nathjb

Gents
I have an extremly bizarre situation occuring with a 2006 facelift s500.
This vehicle suffers from an intermittant non-start and i do mean very intermittant, the reason for the non-start has been found, the cause however has not yet been located, the cylinders are filling with fuel causing bore wash and on one ocassion hydraulic lock. It would seem that the fuel pump is being activated at times when it shouldn't be, but also the injectors must be opening as well for the cylinders to fill with fuel, the ecu has been sent off with no faults found, whether i take this as an absolute proof thats its ok is open to suggestion. But going on the basis that the ecu is ok what else could be turning on the pump and opening the injectors. Other suggestions have been crash sensors that would command the injectors to open in the even of a crash to rid the system of fuel pressure, but this doesn't fit in with the fuel pump running at times. any ideas or suggestions would be great. Thanks in advance

Nickthegreek

is there spark? Also is there a crank signal. The crankshaft position sensors are notorious on mercedes for intermittently going. You need spark to ignite and a crank signal to fire a spark. Also is the pump going with the car parked and the key not commanding a start or is it just when you try and start the car?

greglig

Hi seems no one wants to go near this thread.
Yes very odd but i have seen it on another car before. An older BMW doing something very similar but it was all the time.

It may be that you engine computer is getting stuck in some sort of odd reset loop.
I have done testing of ECU's from about 1997 so have seen just about everything.
Normally ECU designers make sure that such a situation does not occur but on some rare occasion it can happen.

Even though you have had the unit tested I would be skeptical. If someone asked me If I could test for that I would say that I may not be able to pick up that sort of fault on the bench. What cannot be recreated off the car is normal electrical noise that is present when it is on the vehicle.

Normally what the dealer would do is install a test ECU (or New ECU ouch$$) but this is not easy as it's VIN coded.

I will try and give you some direction If I can. Can you give me more details on exactly when the fault occurs or if there is no pattern at all?

Also is there any other odd things happening that you may think are unrelated?

Cheers.

alx

you mentioned that the computer does not report any errors.

what does that mean? have you read the car with star/das? if you are dumping a lot of fuel that means the control over the injectors suffers.

this can happen due to one of two reasons:

1. bad ecu pulsing injectors with the engine off (crank sensor when fails fails in a way that "shows" engine as not running so it is not a sensor, but the ecu)

2. sticking injector - they do happen. the ecu pulses the bad injector and it appears alright while in reality it is stuck open and filling the cylinder with fuel.

#1 can be tricky to prove unless your ecu throws strange errors you can log/ catch

#2 can be easier to troubleshoot ...

what i would do is i would reset all errors via star/ das. then i will run couple bottles of chevron techron fuel additive and do some aggressive driving. the next time the car fails i would read it and see if anything popped up in the logs as well as observe which cylinders get flooded. if a single one is flooded - i would suspect an injector. if all are flooded - ecu.

greglig

One thing I will Add to alx's recommendations is if the engine computer is at fault it's very likely that other modules on the car will complain and set error codes for a "no communication" or no "CAN bus message received" from the engine computer.
After all the codes are cleared and it happens again check all the modules in the vehicle to see what else comes up and not just the engine computer.

BTW is you car a W221 or W220? As you say facelift but the Facelift for the W221 was in 09.
The reason I ask is the older style computers are I think more reliable than the newer system with the ECU on top of the engine - not the best design idea from Mercedes. I've already fixed one ecu from a CLS500 with a cooling fan fault.

Slightly off topic laugh: I worked on an old e36 3 series BMW years ago that had a no start fault.
Had spark and fuel as normal but not even a cough. Then one of the guys pulled the fuel pump relay and the thing started and run almost normally for maby 20 seconds. WTF???
Put the relay back and nothing. Pulled the relay and the same thing happened again, run for a short while then stopped.
After lots of hair pulling and thinking turned out that the previous workshop that sent the car to us had been cranking it so much (with another no starting problem that was repaired) that they have filled half the sump with fuel and the engine was running off the fuel vapor in the sump so with the injectors also injecting fuel the mixture was too rich to ignite.
That was fun job.

That also reminds me when all is sorted with you car it would not be a bad Idea to change the oil if it has been going on for quite a while as I am quite sure that an excess of fuel in the oil could lead to other engine damage.
Other member please correct me on this if I'm wrong.

Cheers.

Nickthegreek

good point i see what you mean, a bad ecu affecting injectors would most likely be a culprit for all cylinders. The fact that its intermittent will make it that much more difficult.