Instrumentation issue or engine issue?

Put 1600 miles on the d this weekend. Averaged 35mpg, 75mph. Mostly flat driving, some small hills, across TX/LA/MS/AL, ambient temps 50-75 def F.

Here's the rub: On 3 separate occasions indicated MPG (instantaneous) dropped 15-20MPG for about 25 minutes, each time. Engine was warmed up, car was running at highway speed, no other apparent issues - usual great acceleration, no vibration, no noise, etc. Issue occurred on 3 separate tanks of diesel bought from "reputable" locations.

Towards the end of each of these episodes you could see the MPG needle fluttering from 20 to 35 MPG as the episode began to subside. This transition would last for about 2 minutes, then indicated MPG was back to normal.

Anybody else experience this or have a guess as to what's going on? Thanks

27k miles.

DPF burnoff?

Originally Posted by Pasa-d

DPF burnoff?

This.

I've never noticed something like this, especially when running at a steady load. I suspect some sort of instrument issue or interference, emi (electro magnetic interfernece) perhaps.

Ah....DPF burnoff. That's was a new one to me Now I can go back to enjoying the car and not worrying about a gremlin. After reading more about DPF burn-offs thats pretty much exactly what's going on. Thanks guys.

It sounds like the engine timing is altered to produce hotter-than-normal exhaust gasses during the burn-off which leads to reduced mpg....that right?

Does 3 burn-offs during 1600 miles of highway driving seem excessive? Normal drving routine is routes <10 miles highway at a time.

Originally Posted by Flyingman

I've never noticed something like this, especially when running at a steady load. I suspect some sort of instrument issue or interference, emi (electro magnetic interfernece) perhaps.

Me too. That is a huge drop in mpg.

My question is how it is that you're viewing the instant MPG, unless you're talking about the bar that's underneath the tach. Did you get your car coded to add this (I plan to do exactly that, along with the digital speedo, along with some other goodies)?

Originally Posted by Stugots

My question is how it is that you're viewing the instant MPG, unless you're talking about the bar that's underneath the tach. Did you get your car coded to add this (I plan to do exactly that, along with the digital speedo, along with some other goodies)?

Digital speedo? Come on Stugots....we need more details on this. Pretty please?

Originally Posted by Stugots

My question is how it is that you're viewing the instant MPG, unless you're talking about the bar that's underneath the tach. Did you get your car coded to add this (I plan to do exactly that, along with the digital speedo, along with some other goodies)?

No fancy instrumentation/coding (yet). I sure would like to see some more engine data - temps, DPF burnoffs, and a lot more. I was using the bar under the tach (but clearly the same data was being fed to the avg mpg calc as it was decreasing rather quickly).

Since I was cruising on flat interstate around 75mph, before the events mpg was low 40s. During the events mid-20s. After the events mpg was somewaht better, getting close to 50 which I think is why the trip average still managed to be around 35mpg.

The wierdest part was the mpg needle fluttering a little bit (between 25/40) before returning to normal.

I suppose a DPF cycle could cause some unusual shifting in intrumentation.

I've just never noticed it after 30k miles and 2 years of driving this car.

Originally Posted by Flyingman

I suppose a DPF cycle could cause some unusual shifting in intrumentation.

I've just never noticed it after 30k miles and 2 years of driving this car.

I'm going to go out on a limb and guess extra fuel is injected after the main compression cycle, not burning in the cylinder but instead gets pushed to the DPF where it burns to do the cleaning. This extra fuel decreases MPG, but does not impact the operation of the engine itself. That's my story and I'm sticking to it untill someone comes up with a better one.

Originally Posted by KeithS

I'm going to go out on a limb and guess extra fuel is injected after the main compression cycle, not burning in the cylinder but instead gets pushed to the DPF where it burns to do the cleaning. This extra fuel decreases MPG, but does not impact the operation of the engine itself. That's my story and I'm sticking to it untill someone comes up with a better one.

specifically:

"Soot particles have a relatively high ignition temperature. So, the
exhaust temperature must be raised in order to initiate a regeneration
phase. The exhaust temperature is raised by "post injection"
events. The DDE system triggers the injectors after initial
combustion has taken place. This raises the exhaust temperature,
which in turn burns off the accumulated soot particles.
The DDE will initiate regeneration every 300 to 500 miles depending
on several factors. Mostly, the regeneration is transparent to
the driver. There may be a light loss of power for a short period
while the soot is burned off."

from BMW "Introduction to Diesel Technology" pdf

Thanks Steve,

I realized it was heating, just not how that was done. So they are essentially pushing unburned fuel through the exhaust into the DPF to "burn".

Fully explains the momentary drop in MPG.

does anyone know if this fuel is injected into the cylinder or further downstream?

I am not reading anything that suggests that there are 'injectors' except in the cylinders.

thanks, I gather that in some engines the fuel is injected downstream of the cylinders for the DPF regen cycle

It does inject more fuel and some people claim to see a drop in mpg when it happens. No one ever has claimed though to see any where near 15-20 mpg drop. Suppose it is all in how they are monitoring things.

Here is an interesting take on this subject:

I drove from Hagerstown, MD to Phoenixville, PA over the holiday. The trip is approximately 160 miles, mostly flat, I-81 and PA Turnpike. On the trip to PA, I averaged 72.1 mph and got 39.2 mpg. On the return trip, exactly same route, I averaged 71.6 mph and got 36.7 mpg. The only thing I can figure is there was a DPF cycle on the return trip.

I can think of many times I have shut my car down and discovered it was doing a regen. My mpg for those tanks though were no different than the ones around them. Perhaps it just is a difference in overall car use.

Originally Posted by julesandtrish

Digital speedo? Come on Stugots....we need more details on this. Pretty please?

You can code our cars (not just the d) to have features some of the other cars have standard (like the M), in addition to adding other features. Digital speedo is one, instant MPG on your OBD (where your trip odo and your average mpg are) is another, and a host of other things.