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#1
07-10-2016, 08:39 PM
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1978 6.9 Hot Start Troubles
I've been battling a hot start problem with my 78 6.9. When the car is cold, it starts right up, while when the car is warm I have to crank it for 10 seconds and then it sputters and pops to life and then runs well after a few seconds. I believe that pressure leaks out of the fuel system when the car rests, since the intake air plate looses all resistance a short time after turning the car off. So far, I've replaced the fuel accumulator, fuel pump and check valve, and all the injectors. I also tested the cold start injector, it operates like it should, only firing when the engine is cold and it doesn't leak. I'm really grasping at straws now. Could the fuel pressure damper be at fault? What else should I try?
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1987 300SDL 
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#2
07-10-2016, 09:00 PM
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Could be the auxiliary air valve stuck open. To test remove it and submerge in hot water from the kettle. It should close completely. If not it's buggered and bad news is a new one will make your eyes water with the price.
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#4
07-10-2016, 11:31 PM
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6F:
Contained in the fuel distributor (FD) is the fuel pressure regulator, and a co-axial component called a "push valve". The push valve is a check valve to retain pressure in the FD and injector lines when the pump(s) are off. Replacing the o-rings in the regulator/push valve usually cures the symptom you describe.
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#5
07-11-2016, 07:37 AM
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boy you are lucky to have one of those fine engines.Just yesterday I was reading up on them.I did not no Mercedes had a cast iron block V8.
How does this engine recieve spark? Points?I remember early days of electronic ignition,a modulator sometimes would fail when hot.
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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt
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#6
07-11-2016, 10:42 AM
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Thank you for the help Frank, I will dig into it!
Oldsinner, I love this behemoth V8, it's much more refined than its pushrod, carbureted contemporaries. It is a cast iron block with an aluminum alloy head. It has electronic ignition, no points on this machine.
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1987 300SDL
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#7
07-12-2016, 06:54 PM
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I pulled the push valve and found a small crack in the o-ring. After replacing the o-ring with an ill-fitting SAE spec one from my harbor freight o-kit, the car now holds enough pressure to easily restart after 15 minutes. I'll have to get an exact fit o-ring, but it looks like I found the culprit!
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1987 300SDL
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#8
07-21-2016, 04:23 AM
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Frank thank you , this will be my next port of call .FPR replacment mine as no replacment parts inside it...
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#9
08-19-2016, 01:17 PM
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Frank i was thinking of something else .The other regulator by the fuel distributor .Your talking about in side .I will try this .Thank you for your post
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