I had this wee adventure on the way back home to Baltimore from Disney World last Sunday. About ten miles into my trip the car’s message system beeped at me and the center console told me I only had 10 starts left. Above that number was a small icon indicating that the complaint was I had run out, or was about to run out of DEF.

I understand why they’re doing that countdown thing when it comes to the emissions control system, it’s because lots of drivers will simply ignore it otherwise, but I absolutely hate it and I think it’s a potential life threatening thing to just shut down the car when the count runs down to zero. When it started happening to me a couple years ago on my way to California, because my catalytic converter failed, I was in Grand Junction which did not have a Mercedes dealership and the nearest one was in St. George Utah, which meant driving several hundred miles through empty roastingly hot desert.

My car is a 2012 E350 Bluetec that I took delivery of December 2011. I am its first and only owner, not counting the factory and the dealer. It has that OEM 642 engine everyone seems to hate but I’ve had no trouble with it (ironically not counting the emissions control system which I’ve had to have worked on Lots, like the DEF tank heater replaced which cost nearly two grand, and both NOx detectors replaced). This is probably because I give the car every service the factory indicates except the oil changes. Those I did twice as often (every 5k) until the car reached 170k and now I do them every 3k. I do them myself except at the 10k service intervals where I let my mechanics do the oil change too.

And also top off the DEF tank. So that nominally gets done every 10k, but if I need to visit the mechanics in between those service intervals, like I’m taking a cross country road trip and I want the car checked over first, I’ll ask them to top off the DEF tank then too.

My car currently has 209k on it, most of that road trip driving. I am able to walk to my office, and to most day to day things I might need right here in my city neighborhood. So the car does not get commuter miles. I do a lot of pleasure road tripping in my cars and this Mercedes makes a wonderful road trip car..

Only once before have I had a problem with the DEF tank running low and that was because a shady dealer didn’t do it, or change the fuel filters like they were supposed to either, and I had to find out, that time also on the way up I-95 from Disney World. Good thing you can get that stuff at any any truck stop.

So this time I thought it was another case of the mechanics not topping off the DEF tank. But the guys I use now are really good and very trustworthy. So I gave it some thought.

I did the emissions recall two Julys ago, got a check for 2k from MBUSA…and lost 4 mpg (why??). I wondered if the car was sipping DEF more since then and maybe that 10k interval just wasn’t cutting it now. I had the car looked over several times before taking a road trip to visit family in California and asked them to top off the DEF tank then. So I might not have noticed the DEF tank was draining out faster than before since the emissions work.

I stopped for some road food and checked to see if I still had a countdown. I did. Now I only had 9 starts left. I had a reservation for the night a South of the Border, which yes is barely a two star motel but every room has its own private covered car port which makes it worth it to me. I figured I would buy some DEF at a truck stop along the way, get myself to South of the Border and my room. Nine more starts would be more than enough. Once I was there it would be easy to empty the trunk and add some DEF to the tank.

I stopped at a Flying J and bought a cubetainer of two and a half Gallons of DEF. When I started the car again it said I had 16 starts left.

Wait…what…???

I didn’t think that was in anticipation of my putting DEF in the tank and now I’m wondering if it wasn’t some glitch in the system and the tank had plenty of DEF in it. So I kept on driving hoping the next time I stopped I’d still have some starts left. Periodically I checked the messages. I had one message, I had 16 starts left. I checked again. Same story. I stopped to take a bladder break. When I started the car again there were no messages, no indication of a countdown happening. I double checked the message list. Nothing.

?????

Now I’m thinking maybe the sensor that tells the car the DEF tank is nearly empty was glitching and now it wasn’t. Or now it was telling the car the DEF tank was okay when it wasn’t. I had no idea, but I promised my car I’d add DEF to the tank when I got to South of the Border and I was going to do that.

So I get to South of the Border, check in, drive to my room (those private carports are Really Nice, you drive right up to your room door and even if it’s pouring rain you can unload the car for the night no trouble) unloaded the trunk, took out the trunk liner (the car has to be a working member of the household so I bought one of those when I took delivery) and uncovered the DEF tank.

The cubetainer of DEF came with an extendable pouring spout that fit snugly into the DEF tank filler. I started pouring. The tank took nearly all two and a half gallons. I think maybe there’s a pint or less still in the cubetainer.

The next morning I drove the rest of the way back to Baltimore without any trouble or new messages. As I read it, the DEF tank in my car has about a six and a half gallon capacity. So there was probably still four gallons of DEF in it. I don’t think that should have caused any problems.

Anyway…new rule: when I do my own oil changes from now on, I will also top off the DEF tank. And it would be nice to have a feature that lets me see how full the DEF tank is. But the sensor that’s in there might not be capable of that.

The chatter I’m getting on the Mercedes forums is the sensor in the DEF tank that tells you when you’re about to run out is getting flakey. Oh well. I’ll ask my mechanics about it when I do the next 10k service in a few weeks. I love my car, but that countdown thing sometimes makes me wish I had a 1973 240D. With a four speed stick.