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 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.

There are only 3 places for the fuel to bleed off.

Fuel pump, fuel filter(most likely), & the fuel rail(injectors).

The way that I would check for bleed-off is to run the pump & then clamp-off fuel lines 1 at a time untill I found the problem.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

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). It was suggested, 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????

Thank You

Purge has nothing to do with the circuit.

ONLY the fuel filter & the pump since you have eliminated the fuel rail.
Again the filter is the most common problem.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

I realise I may be way off the mark here, but I had a similar hard start problem some time ago and it was fixed by simply replacing a couple of small rubber o-rings in the end of the fuel lines that go into and out of the fuel pump.
These o-rings perish over time and let in small amounts of air which takes time for the fuel pump to overcome. Once engine is started everything runs fine, it just took quite a while to start.
Only difference is mine is diesel ML270.
Perhaps they share similar o-rings in fuel lines to cause similar problem.
hope that is of some use.
regards

Fixit, were you able to find the problem after all? I already changed the filter and cleaned and checked the pump. Will change the 3 lines between the pump and filter next. Need advise...
Thanks

I have the same issue with the vehicle starting, however mine is intermitant.

Diagnosis;
Car will stat sometimes ok, sometimes not or sumtimes run then stall.
Every morning for the last week I have stuck my head under the bonnet to check things before and after starting. On the days it wont start there is a bubble of air in the diesel fuel line just after what i believe is the fuel pump (but im uinsure what is is really, its jsut a little black block at the the top front of the engine that is directly after the water filter and directly before the injectors by way of fuel line.)
On days it starts fine there is no air bubble at all.
Yes these lines are connected by a simple O ring and clip, however I have to wait a week to get parts from mercedies as the mechanic does not want to replace the O ring without the complete part as the clip is so brittle from the engine heat.

About 6 month ago I spend all day trying to find the root of the problem. I have installed a new filter about a year ago without fixing a pressure loss in the fuel rail. I then thought that i need to also change the fuel hoses connecting the filter to the pump.
So 6 month ago i changed the fuel hoses as well. No luck, the car would still need to turn for 10 seconds and then it would start on second try which was kinda embarrassing at times. After mingling with the pressure miter and sniffing a lot i finally decided to add an auxiliary switch on the dashboard that would allow me to manually turn on the pump to pressurize the rail before starting the engine. The whole install took about 30 minutes and it had been working fine for 6 month.
I just dont want to spend another 100 some bucks on a new filter with pressure regulator which could be same piece of junk as the previous one.

Just to add to the info here, there is a check valve inside of the fuel pump itself that can and does fail.

__________________
Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles