Fuel pressure drop when ignition off. - Mercedes-Benz Forum
 
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post #1 of 3 (permalink) Old 01-07-2019, 10:28 AM Thread Starter
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Date registered: Nov 2017
Vehicle: 2000 E55 Wagon
Location: Vancouver
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Fuel pressure drop when ignition off.

Hey all,

My wagon has been starting harder and harder of late, so I replaced the fuel filter with a Mahle, made in Germany. However if anything, it's gotten worse since the swap. It takes two full cycles of cranking before it erratically catches and starts running. Once it's running, it's fine and runs strong.

I bought a fuel pressure gauge to check what was up. Once you the car started the pump seems fine, hitting over fifty psi when running and not dropping when revving the engine. However when the car is shut off, pressure drops away very quickly, hitting 0 psi in something less than a minute. My understanding is that pressure is supposed to stay up. I'm presuming a lot of the reason for the poor starting is related to this. Some earlier research leads me to believe that the ecu is expecting there to be residual pressure and not running the pump constantly before starting, but I'm not sure if that's actually the case.

I've looked around the obvious areas for gasoline smell or puddles of fuel indicating an outside leak, but nothing seems apparent. My next step is to pull some plugs and look for fuel, in case of a leaky injector. Any other troubleshooting suggestions? I'm trying to avoid throwing parts at this.

Also, is there a good diagram/explanation of the fuel system online somewhere? I feel a less knowledgeable about how this is expected to work than I want to be.

Thanks,

Mark
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post #2 of 3 (permalink) Old 04-07-2019, 09:53 AM Thread Starter
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Location: Vancouver
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493 views and no replies...

Following up, for those in a similar predicament. It was the fuel pump. I can't remember exactly where I found the information, but I found a post of someone stating there's a check valve built into the pump that fails and allows fuel to flow back into the tank. I was worried it was hearsay, but in desperation, I ordered a Pierburg and, once I found the old pump's location (on wagons it's under a plastic shield in front of the fuel tank), I replaced it. It was a bit scary because I didn't want to clamp the super short and rather hardened line to the tank and risk it splitting, so I was worried about dousing myself and the floor beneath me with the entire contents of the tank. It turned out to be not so bad - I pulled the pump off and jammed an old fuel injector I had lying around into the line to plug it, and that worked great with very little of the expensive go-juice wasted.

Once reassembled, the car started right up. Monitoring the fuel pressure once shut off showed very little pressure drop, so thank you, whoever figured that out that there's a check valve in the fuel pump.

Mark
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post #3 of 3 (permalink) Old 04-12-2019, 05:55 PM
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Date registered: Nov 2015
Vehicle: w163 02 ML500 W210 97 E50Amg w202 99 C230K w124 - need more space, time to build again
Location: Canada
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Originally Posted by therealmark View Post
493 views and no replies...

Following up, for those in a similar predicament. It was the fuel pump. I can't remember exactly where I found the information, but I found a post of someone stating there's a check valve built into the pump that fails and allows fuel to flow back into the tank. I was worried it was hearsay, but in desperation, I ordered a Pierburg and, once I found the old pump's location (on wagons it's under a plastic shield in front of the fuel tank), I replaced it. It was a bit scary because I didn't want to clamp the super short and rather hardened line to the tank and risk it splitting, so I was worried about dousing myself and the floor beneath me with the entire contents of the tank. It turned out to be not so bad - I pulled the pump off and jammed an old fuel injector I had lying around into the line to plug it, and that worked great with very little of the expensive go-juice wasted.

Once reassembled, the car started right up. Monitoring the fuel pressure once shut off showed very little pressure drop, so thank you, whoever figured that out that there's a check valve in the fuel pump.

Mark
Hi Mark,
Welcome.

Sorry to hear this Mark,

The # post views may be related to the forum itself? Someone correct me if my reasoning is wrong.

Anyways, I myself dont frequent this forum - usually attend the w210 thread as there is more activity over there and
many of the related issues are also tied to your E55T.
It's a safe bet you can post your queries in that thread, and probably find related answers to your Qs in that forum too.
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