leak down test question

diver

Diver
First to say This forum is a great resource and I am very thankful for all the givers
I have done the injector test a few times and am perplexed about one part of the process
To keep the engine from starting we are asked to disconnect the cam sensor
I always thought that the only way to shut off a diesel is to cut off the fuel or turn off the injectors, if that is the case how can we still get a reading on the test if the injectors are not firing or not getting fuel? My 2003 MB engine is giving me problems starting, it won't fire up without starter spray.
Was wondering if the cam sensor is the issue, ordered a new one to see if that is the problem
Thanks
 

NelsonSprinter

Former Nelson BC Sprinter
The injectors will be working with the cam sensor unplugged, all the other components will not be engaged in a start from the ECU, this is a computer controlled diesel that will not start until all sensors are known to be working within spec.

A 2003 will not start if the fuel pressure is too low, from a leak in an injector or rail pressure solenoid, and this test sees if it's an injector leaking back to the return. Starter fluid will speed up the engine to gain enough fuel pressure for a start with it's firing a few cylinders
 
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random

05 140" SHC 2500 Cargo
Hmm, this is a good question, really. Nelson I see what you are saying, but what is the actual mechanism that prohibits running? I mean, the HP pump and injectors have the fuel and psi to run, is it a question of improper injector timing then?
 

NelsonSprinter

Former Nelson BC Sprinter
I see, the injectors will be receiving HP fuel, but not getting the signal to release it into the cylinders when the camsensor is off
 

surlyoldbill

Well-known member
Diver, what are the conditions when it won't start? Cold in the morning? After sitting X-amount of time? When the engine is hot?
 

random

05 140" SHC 2500 Cargo
Diver seein as you have a 2003 I assume you are familiar with the air in the lines phenomenon? This could cause hard start and is easy enough to check for.
 

diver

Diver
Ok so the injectors are not ingauged but shouldn't they just bypass all the fuel and if some was leaking into the cylinder wouldn't you get some ignition? I assume the test works somehow or it would not show bad injectors. My sprinter will not start at all without the juice. I replaced 4 injectors about 5k ago along with all the glow plugs. The last digonistics showed a low pressure on the rail but we found a loose fitting on one injector and it ran great for a few months now it is doing the same thing again but no leaks this time
 

diver

Diver
Oh yeah I forgot to mention I put a small fuel pump and extra filter out of the fuel tank . I know this year doesn't have one but it seemed to help for the last year with the starting and the in line filter was much cheaper to replace on a regular schedule. I purged all the air from the filter and there are no bubbles in the clear lines
 
Random. Where did you hook up your fuel pressure gauge when you tested your low pressure pump ? Did you add in a tee with a port on it to hook up too?


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NelsonSprinter

Former Nelson BC Sprinter
Ok so the injectors are not ingauged but shouldn't they just bypass all the fuel and if some was leaking into the cylinder wouldn't you get some ignition? I assume the test works somehow or it would not show bad injectors. My sprinter will not start at all without the juice. I replaced 4 injectors about 5k ago along with all the glow plugs. The last digonistics showed a low pressure on the rail but we found a loose fitting on one injector and it ran great for a few months now it is doing the same thing again but no leaks this time
the injectors do not leak into the cylinders, it's a leak past the piezo piston assembly inside the injector to the fuel return line. Again the test measure the amount of fuel not getting thru the injector into the cylinder because of this leak from worn parts or clog
If you have 4 newer injectors, you probably have a leak in the fuel rail solenoid's seals at the rear of the fuel rail , a $12 kit is available from MB and Dodge https://sprinter-source.com/forums/showthread.php?t=972&highlight=solenoid
 
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220629

Well-known member
the injectors do not leak into the cylinders, it's a leak past the piezo piston assembly inside the injector to the fuel return line. Again the test measure the amount of fuel not getting thru the injector into the cylinder because of this leak from worn parts or clog
If you have 4 newer injectors, you probably have a leak in the fuel rail solenoid's seals at the rear of the fuel rail , a $12 kit is available from MB and Dodge https://sprinter-source.com/forums/showthread.php?t=972&highlight=solenoid
That's basically what I understand too.

I believe MB calls it a Leak Off Test. A Leak Down Test generally refers to cylinder pressures.

The high pressure pump produces pressure in the fuel rail whether the ECM sees the Camshaft Rotation signal or not.

:2cents: vic
 
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diver

Diver
I did not put a port for a gauge the pump I used developed only 4 psi so it was not even worth measuring but I was more interested in increasing the flow and seeing the condition of the fuel with a see thru filter.
Back to the injectors, why do they even bypass fuel why not just maintain pressure instead of bleeding it off does it need to keep the high pressure pump flowing fuel?
What if I measured voltage at the injector when I tried starting to even see if they are working at all?
Thanks for the info on the regulator looks like the way to go, will report on the progress
 

random

05 140" SHC 2500 Cargo
Random. Where did you hook up your fuel pressure gauge when you tested your low pressure pump ? Did you add in a tee with a port on it to hook up too?


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I never tested it. I don't have a pressure gauge plus all seemed well. They rarely fail.
Fwiw, diver may have a leaky fuel rail solenoid, which is usually fixed with new o rings. But we have the om647 with non serviceable solenoid.
 
I'm wondering what sensor is actually throwing the code for my fuel rail pressure ? I was watching the live data today and I saw the rail pressure go from 18,000 to 0.0 for a few and back to registering again. So the computer is seeing a fault with the rail pressure but what sensor? The one in the front of the rail or the back :/


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Of course this only happened when I first started it today. The other three key cycles were perfect. Crazy power, rail pressure at idle around 4500 psi. Getting on it around 21000 or so plus or minus


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diver

Diver
Ordered the o rings for the fuel rail regulator
Changed the cam sensor, did the leak off test ( all good ) and cleaned out the fuel filter now it starts up every time, Thanks everyone:thumbup:
 

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