Since getting the field measures done in May for the Bluetec class action stuff, my 2009 ML320 has been running great. We've done a bunch of camping this summer, towing our trailer, but the "check Adblue level" notice came up much sooner than I would have expected about a month ago. I refilled, did a trip from Calgary to Yellowstone and back and had the notice come up again, which is odd since usually I've only had to refill it a couple of times a year. Went from Calgary to Winnipeg a few weeks later and had the notice come up again once we were on the road, and in the course of the trip there and back (about 2500km) I had to refill 3 times. The first couple of times the car continued to work as usual, but then the CEL came on. The last time I had to refill it went into limp mode, and my code reader (Foxwell 510) communicates with most of the car's systems but not the Adblue system, so even though there are no other pending codes/issues, I can't clear the CEL and the limp mode returns any time there is any substantial load (either accelerating quickly, going up a large hill, or cruising at over about 90km/h). What should have been a 7-hour drive on our last day home ended up being 9 with having to drive slower and then also to stop over and over to turn the car off and on to get out of limp mode.

The trailer is one we've pulled with this car for the last 3 years, and it weighs about half of the towing capacity, so usually cruising at 120km/h on the highway (with hills) is no problem, so I'm pretty sure this sudden onset issue isn't caused by towing.

I have it booked in at the dealer, but can't get in for a couple of weeks and I'd like to do some of my own troubleshooting first, but I'm not sure where to start, especially without having any codes to jump off from.

The fact that when I get the "add DEF" notice it always takes a full jug of DEF would indicate to me that it's not an issue with the sensor for the DEF level... the tank really is empty. So either it's using way more than usual for some reason, or there's a leak somewhere and the DEF is draining, rather than being used.

To troubleshoot, I'll check for any leaks under the car (haven't noticed any, but I haven't looked closely yet), and it won't hurt to check the Adblue-related fuses (rear left-hand side of the car?), though I don't see how a blown fuse would cause hugely increased consumption.

What's the easiest way to view any hoses or whatever that could be corroded and leaking? Do you get to them from inside the car or underneath?

Anything else I should be checking?

 

Any adblue leaks will be accompanied by massive amounts of white crystallization of the urea.
The adblue tank is a bit larger than 7 gallons - one jug is about 1/3 of the tank capacity. Towing a trailer will significantly increase adblue useage - we towed an RPod camper about 3K miles last year and used about 7 gallons of DEF.

Getting a scanner that can talk to the whole car - adblue system included - is money well spent. I've got a Launch X431V+ that has yet to put up any roadblocks to any vehicle I've connected it to. Best $900 car related tool I've spent money on.

 

Thanks - didn't realize that the tank capacity was much larger than the jug... in any case, I've been towing with it for three years and while it may have upped my DEF consumption a bit, it's certainly not been anything like this. I do have some crystallization on my garage floor, so after we go out this weekend (not far from home and shouldn't be too many hills, so I should be able to manage it even if I have to stop/restart a few times to get out of limp mode) I'll climb under to see if I can spot where it might be coming from. I have it booked to go into the dealer in a few weeks, so I expect we'll be able to live with it until then if I can't find the source and easily fix it.

I had wanted a full function scanner, but couldn't get the cost through the house budget committee, so ended up with the best compromise I could find... Foxwell N510, which is bi-directional and, up till now, been able to pull and clear every code I've encountered. It's not going to measure up to the launch, but at about 20% of the price it gets me more than 20% of the way there, and was the best I could get.