View Full Version : Testing high pressure fuel and inj.

05-03-2012, 06:44 AM
Hello, I work at a shop. I don't work on diesels much so not much exp. with them. A long time customer has a couple of these. We have the MB scanner so I can do alot with it.
It's a 03 sprinter. I have codes for water in the fuel, #1 glow plug and fuel pressure leakage. I don't have the numbers here, at work. Starts ok cold but when hot and sitting 5 min. long-long crank. I've checked basics, cam and crank, temp... I do see bubbles in the fuel lines. They are not coming from tank.
While looking through the scanner I found inj. test. It wants you to put graduated cyl. or metering tubes in the top of inj. We don't have these so I tried vinyl tubing. While running test a lot of bubbles and fuel poured out of #4 inj., there was some bubbles out of #3 while the others just slowly filled tube. If you let them sit this way #5 inj leaks the fuel down.
I don't fully understand what this all means but I think I have some or atleast one bad inj. Any help would be great. Is there a sticky or something that can help determine a bad inj?


05-03-2012, 08:08 AM
Hi. Sounds like #4 injector is bad. Don't know about #5 - if it was the same height of fuel as #1-3 then it might be OK.

Probably a good idea to change the fuel filter.
Maybe there is an o-ring on it that has gone bad allowing air into the fuel line.

You would have to follow the glow plug testing procedure to determine which glow plug or rule out the module.


05-10-2012, 03:22 AM
The boss decided to have a dealer take a look at it. We'll see what happens. I replaced all the o-rings and filter a month ago.

06-25-2012, 01:25 AM
Well the dealer said it was the high pressure pump, 1600. Still has the same problem. Took it back. Now they say it's the injectors.

06-25-2012, 03:39 AM
I hope they gave your $1600 back for misdiagnosis and changing a random part to see if it made the problem go away.

Has anyone done a leak-off test?

06-25-2012, 04:24 AM
As of right now, no the steeler as done nothing about the 1600. The case is going to the BAR they will mostlikely get it back.
I have only run the test stated, how do you do a leak-off test?

06-25-2012, 04:33 AM
You already did it with the vinyl tubing. At least #4 is bad, plus you have a leak of air into the fuel system probably somewhere around the filter. The low pressure pump sucks fuel from the tank, so any leak will let air in - like a hole in a straw.

06-25-2012, 05:08 AM
Oh, ok.
When I first got it, I replaced filter and all the o rings in it and the hose going to low pressure pump because the clip wasn't so good. It was fine for awhile but came back with air in lines and long crank.
I put vinyl tubing going to and from tank. No bubbles there. If I remember right the high pressure line dumped into the filter as well. I tripled check low pressure lines. Seemed like the bubbles were coming from high side which is why I was looking into inj.'s.
The long crank is much worse now.

06-25-2012, 02:11 PM
Reading your response, I'm no longer sure you actually did the injector leak-off test. Look at this link for the test:


06-26-2012, 02:05 AM
Ya, that's the test. Except with DAS it disables the engine for you.

06-26-2012, 03:10 AM
OK then, an injector that leaks off too much like your #4 will cause it to be hard to start because the high pressure pump has a hard time supplying enough pressure at cranking rpms.

06-26-2012, 05:20 AM
Great, what do you think about inj #5? We ran the same test on another Sprinter and #5 leaked down too? I would think that all the inj should do the same thing, slowly build up and then hold.

06-26-2012, 02:51 PM
Great, what do you think about inj #5? We ran the same test on another Sprinter and #5 leaked down too? I would think that all the inj should do the same thing, slowly build up and then hold.

Don't know exactly what you mean here, but it seems you're saying that when you let the fuel sit in the tubes AFTER the test, the fuel in the tube connected to #5 goes away? If yes, then I don't know if there is any significance to that observation. Again, if yes, maybe the tube wasn't fully seated on the top of the injector?

06-26-2012, 11:58 PM
first things first get the air leak fixed..If you didn't get a benz fuel filter get one, aftermarket does not work. The clear hose at the filter, the oring needs replaced with benz unit, if is aftermarket it will skin the oring on installation.
the water in fuel sensor oring does not like to be reused,I had similar problem, wrap sensor with teflon tape , install in filter.. restart motor... see if fuel line clear.. make sure all the white plastic fuel line locks are installed.. test fuel pressure 4000lb at cranking 4500-/+lbs @ idle 12000lbs on a quick rev you have a MB reader any codes... need more info for diag.. A good dealer is good to find, watch out for bad parts guys ,they will send you away with the wrong part to save themselves time from looking the right one. some sprinter parts are tough to look up . I had hard leak to find, it pissed me off so I took the low pressure pump loose and tilt it and put a 3/8 headed bolt in an air drill. spun to pump track where leak had happened,, split oring on top of filter,, wrong oring...I down loaded parts manual found right oring. I won!!!

06-27-2012, 02:31 AM
The boss has decided to do all the inj.
We get all our parts from Worldpac. OEM is all I know. I mostly work on gas, don't have a fuel gauge to test high side.