Subaru diesel Deceleration Fuel Cut-Off

[Garona]

I have a strange problem with my Subaru Legacy, 2.0 diesel, 2012 year model.
When braking with engine, fuel injection is switched off. When I push the gas pedal again, just a little bit to maintain a constant speed in a city, there is very annoying gap before injection unit starts injecting fuel again. Normally, you can easily keep constant speed by decelerating, accelerating and you don`t even notice the gap between. But this Subaru is like driving with a switch. Injection on, injection off with an annoying gap between. Of course Subaru service says there is nothing wrong because no any fault messages are seen in diagnostics. I don`t know how it works with Subaru diesel, fuel cut-off. Is ECU reading vacuum pressure from somewhere when decelerating? Could it be that some valve is not working correctly? ECU is updated with the latest software. Subaru diesel works with a Common rail injection.

[Arragonis]

The Sube Diesel is not a TDI, which might be both a good and a bad thing.

We need more info - e.g, what engine and road speeds are involved when thus happens.

[oldtamiyaphile]

From what I've heard, Subaru's diesels are not very refined. They only just got into automotive diesels recently. Perhaps you can test drive another one while at the dealer to see if the issue really is universal.

My petrol Renault has the same issue. Perhaps if I wasn't hypermiling it wouldn't be as apparent, but I'm always on the edge of that DFCO envelope. My TDi on the other hand has no such issue.

[Garona]

Subaru has this particular diesel engine in production since 2009. I have had 2 another Sub. diesels before and tested many others. All of them were running as smooth as diesel can run. This car has been running for 50000 miles. I am the second owner and as far as I can remember, this problem started a little by little, around 10 000 miles ago.

[Garona]

Speed doesn`t make any difference, it`s always there. I just can feel it better at low speed in urban areas when driving in first or second gear. Btw. this car is with 6 speed manual gearbox. Here is one typical scenario. I`m driving in traffic jam. Gear 2, engine running 1000-1500 rpm. Constant speed behind another car. Giving a little gas, braking with motor, give a little gas, braking with motor... This is really nerve wracking situation. Instead of gas pedal, it`s like using a switch to switch injection on and off! None of my Subaru diesels I have driven, has never had this kind of problem before. I was already thinking to replace the whole ECU unit with another one, but would it change anything if a new ECU software doesn`t change anything either. This problem started a little by little, around 10000 miles ago. That`s why I was thinking, it might be a hardware problem, not a software.

[Gasoline Fumes]

Assuming it's drive-by-wire, I'd suspect the gas pedal.

[Garona]

I was also thinking about a gas pedal. But if I let the engine run at idle, then there isn`t any gap. It runs ok and reacts smoothly to the pedal. It must have something to do with Deceleration Fuel Cut-Off when braking with engine.
I found one interesting post about this problem. I just cannot insert any links to this post, but I think ever green has a solution to this problem. Only his car is gasoline and automatic gearbox. Can make a difference... dunno.

Ever green says:

So I opened my ECU ROM in Romraider (a subaru open source flashing system) and found overrun fueling tables. I just raised overrun fuel injection rpm from 1100 to 4500 rpm and now my car doesn't enter fuel cutoff at all.

[Arragonis]

You probably can't post links yet as you don't have enough posts - this is to prevent single sign up SPAM-bot type things. You will be able too quite soon (6 or 8 posts I think maybe)

The ECU decides on what to do based on sensor readings such as engine temp, pedal position, gear, engine speed, air temp, boost (it has a turbo) and road speed amongst others. This car doesn't have a vacuum as it has no throttle.

It reads like one of those sensors is out, probably the pedal position sensor not detecting you pressing on it until it reaches the "on" position. Your "idle" diagnostic is a good one. How is the sensitivity - e.g. if you are at idle when warm and you press gently can you get it to rev at 1K, 1100, 1200, 1300 etc. quite easily or are the steps quite large ?

If that is OK then maybe a different sensor might be out.

Most ECUs record sensor faults but if they are intermittent (i.e. less than x times in y trips or y days) then they won't record it as a fault and turn a light on. It might still be recorded in the ECU somewhere if it can be scanned.

Originally Posted by oldtamiyaphile

From what I've heard, Subaru's diesels are not very refined. They only just got into automotive diesels recently.

Sube Diesels are "OK" in that they are a generation behind most European ones but that is true of most Japanese Diesels as they don't sell them in the biggest markets - North America and Japan. That tapped they get positive reviews in terms of FE and performance :

Subaru Legacy Diesel 2008 Road Test 1 | Road Tests | Honest John

[cosmick]

Once you get this resolved, I'd start playing with getting big boost in at lower RPM, even if this results in peaking before redline. This should help MPG just as much in a 2.0 as in a 6.2L diesel, assuming yours isn't grossly overgeared.

[Garona]

I decided to take this car to the Subaru "doctor" in another city. Already called him and he would like to dig deeper into the settings of Ecu. He will run the car on Dyno and see how it behaves. Hopefully he can find some miracle value for the right settings.