Water in Fuel sensor - 07 ML320 CDI - PeachParts Mercedes-Benz Forum

dmcginn2 · #1 02-24-2015, 03:43 PM

Ok, I am wits end with this car.

07 ML320 CDI, had a small fuel leak on fuel filter. Replaced hose clamps, the one small fuel line, and O'rings for water in fuel sensor (and the filter while I went that far).

Leak went away, couple months later, at oil change time, I notice fuel on filter again, so I replaced the other fuel line. It is now evidently coming from the water in fuel sensor.

Leaves spot on driveway kind of thing, not gushing, more like seeping out.

Any thoughts on what I do next? (besides a match...) Maybe replace the sensor itself? Or maybe there is a way delete the sensor or to jump wires at the plug, use the other fuel filter with no hole for a sensor in it...

jay_bob · #2 02-24-2015, 08:19 PM

Water in fuel sensor on a CDI is critical.

Any water that gets past the filter and into the high pressure pump and injectors is bad news. It will destroy the pump and injectors.

dmcginn2 · #3 02-24-2015, 08:26 PM

Quote:

Originally Posted by jay_bob

Water in fuel sensor on a CDI is critical.

Any water that gets past the filter and into the high pressure pump and injectors is bad news. It will destroy the pump and injectors.

I wondered about that, this has over 120k miles on it and never has had any water in the system. So I kind of figured it as a nice to have...and it wouldn't be missed.

Flip side same token, the 83 300SD doesn't have one. Is it more crucial to common rail direct injected?

jay_bob · #4 02-24-2015, 11:29 PM

Quote:

Originally Posted by dmcginn2

Is it more crucial to common rail direct injected?

Big time.

Injection pressure on IDI is around 2000 PSI.
CDI rail pressure is 10 times that. Yes 20k to 30k PSI.

When water passes through your (very $$$) high pressure pump, the pump surfaces, that are intending to be lubricated by the diesel fuel passing by, are not getting lubricated. An IDI pump has some pretty tight tolerances. CDI high pressure pump is orders of magnitude more tight.

When water goes through the injector and approaches the tip, it flashes to steam due to the high temperature at the nozzle region. Water to steam is a 1:1600 volumetric expansion. That steam has to go somewhere, up to and including blowing the nozzle apart. This is much more likely to happen in a CDI nozzle owing to its operating parameters and the way it is constructed.

Water is vital to most life forms. It is lethal in the fuel supply of a Diesel engine.

dmcginn2 · #5 02-25-2015, 07:04 AM

Yeah, I know water is bad in diesel, just wasn't sure why it was so different from the older vehicles.

But don't some of the sprinter's and some other modern vehicles operate without a water in fuel sensor?

Other than that all I have to say is I have never liked the set up of this vehicle.

Maybe what I do is get an old fashioned diesel fuel water separator, and put it in line with another type filter, or the MB filter without the sensor in it. I think for a slightly higher up front cost it would be cheaper on filters in the long run. And relocate the filter for better access/maintenance. IF I do that, the question becomes how do I keep that sensor from causing a fault if it's not used?

This car has been ridiculous with the electronics, sensors and who's bright idea was it to put the air filters in there like that along with locating the fuel filter so you have to mess around with that intake duct? And dont' get me started on the plastic intake duct...

My 83 300SD has had far fewer quality issues at 295,000 miles than this car has had within it's 120,000.

pawoSD · #6 02-25-2015, 09:14 AM

The sprinters also have CDI engines, very likely set up just like the engine in your ML.

barry12345 · #7 02-25-2015, 12:19 PM

Aftermarket water in fuel filters are cheap and have a warning light set up of their own. Especially important if you get a load of water it shows fast before you have gotten far from the supplying station.

You can usually get back to the suppliers station before the contents of the main filter are used after draining the glass visual indicator down of any water.

Of course if the glass is full of water it is too late. On A cdi engine I would call a tow truck then. The towing charge is litterly just nickles and dimes compared to repairing a damaged cdi type injection system.

It has always bothered me that they were not standard issue on the 123 series of cars. Even the manufacturers of much cheaper diesel cars almost always included them. I purchased mine from ebay when they were les than thirty dollars delivered.

ILUVMILS · #8 02-25-2015, 12:27 PM

The last CDI we had in the shop with water in the tank cost about $8000 to fix. Filling station's insurance company paid the tab.

The high pressure pump wouldn't produce more than a few hundred PSI while cranking. Once the new pump was installed the pressure was good, but it took more than 30 seconds of cranking for the engine to fire up. New injectors fixed that. The rail pressure sensor and water sensor were also damaged (hard fault codes that wouldn't clear).

dmcginn2 · #9 02-25-2015, 03:55 PM

Ok, so water in fuel bad.

Now let's talk about aftermarket setup. Recommendations? 7 micron, 2 micron?

See lots of options out there.

And what do I do about the stock sensor?

dmcginn2 · #10 03-08-2015, 04:50 PM

Ok, so I gave in and bought a new Water in fuel sensor, pulled the old one out.

Seemed like the hole on one side of the sensor of the torx screw was elongated.

The new sensor only had one o'ring on it...selected a Viton one that was close, maybe slightly thicker. So far so good after 20 miles.

IMHO still a stupid setup...if it happens again, I am going to do my own thing.

With that in mind, I guess my question remains, about the sensor, if I leave it plugged in, wiretape the probes, will the sensor trip the computer?

It uses AC voltage to sense the resistance when there is water in the filter...wiring diagram shows DC input, so I am assuming the square thing on the sensor just converts to AC and sends a signal back to the computer when there is water?