1998 ML320 150K miles. Hot or cold the engine takes 10 to 15 seconds of cranking before it will start.
When I bleed the pressure from the fuel rail, there is a combination of vapor and gas.
Each time I bleed the gas vapor out of the fuel rail, the truck will start up normally several times before it again becomes hard to start.
The truck runs fine after it starts.
The check engine light is not on.
Fuel pressure with engine running 55 lbs.
When I turn the engine off, the fuel pressure will bleed down to near zero in 4 to 5 min.
Occasionally when the key is turned on, the fuel pressure will go up to 55 lbs. instantly, but most of the time the pressure will not come up until I start cranking the engine and even then it might not happen for several seconds.
From reading the forum I am under the impression that the fuel pump does not come on until the starter is engaged.

From reading the forum, I tried the following test which were successful:
When I turn the key to on position the red LED DAS light flashed.
Also when I turn the key to the on position and wait 5 seconds before turning to the start position the starter does engage.

I have tried three different keys.
I changed out the fuel pump relay but that did not change anything.
The dealer installed a new fuel pump and filter, and the problem still persist.
I do not think it is normal for the fuel pressure to bleed down after turning off the engine and since there is a new check valve in the fuel pump/filter change, my guess is that there is a fuel injector leak.
I have never pulled the injectors from this truck, so any diagnostic suggestions will be helpful.
I will appreciate anyone's thoughts about my problem.

 

YOU MOST LIKELY HAVE LEAKING FUEL INJECTORS. 4 E10 BOLTS AND PRY THE RAIL AND INJECTORS OUT OF THE INTAKE. MAKE SURE YOU HAVE PRESSURE AND INSPECT INJECTORS FOR LEAKS.

 

Today I disconnected fuel line to the fuel rail and pressured it up to 55lbs. with air and it held.
Next I pressured up the fuel line back to the pump and it help the first time but the next time it bled down.
This hard start problem has been going on for over a year. I mentioned that I had the fuel pump and filter replaced, which was about a year ago after this problem began. So last week when I saw the pressure bleeding off I assumed it was a fuel injector(s). I had someone suggest I clamp off various fuel lines at the tank so I can see if it is the filter or pump. Can any one provide a diagram of the filter/fuel tank lines so I can figure this out. I believe there is a purge valve line
that ties into the filter or tank. I would not think that a bad purge valve would account for the loss of pressure???? Where is the back pressure valve located, fuel filter/regulator or in the pump????

 

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sorry i should have read your complete post before i responded cause i said to replace the fuel filter. I see that you had that done already.

 

It was about a year ago that the fuel pump and filter wererepleaced so after my recent pressure testing and based onyou telling me that the regulator is in the filter I amguessing that the filter needs tobe changed again.
When you said fuel sending unit did you mean the fuel pump?