OM648 W211 - Does Crank but no Start - Diagnostic logic check

[Richteo]

Hi, sorry for the long post. I've had this 2003 W211 E320CDI for over 10 years now, and it currently has around 253k on the clock. It has been relegated to being a spare car but is still in regular use a few times a week.

Yesterday my wife had been driving it with multiple stops and restarts with no issue until later in the day when it refused to restart. It cranked but no joy, not even a splutter. I had an icarsoft in the boot and looking for fault codes nothing was present (expecting crank sensor maybe). I called recovery and it started almost immediately with a squirt of easy start. The mechanic suggested it could be a clogged fuel filter. Turning it off at that point and trying to restart resulted in another no-start. A squirt of easy start and off it went again. At this point I drove it the 8 miles or so home without issue. There is about 30 litres in the tank, corroborated by fuel gauge, icarsoft values and based on mileage since last fuel up.

Today I've tried to get to the bottom of it.

1. Using the icarsoft in the 'cranks won't start' section with the ignition on there was zero bar pressure.
2. On cranking it went up to 40-50 bar, not enough to allow a start condition.
3. I bought and fitted a new filter (and some easy start), and while changing it turned on the ignition to confirm operation of the tank lift pump. I have no means of measuring the pressure but got a healthy soaking in diesel.
4. Plumbed in the new filter and no change even after a number of purge cycles. 50bar or so on cranking, no start.
5. I squirted some easy start into the intake and as the night before it fired into life. icarsoft shows 350-400 bar which I believe is normal.
6. I then ran a leak off test, clamping off the return line that goes back to the rail. Just cranking resulted in some diesel flowing from injector 1.
7. I repeated the leak off test following a squirt of easy start and when running there was increased flow on injector 1, and a very small amount on injector 6. See pic below.

Based on the above I think I have a faulty No. 1 injector that is leaking off causing an inability to get to the required starting pressure. Is this logic sound or is there anything else I should check?

The injectors in the car are around 50k old. Proper Bosch centre refurbs. What caused me to replace them last time was missing/uneven idle, with 2 or 3 failing a STAR test. When running the idle seems fine, plenty of power, no smoke etc. The other day on cold start it did sound a bit rattly. My initial thought was timing chain or tensioner finally starting to wear but now I wonder if the injector had dumped fuel into that cylinder.

Any help or thoughts welcome. Thanks.

 

[mersum1es]

How fast that leak came? Problem is that ALL injectors should leak a bit. Leak is coming from pilot stage, and when its actuating as is should, it is always delivering some fuel to tank line. You said you have about 30L fuel in tank. How about adding 20L more? You may have saddle tank syndrome and your low rail pressure is due to fuel starvation in feeding side of the tank.

 

[Richteo]

Thanks for the response. It was within 30seconds of running, probably less.

Yes, I will try that tomorrow and it was something I'd read about. I had discounted it because of the very healthy flow to the fuel filter with the hose disconnected. It sprayed all over the engine bay! I had assumed if the saddle pump issue was present I would see nothing.

 

[mersum1es]

Ok, if your primer pump can feed lot of air free fuel, it is probably not saddle tank... but your #1 injector is definitely not 100% health, leak limit is 20ml/min cranking or running.

 

[Richteo]

thanks again, I will fuel it up anyway but a bit wary of it not restarting again at the fuel station. Once the HP pump gets the command to deliver fuel does the saddle pump issue somehow become irrelevant? I can't quite get my head around that unless enough of a depression is created with the engine running via the HP pump to suck from one side of the tank to the other.

 

[mersum1es]

Not exactly. But on high flow demands, it might occasionally get some air drawn in if balancing is not at fully ok. Delivery pump is engaged when ignition is turned on. It feeds HP pump. HP pump is mechanically connected to engine camshaft and is pump all the time, at full stroke. Fuel quantity valve is restrictin fuel flow in HP pump and controlling fuel flow. Pressure is build according flow and balance how much injector are delivering fuel to cylinders, so FQV can be used controlling the pressure. Rail pressure relief valve is controlling the pressure at start and certain situations, but usually it is just for over pressure protection. Tank balancing system is passive venturi type device, which works by returning fuel flow, pulling fuel from passive side to feeding side. So if HP pump cannot return enough flow, venturi is not working effectively and eventually feeding side is dry but other side half full. Of course balancing system can be clogged by contamination. Like said saddle tank syndrome is easily ruled out by adding some fuel.

 

[Richteo]

thanks, great info. Very helpful.

 

[supernoodle]

There shouldn't be any leak off if the injector until the injectors are being energised. I wouldn't expect them to be energised until rail pressure reaches 150bar (during crank they are operated at lower pressure than normal in the expectation the rail pressure is rapidly rising).

As mentioned already, there should be leak off from all injectors when they are being energised. The leak off is from the control valve. The needle is pressure balanced, ie has fuel at rail pressure acting on both sides. Energising the injector just causes solenoid (or piezo via hydraulic amplifier) to open a control valve allowing the fuel behind the needle to spill, lowering the pressure behind the needle, the higher pressure in front pushes the needle up, allowing the fuel out of the nozzle. So every injection event, ie pilot, main, post there will be a leak off of the control volume.

I'm guessing the easy start here is combusting, raising engine speed and hence pump speed allowing a higher fuel pressure to be reached such that the min injection is reached.

During a crank I expect the quantity control valve and pressure control valve to assume open loop control positions, ie not try and control closed loop.
Once engine is running they would enter closed loop control.
When fuel temp is low it will operate in pressure control valve mode, ie quantity control valve fully open. This pressurising of fuel and spilling via pressure control valve on rail aids warm up.
When fuel temp is warm pressure control valve will normally be closed and pressure regulated by quantity control valve.

 

[Richteo]

I've topped up the tank and it's exactly the same so I'll order an injector and report back once I've fitted it.

 

[Richteo]

I replaced the injector earlier today and all seems back to normal, including a cold start after 8 hours. Thanks to all who contributed.

 

[Uncle Benz]

Nice one! I commented on your same thread on MBClub, but the forum here has thwarted me all week. When I try to post it just times out

 

[Uncle Benz]

Holy crap! It’s worked this time!