DEF usage on an X-15 605 Cummins engine

[Gary.Jones]

While on this past late summer and fall trip up into the NE, I experienced something I had not experienced before and that was for my dash DEF readings to have little relationship to the actual DEF levels in the DEF tank. My dash gauge would go orange (warning) when the DEF actual level was at 75% full tank, and then it would go green and show full just by adding a 2.5 gallon jug of DEF. So got an appointment at Cummins NorthEast in Dedham, MA (Nice people) to have it examined. They determined that the problem was a failed / failing DEF pump. So $1700 - $2700 dollars later (dont remember which figure it was), I had a new DEF pump and I was on my way (surprisingly expensive repair, with just a one hour labor charge .... that DEF pump must be made of gold.. ).

Anyhow, off we went and drove quite a ways and frankly I hardly have ever paid any attention to DEF readings on the dash since it seemed like I could drive a long long way before worrying about putting any DEF in the tank, but because I had had problems, I paid more attention and it was going down pretty fast, so at the next fill up, I decided to verify the dash reading by visual examination, and wow, I had burned through a lot more DEF than I had ever burned through before ...... level was way down.

So, I called Cummins to report the problem. The guys I talked to there said that I should be burning 3% of what I was burning in diesel. I told them my own expectation in the first year I drove this coach/engine was a gallon of DEF every 200 miles. They said, 3%. I had never tried to figure it that way, but I was a long way from home, so decided to run tests. They also asked if the new pump had been "calibrated" again somehow/somewhere (when replaced in Dedham). I had no idea. I saw the mechanic replace the pump but had no idea if he calibrated anything anywhere. In the first test, I filled the DEF to the "Do not fill beyond this line" on the tank and then put on about 775 miles. I burned 120.5 gallons of diesel over that mileage (6.43 mpg by the way)(I am using the digital dash reading of Actual Diesel consumed for this number. If you divide that out (or multiply that out) then you come up with a number close to 5.5% (5.5% DEF consumption given 120.5 gallons of diesel consumed). So my consumption was almost twice as high as the figure that Cummins gave me, and conformed to my own seats-of-the-pants estimate that I was burning a lot more DEF with this new pump than I had the first year I owned the coach.

So I repeated the experiment. I filled the DEF up to the "Do not fill above this line" line again and fired up and put on another 736 miles and consumed 108 gallons of diesel on this run (6.81 mpg) (again using actual fuel consumed from the dash), and this time, filled back up to the target line, and it took 5.5 gallons of DEF. I again calculated the DEF percentage burned and this time came up with close to 5%. The earlier percentage was taken while going up and down and up and down the Appalachian and Smokey Mountain chains, and the latter was done largely on the flat-lands heading from Tennessee to Louisiana, but overall pretty good agreement..... but again seemed much higher than before. So I called Spartan and this time Spartan said they had asked Cummins before and Cummins had said that the ISX-15 and the X-15 burned much more DEF than the ISL-450 and rates up to 5 to 6% of fuel consumed were not a problem.

Now, it has been reported on here that these ISX and X-15s do consume more than the 450. However, is this about the rate the rest of you 18 and 19 and 2020 coach owners are getting? Does this conform with what you are finding? My primary worry was if the Cummins was drinking too much DEF that it would hurt the engine or throw a code. (actually it is the "after processing system" that consumes the DEF.... I had always thought DEF was added to the combustion chamber to make a more complete burn, but was told, "No, You have it wrong.... it is added as needed to the after processing system" (the catalytic converter) only when it is called for). Anyhow, I am learning every mile I drive this thing... but what do you other owners think?

Cummins engineers assured me that too much DEF being burned was not a problem... it would not hurt the engine or after processing. OK, that is important. if too little is a problem, I was concerned that too much was an equal concern.... apparently not.

What do you think? What is your experience?

Gary

[Rollin101]

Gary, I have questioned the ISX def useage also. They keep saying the usage should be around 3% of fuel burn. My problem with that my 2017 ISX got about the same mileage as the 2014 ISL. So going by their calculations it should have burned approximately the same amount of def as the 2014, but in actuality it used at least twice as much. I haven’t had the 2020 long enough to get a good check, but I can tell you it certainly doesn’t use any less than the 2017.
On the ISL my usage was about a gallon per 350-375 miles. On the ISX the usage was about a gallon per 150-175 miles and I don’t expect the X-15 to be any better.
I believe the usage on the big engine is closer to 5% than it is to 3%.

[Turbojack]

I have ISX15, friend has X15 We both took a 3,000 trip together. At the end of the trip we both used the same amount of DEF. The bad part is I do not remember how much DEF we had used.

[Turbojack]

Gary, I am surprised you had to pay for this repair. Was it not under warranty?

[Gary.Jones]

I’m sorry if I left the impression that I paid for the repair. I didn’t. The pump replacement was paid for by either Cummins or Spartan on warranty, not sure.

Sorry I missed-stated or was not clear. (If I had paid for it, I sure as hell would have remembered if it was $1700 or $2700). ;- !!!!!

[Divingbiker]

Gary,
First off, as a blanket statement, I really appreciate your post. As the owner of several large offshore commercial boats and commercial trucks, I'm very experienced with diesel engines and the maintenance required to keep my fleet running. These motor homes are a different beast and I have found that they require just as much attention as any of my boats......On a personal note, I find that I learn something every time you share a new experience with your coach. I'm down in Lake Charles and hope to meet you in person one day.

My smaller 40 foot Insignia has the L9 380hp Cummins. So far, I'm VERY pleased with the economy, torque and performance of the engine. I just completed a 4000 mile round trip out west. I was NOT pulling a toad.

Average cruise speed for this trip was 68 MPH
Combined MPG (flat and in the hills) was 8.5 MPG
Total DEF used was approximately five 2.5 gallons jugs of DEF (12.5 gallons)
My Aquahot is propane, so no diesel consumption when its being used.

So,
I consumed almost 500 gallons of fuel (actual was 470 gallons diesel)
3% DEF would yield a burn rate of 14 gallons (I used a little less then this at 12.5 gallons)

Because of the big difference between the size of the engines in our two coaches, there is no comparison as to fuel or DEF consumption. But you asked and this is what I'm getting.

[Gary.Jones]

DivingBiker.

Nice to meet you and given that you are in Lake Charles area, we are down in that area a couple of times a year (most often at the BreaxBridge Crawfish festival. High probability that we will bump into each other some day. Looking forward to it.

Since you are driving a different model and engine, it will be interesting to compare notes here on iRV2 for different performance and conditions. That is what is so great about iRV2.... we are all establishing essentially an encyclopedia of issues/equipment/fixes/experience which helps everyone. Thanks for providing that information. Not a lot of people on here running that particular engine so that is a good foundation for those that follow.

That is the first I've seen DEF burn rates for that rig.

Drive safe

Gary

[TXBart]

We have put 13,158 miles on our X-15, burned 1,853 gallons of diesel and 75.2 gallons of DEF. That gives us DEF mileage of 175 mpg, diesel mileage of 7.1 and DEF/diesel burn rate of 4%. I haven’t done a comparison with our ISL but it seems like we burn a little more DEF in our Cornerstone.

[photraveller]

Gary, I think we are using closer to 2% on our ISL9.

The only time I really tracked our DEF use was on our 10K mile trip to Alaska last year. I started with full DEF tank and five 2.5 gallon jugs on board. I bought two additional jugs in Anchorage to be safe.

Don't remember exactly but we were probably at least half full (those two jugs) when we got home. So, roughly 30 gallons of DEF for 1,360 gallons of diesel. (2.2%)

I've always used the 50:1 ratio and it seems pretty close. But I only top off the DEF tank by accident. I add a jug or two when it starts getting low.

Steve

[Gary.Jones]

Thanks Steve.... interesting.... that is the lowest percentage I've heard but would make sense given that engine.

Gary

[hdgoldie]

Has anyone noticed a difference on DEF use when running bio-diesel vs. regular (non-bio) diesel? Rick thinks we burn more with the bio fuel but we haven't tracked it to be sure.

[Gary.Jones]

Linda

Its been so rare for me to find a station that claims to not have biodiesel that I have never been able to track that issue. Maybe someone will chime in and give us some real data. Probably 100% of the stations I fueled in the last 6,000 mile trip to New England was with pumps that had the biodiesel sticker on the pumps..... now, it would not surprise me if Rick is right, and that biodiesel would lead to a stronger burn of DEF. Maybe someone really knows.

Gary

[Matt D]

Gary, Our experience with Bio and #2 diesel was that #2 got far better mileage, but burned more DEF.

Bio gets much worse mileage and uses less DEF.

Our experience was with #2 up north during an early spring trip. Everywhere else, we have to use Bio.

My suspicion is that Bio (B5- 20) adds volume to the fuel without a proportional increase in BTU per pound of fuel. This is great for emissions, as less hydrocarbons burned, less emissions to resolve before exiting the vehicle.

The downside is less fuel economy and total lower output.

Pure #2 without a bio blend has a higher BTU content per pound of fuel, makes more power, and also produces more emissions to be eliminated, thus the higher DEF usage.

I informally track our mileage and during this trip was almost a full point bump in fuel economy. That is what captured my attention.

What has happened with biodiesel is almost exactly what happened with the Ethanol gasoline blends. Better emissions at the cost of reduced power and overall economy.

Unfortunately the policy makers only care about the tailpipe emissions, rather than the overall science, and how proper fuels can produce less usage overall.

-Matt

[Gary.Jones]

Thanks for that information Matt. I have never factored that in at all.... now I am going to have to pay attention to another issue to get a handle on this DEF bur n rate issue. I have a ticket to get the rate "calibrated" next time I am close to a Cummins shop and have the time. I spent a lot of time up North for this past 3 months. I did not notice if the pumps was saying Bio or #2. Frankly, I assumed that whether it had a sign, it was going to be biodiesel.... not pure #2.

On a slightly different issue, in NH, I started running into pure #2 and it was almost a buck less a gallon as it had no state or federal tax included in it. I asked what the deal was and was told that straight diesel was not taxed as it was not for highway use and was only there for a bunch of people who heat their barns or houses or whatever with diesel or maybe farm use. They told me it was a different color from highway diesel (maybe tinted red ????) and if anyone stopped me and sampled my fuel (who would do that?), that I would be ticketed as it was illegal to run raw diesel on the highway...

Well, I have been through MANY problems with after-processing on both this coach and my 15 Anthem and they were significantly expensive warranty repairs, so I had no desire to screw around with any of that " so I was not going to take any chances of throwing codes or generating problems over 50 - 99 cents a gallon difference in price so I always bought the high priced, "street legal" stuff, but I had never seen this issue discussed on here, and was wondering. Never seen that down south below the Mason-Dixon line. Who knew?

Gary