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  #1  
Old 03-02-2002, 07:34 PM
scott schuett
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380SEL fuel supply problem

I have a 1983 380 SEL and it starts up great when the engine is cold. When the engine is warm and sits for 20 min. or longer the car will not start unless pressing on the accelerator. The fuel is not reaching the fuel injection system. Any ideas?? A faulty fuel pump??

Thanks,

Scott
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  #2  
Old 03-03-2002, 09:59 AM
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Fuel Pump

Scott,

This problem occurs particularly after the engine has been running due to the pressure relief valve located at the back of the pump inside. It is activated by a small spring and a metal cone which in time weakens and do not allow pressure to be relieved.

As the pump is driven by a permanently excited electric motor (when the engine is running) the pressure is reduced. After stopping, the fuel reservoir maintains pressure in the system to facilitate starting and with this pressure if the valve is stuck it develops an inner leak.

When this happens, if you tap the pump firmly it releases the pressure valve and the engine starts again. When the tapping affair becomes more frequent, it is time to replace the pump.

However, Mercedes have realized this and they provide a Check Valve which can be installed by replacing the existing hollow screw to the pipe connection to the pump, effectively providing a secondary pressure relief valve and avoiding having to buy a new pump. ( $7.50 for the Check Valve).
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Old 03-03-2002, 04:37 PM
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Hard to pick the real from the obtuse in that reply.

The problem is caused by the fuel boiling in the rail and injectors if the fuel pressure is lost while the engine is hot (vapor locking). Once that has happened the fuel pressure check valve plays no part. Its sole job is to maintain the system pressure after the pumps stops.

While an open check valve will cause the pressure leakdown the most common problem is a bad pressure accumulator which not only causes a leakdown, but without its accumulator action makes even small leaks major.

If tapping on the pump helps start the car the problem is the pump itself. Residual pressure IS NOT necessary to start the car, only to prevent fuel boiling. The car will very seldom have any residual pressure on cold starts and does fine.
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Old 03-03-2002, 05:11 PM
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"Whoever undertakes to set himself up as judge in the field of truth and knowledge is shipwrecked by the laughter of the Gods."
- Albert Einstein -
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Old 03-03-2002, 05:34 PM
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Truth may be up for discussion, but facts are facts. This is not figure skating. The way the system works is not debatable. If you think I am in error be sure and point it out.

I would much rather be wrong than leave a permanent record in the archives full of misinformation. Lets get it all out.
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  #6  
Old 04-30-2002, 08:27 AM
scott schuett
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found the problem

Steve you were right. It was a defective fuel accumulator that was causing the fuel pressure leakdown. This was a problem with the PO for about 6 years. Thanks for your help.

Scott
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