virgisr

My car has got 642 engine CDI, 2007, gerabos 7g tronic 722.9

When the car is moving, and i want to speed up, i start accelerating and it does not downshift properly, i have to kick it harder, a couple of times, then it downshifts.
Also, on cold start it cranks the engine for too long before the engine starts up. The fuel pressure builds up i guess. The fuel system does not maintain the residual pressure. Not sure where that prassure is failing to be maintained.
I tried to switch ignition on and leave it for 30 seconds, the pump at the tank prebuilds the pressure in that time. Then the cold start is ok, no long cranking before it starts up.

I have read some old thread in this forum, that problem with the downshift and long cranking are related. Possibly cause of the fuel pressure is not maintained properly?

Any advises are appreciated.

Maj. Dundee

Please UPDATE YOUR PROFILE in USER CP to include the year and model of your ML.

virgisr

I did not include ML model, cause my car is nor ML, it is R class, but the engine and the gearbox is shared units. I am expecting the users of ML mercs possibly faced similar problems and could advise me what could be the reason?

My merc is R320 CDI, OM642, gear box 722.9, year 2007.

Sorry to post my thread in the ML section, i hope to have more chances to receive an answer to my problem.

Best regards,
V.R.

Maj. Dundee

I would check the main fuel filter at the engine if it has the threaded connection. If not, it must be replaced.

virgisr

On my car filter there is no threaded connection. I did replace the filter, thought it could be clogged. It did not make any difference. So that can be excluded.

Thank you for your post and the attachment. Appreciated.

In addition to my post at the top i will add that the scan does not give any errors, which does not help..

Maj. Dundee

Then check the fuel pressure at the fuel rail.

virgisr

Some time ago, when that problem appeared, I did check the fuel pressure of the fuel low pressure line. When testing, with engine running idle, the pressure was building up up to 4.5 bar, which as computer said, was ok. If i remember right when i stop the engine, the pressure did go down quite quickly. Doesn't it have to be maintained at same or similar level?
If it is not maintaining the pressure and it should, will it be the pump at the fuel tank?

Thanks for your reply.

Maj. Dundee

Some time ago has nothing to do with your present problem.

virgisr

I ment i performed testing of the pressure when this present problem appeared. The present problem appeared some time ago.

virgisr

Are you suggesting to test only the rail pressure (high pressure line) only? Thanks for your advises

Maj. Dundee

Check both.

virgisr

Is the the pressure drop in low pressure line after the engine is stopped is normal though?

Maj. Dundee

Yes.

virgisr

so, i checked the high fuel pressure line pressure, when engine running idle, then when engine stops, how much it drops down.
I used the OBD bluetooth adapter which gets plugged into the port in the car, gives live data on the app on my phone. I would think it is reliable enough (unless the pressure sensor would be faulty?).

The logged data below for the high pressure line in the rail:

- when the engine is running idle - 284 bar

- when engine stopped - the pressure goes down from 284 bar to 6 bar in matter of seconds

- ignition switched on (the fuel pump in the fuel tank starts building the pressure in low pressure line for 30 seconds) - the pressure in the rail goes up to 11 bar

- ignition on, past these 30 seconds the pump stops building pressure - the pressure in the rail goes down to 6 bar again


I did not check the pressure when the car stays off for longer yet. Will do that tomorrow morning. I am guessing the rail loses pressure to barely anything (will see tomorrow morning), cause the engine is cranking for a good while till it starts. With the figures noted above the engine starts up ok.

To my understanding, the pressure in the rail should be maintained to certain level for the car to be started and till the pump builds the pressure in the low fuel line?

What keeps the pressure in the high pressure line? Fuel pump itself? Pressure control valve (Y74)? Quantity control valve (Y94)?

If that would be any of the injectors faulty, if the pressure drops that quick, the car would be smoking? But when the car starts up, there is no smoke.

Maj. Dundee

If the fuel pressure drops quickly after shutdown, then it is usually the fuel pump. If it drops at a slower pace, then usually it is the injector/s.

You make the call as I am not a diesel enthusiast.

virgisr

Hi, thanks for your reply. I would think its the pump. i will keep digging, want to make sure first before replacing it.

Maj. Dundee

I agree with keep digging.

Do you have the procedures to remove the pump?

virgisr

Yes i do have access to it. Thanks.

S-Vibes

it been a while. I hope this is resolved satisfactorily and you’re able to clue us in on what you did to solve your issue.

virgisr

nope. The problem is still there. Replaced the pump, no change. Replaced LH rail with pressure regulator, no change. The injector leak off test showed me two of them are up to 3 times over than the rest of the injectors. There is a plan to replace these. But still did not get to that. Some time this year probably..