamp on the reutrn line but it didn't boost my pressure. But I thank you for the effort! What should the pressure be with a FPR pins jumped (relay out) not running with key in position 2?
 

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Registered
1990 560SEC 334K miles as of 5/19, 1990 500SL (Euro Build) 67K miles as of 5/19
Joined
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126 Posts
Discussion Starter #4
Update, fuel pressure dropped to 22 psi overnight. Stays there even when I clamp the fuel return line from the FD and jump the relay. Does this point to a bad fuel accumulator? Is it pumping fuel back into the tank thru the busted diaphram, thus no pressure? I will do as Heikkif mentions and clamp that accumulator line and check it.
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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4,000 Posts
It sounds to me like things are working properly. When you energize the fuel pumps, you get the 6.4 bar. When they're off, the pressure drops to just below the injector spraying pressure. It should drop to about 3.4 bar, according to the manual so you might be a little low but I don't know if that will cause any problems.

You asked what the pressure should be with the FPR pins jumped. Doesn't matter what the key position is, you bypass everything this way and you should see system pressure, 6.2-6.4 bar.
 

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Registered
1990 560SEC 334K miles as of 5/19, 1990 500SL (Euro Build) 67K miles as of 5/19
Joined
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126 Posts
Discussion Starter #6
Mike thanks. No the pressure peaked at 6.4 bar. I then stopped jumping them to power. Then energized the pumps again, the pressure had dropped down to 2.2 bar. I left it to investigate online why the pressure was not going back to the 6.4 bar when pumps are on. I left it for the day/night at 2.2 bar. When I checked it today fell to 1.6 bar. I clamped the return flow of gas to the tank via the FPR, energized the pump pressure remained at 1.6 bar. I have not tried clamping the return flow of gas to the tank from the Accumulator to see what effect that will have. Open to ideas.
Thanks!
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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4,000 Posts
Hmmm. I assume the car won't start now. It shouldn't be able to at 2.2 bar since that's below injector spring pressure. Now I'm wondering if your pumps are actually both working? Maybe one has quit on you?
 

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Registered
1990 560SEC 334K miles as of 5/19, 1990 500SL (Euro Build) 67K miles as of 5/19
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126 Posts
Discussion Starter #8
Hi Mike, found the answer from HD in another thread. But for your reference. "here is how it all worked out. I had original pumps on my car, I noticed corrision on one of the pumps connection and lound sounds from the. I cleaned the connection but the pumps failed. No pressure. I replaced both pumps. I jumpered the pumps. Pressure peaked ar 6.4 bar and went down from there. Still no pressure to speak of. I did as you suggested and attached a clear line to the return line of the FPR. Jumpered the pump for 5 seconds. Fuel continued to flow after the power ceased. I waited until the flow stopped, I cleared the line into the bucket and measured. 250cc, So I surmise that my car had been operating on failing fuel pumps. The fuel pressure that did get to the FPR was not diverted back to the tank, as it was low already so all of it went to running the engine (poorly i might add). So when I put new pumps on the newly rebuilt FD, the FPR got correct pressure and then failed catastrophically. However, only you FD gave me the test to isolate the FPR as the cause, and I have scoured the boards for a test like the 125cc test.. I discovred the potentiometer problem doing duty cycle static tests. I have installed the Bosch Poteniometer part I will update when the new Bosch FPR is installed. Thank you! "
 
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