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Discussion Starter #1 (Edited)
Hi guys
Have a 1998 SL500 last year of the m119 engine that will crank , almost fire up , then immediately die out, as if it’s not getting fuel . After several tries cranking it and pumping the gas it will finally start. Then Once it starts it runs perfect , no stalling no nothing .

But getting it to initially start is very hard.

My mechanic has changed the maf, crank sensor , fuel pump and fuel filter , it has not fixed it . He put a pressure gauge on the fuel rail and says it’s too low at startup

Anyone familiar with this issue ? My mechanic thinks it’s the computer , Is there a fuel pump relay or cold start device or something else that affects fuel at startup but not once running ? Fuel pressure regulator ? I saw another thread talking about MAS , what’s that ? do the 1998 models have them ?
 

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I know the answer will send you it tomorrow it's very simple don't spend a lot of money. I have had the problem in a few r129 Mercedes all it requires is fuel treatment. There are two type one petroleum-based and one detergent based it takes a while running them through with high test gas but that's what does it. I promise you don't listen to other people it's simple and it works but it takes a little while and all of a sudden the car will run perfect I guarantee you. You have to use both types one at a time when I think of the name of the other one I will send it to you.
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96 SL600, 07 BMW M5 ,04 545i sold, 2013 C300 Sport (for wifey) 03 CL55 s/Chg sold,91 500SL, sold
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Sounds like a fuel pressure regulator issue. Especially since it's low on start up.On a cold engine the ECU injects more fuel as enrichment so as to keep the idle happy..Then tapers off as engine warms up. If you are having hot start issues it's possible the fuel pressure regulator isn't holding enough pressure, which it should for atleast 30mins upon last shut off.
I had a similar issue with with start up once before, car will start up then immediately will try to stall. ECU will compensate by raising the idle. I also noticed black smoke from tail pipe on start up. Apparently I was leaking fuel into the intake manifold upon engine shut off. The diaphragm in the pressure regulator had a rip in it.
Changed fuel pressure regulator, never had any more problems.
 

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'03 SL500, '03 SL55, '97 SL320, (2) '91 300SL, (2) '91 500SL, '00 S500 -- all for sale
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I saw another thread talking about MAS , what’s that ? do the 1998 models have them ?
Early 129s have a MAS module which controls the fuel pumps among its functions.

I like mb rider's suggestion that you replace the fuel pressure regulator.
 

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Discussion Starter #5
Turns out it was the fuel pump relay , green relay in the trunk by the battery . I thought these only fully worked or fully not , but apparently they can “kinda work” sometimes .
 

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'03 SL500, '03 SL55, '97 SL320, (2) '91 300SL, (2) '91 500SL, '00 S500 -- all for sale
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Turns out it was the fuel pump relay...
I wouldn't have thought this was possible. Had I been in your shoes I suppose I would have ended up with a car mothballed until some indefinite time far into the future.
 

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Discussion Starter #7
Yeah I also assumed a bad relay either would not work at all, resulting in the car never running - or else it would work fine .

Funny thing is this Happened to two of my mb , this 1998 sl500 and a 1995 c36 amg . my mechanic was fixing the c36 and said it might have been the same issue in the sl500
 
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