virgisr

Hi everyone,

i was testing the pressure of the fuel rail on my car, engine OM642 (car mercedes R320CDI, 2007) and found that the high pressure - 284 bar on engine idle drops to 6 bar when the engine is shut down. And the pressure drops within few seconds.

The question is: is the pressure drop in the fuel rail as i described above is normal? I would doubt that.
What would be the first guess, why it drops like this? Fuel pump is my guess, i had one more opinion from one member it would be the pump.

I want more opinions regarding this, knowing this is diesel section, there will definitely be some diesel enthusiasts, who know the answer to my question.

The reason i am looking into that is when cold, engine cranks for too long for 10 seconds or so. But if i switch ignition on and wait till the pump in the tank builds pressure for 30 seconds, the engine starts with no problem, no long cranking. The engine runs ok when started and no smoke.
To my understanding the high pressure line loses pressure, possibly through the high pressure pump? back to the tank?

Any advise and opinion is appreciated.

kajtek1

This is not a subject that MB would educate you about, but from my experience the pressure is bleed by electric valve.
I was observing rail pressure on car with ECO stops and during ECO stop the rail hold the pressure (around 5000 psi from my memory).
When you shut the ignition off, the pressure drops like a rock.

virgisr

I see. Thank you for your reply. I wonder what it drops to?

kajtek1

Don't recall what residual pressure was, but on system operating at 5000-20,000 psi anything below 100 is technically zero.

virgisr

What would be the residual pressure in the low pressure fuel line when the car is shut down? When i was checking my car low fuel pressure line, the pressure on idle engine was approx 5 bar (73 psi), when switched off, the pressure was falling to barely anything. Should it maintain some residual pressure?

kajtek1

Don't think your engine would idle at 5 bars.
All the common rail engines we have in US idle at 5000 psi = 330 bars.
And again, the pressure is bleed with engine shut down.

virgisr

no, in this message i am talking about the low pressure line, the line before the high pressure pump. the pressure of 5 bar was in the low pressure line.
In high pressure line the pressure on idle was 284 bar (4110 psi). is that not high enough then? i would assume in US or UK or anywhere else the pressure would have to be to the same standard?

I am trying to figure out why my car when cold is cranking too long before it starts up, was thinking the low pressure pump in the tank is not building enough pressure, but when performed guided test through SDS, the computer said it was within specs. The high pressure as mentioned above.
There must be some problem with pressure build up or maintaining it, cause when i leave ignition for longer and then startup, it starts as it supposed to.

kajtek1

OK, now I got it.
The tank pump is priming pump only, so 5 bars is good pressure.
But the 284 bars rail pressure looks low to me. How high it goes when you rev the engine?
Any codes? Can you test glow plugs?

virgisr

I revved up to maybe 1500rpm, it was showing approx 462 bar (6700 psi)

mersum1es

do a leak back test for injectors (cold engine of course)

virgisr

Thanks. I will do that.

gone_walkabouts

my gl350 does the same the pressure at idle is same as your which is in range but the pressure drops to seven when turned off

i take it time starting particularly when hot - I suspect it needs to build up pressure on the common rail -

a friend did the same test on his Ml350 and the pressure was maintained on the common rail when the motor stopped

virgisr

i did not get to testing my car's leak back test yet. Ordered the kit for it, took a while for it to come, then when it came it was broken, i had to reorder it, so it extended to a long period of time. I have the kit now, still planning to do that. It is good to know the pressure maintained after the engine shut off, thanks for your message. Cause i can not find the explanation anywhere how exactly it suppose to work.

The reason i am digging into that is when my car is cold, stationary for a day, when starting it, it revs for too long before it starts. But if i switch ignition on for 30 seconds, let the pump in the tank prime the fuel to the engine, it starts in few cranks as it supposed to. So i am guessing the pressure is not maintained in the system after shut off. So checked the low line, seems the pressures are ok and apparently it is ok if the pressure in the low line drops. The high pressure line, seems the pressures are not out of range, but the residual pressure drops which is on the question.
If the pressure has to be maintained to certain levels after shut off, and it is not, what are the possible causes?

Thanks anyone for your insights.

gone_walkabouts

Very similar situation- my OM 642 - if I let the tank pump prime a couple of time I generally get a much better result - a start in four turns instead of 8 or 20 -
sometine if it is hot and has been left for a few minutes it might fail to start

These symptoms gel with this

http://www.mercedes.gen.in/sprinter-...poor-starting/

virgisr

Mine is playing slightly differently, when it is warm, there are no starting problems at all.

Thanks for the link. My high pressure line idle pressure apparently is too low then. So if i perform the leak off test and find out the injectors are ok, there will be pressure regulator to be checked/replaced. I will get on it.

virgisr

So, performed the injectors leak off test. Two of them were showing excessive amount of fuel.

Below is the test result. I was running the engine cold, for approx one minute:

cylinder 1 - 3.190ml
cylinder 2 - 3.218ml
cylinder 3 - 3.429ml
cylinder 4 - 6.506ml
cylinder 5 - 3.274ml
cylinder 6 - 7.954ml

So looking at the result, the 4th and 6th injectors are faulty? I can not find the nominal leak off volumes, but if i am right, if the difference is greater than 20%? the excessively leaking ones have to be replaced?

dhurley

Correct. Replace 4 and 6.

GoodByeHonda

That's one accurate leak test kit! Down to 1000th of a mL. Mind sharing the source?
Is there an inexpensive source of injectors (new or rebuilt) you guys are aware of for OM642?
Thanks

virgisr

all i have done is attached the silicone pipes to the leak off ends on the injectors, raised them up, let the engine run for a minute. Marked the distance on the pipes on each of them, where it is filled up with fuel. Then measured the inner diameter of the pipe, used it on some website where it calculates the pipe volume by the inner diameter. Using google search it wont be hard to find one. I did not save the link. And i got the results.

I asked around a few rebuilders of them injectors. Cause they have that pjezoelectric element, it is complicated to be rebuilding it and according to them, bosch is rebuilding them themselves. You can buy refurbished one for 90-180GBP or a new one for 220GBP, that is with part exchange of the old one.

wzbhnn

Sorry for digging up this thread but I am experiencing the same issue: long crank after sitting. But if I let LP Pump prime in ignition before starting. It would start right up.
Did you solve your problem by replacing the two leaky injectors?

virgisr

no worries. No, did not fix it, id does not get any worse, so I am just priming it same way before first start on the day.

wzbhnn

Thanks, guess I am not at the point of giving up the search yet lol.
I will do the leak-off test once the kit arrives. I removed the fuel return line and let it crank and idle to see if the fuel coming out of the 6 holes was visually different. So far they look the same.
However the return line itself is pouring diesel out of all six ports. Is this normal?

virgisr

probably you need to use the kit to see if there is any difference. I dont remember return collector to be pouring diesel. Not sure, dont want to mislead you.

Good luck with it. Keep us posted.

wzbhnn

Will do! In the meantime I also ordered a new fuel filter and will install it as I wait for the kit.
I read somewhere there is a one-way valve inside the fuel filter. So if that fails low pressure will bleed off from the filter when the engine is off.
I have never heard of any filter with check valve built in, but also I couldn't find any information online about fuel regulating valve on OM642 so maybe it's a thing.

virgisr

Thinking about it now, i remember after i replaced my fuel filter (with this hard start issue already present) it felt like it made a difference. I got brave not to be waiting for it to prime all the way. Just probably half way, time wise. It felt like it improved, but after some time i experienced same thing again, so started priming it fully again. Filter was bought from merc dealership. So if that has check valve, on my engine there is problem somewhere else. Hopefully on yours filter will do the job. Also, when i was looking into all that, i was finding information on HP fuel pump, which if i remember right, has check valves inside of it. So replaced HP pump at some point. There was no no change at all, so pump was out of equation.

I was considering adding a check valve i a line manually, even bought one. But since priming prior starting made difference, i have left all alone.
I might even get back to it and try adding this check valve, since you reminded me of it