Mileage Loss After M-Unit Replacement

Hello, recently replaced mechatronic unit on wifes daily ride. Two tanks of diesel later and mileage drop is noticeable. Noticeable, as in it dropped to 38mpg. After replacing the M-Unit I limped to local dealer and they programmed the trans? Does anyone know if a possible reason might be that the VW dealer programmed it to do this? In other words, what just happened to my Jetta's 43 - 46 mpg capability. To my knowledge this will be the first time the little TDI ever dipped under 40mpg.

Again, any info is appreciated. Either way I'm calling VW tomorrow.

Not certain why everyone shy's away from my questions on this forum. Okay, maintenance is always meticulous on this car. Therefore, fluids and filters are always monitored and replaced. That being said I do not have oil analysis done. I change the oil every 5k...period. Other fluids and filters are always changed within specs or well before.

Back to the noticeable mpg change. Day of mechatronic unit failure the car gets it same reliable mid 40's fuel economy and has since day one however, after the M-Unit change and adaptations at VW, the best the car can muster is 38mpg to the tune of 3 tanks of diesel now so its a pattern of consistency. To me, this is unacceptable in a 1.9 TDI. I want my baby back. We check fuel efficiency on every fill-up, on every vehicle we own. I know its winter...we usually take a small hit in economy every winter...everyone does. That isn't the problem. VW insists it is...it isn't...period. If THAT was the problem we would have experienced it at least one other time in the last decade.

Anyway, no strange noises or problems except the car does occasionally downshift more harsh than it ever used to...adaptations? When I say "harsh," it is nothing profound or alarming. It is just different than what the car did consistently for a decade. Other than that??? That is why I'm asking here, any valid ideas? Anyone?

Patience waiting for a response. People are busy!
How many miles on the engine?
Have you checked parking brake engagement? Is it releasing completely? You'd probably notice it dragging, but mine was dragging for many miles before I noticed how hot the left rear wheel was after a long drive.
My own experience with mechatronic replacement (early 2009 JSW), I saw no difference in MPG. I can't imagine, unless the trans shift points are dramatically different, that the mech replacement is the culprit.
I suspect some other issue. Are you certain all the turbo plumbing was reattached correctly? Have you checked carefully for loose connectors?

Hello, tdiatlast

Thanks for responding!

225k

I should have listed that brakes are a first check item. Especially on the Jetta with the gimpy rear caliper problems. I spin the rear wheels freely and parking brake works as it should. This car is used as a daily driver so the parking brake is set daily as well. Another feature of the DSG is excessive movement of the car when the parking brake isn't used so its a must in my opinion so it doesn't damage anything inside of the transmission.

I have not checked the plumbing on my turbo as a possibility. There is no loss of power that I can tell. To this date, the turbo has never been apart. I did have a v-band fail, but memory fails me as to how long ago. It would be amazing timing if the turbo was the issue as it happened at the exact time of M-unit failure. I was wondering if anyone else has experienced the mileage issue after changing one...so thank you very much for responding back.

I am still open to any suggestions as to the profound drop in mileage.

Two tankfuls is hardly what I would consider a trend.

You can go through my sticky at the top of the Fuel Economy section if you want to check off a few things. I doubt it is related to the M-unit. They do come with software already in them, so it would have whatever is the latest and greatest, but I do not even think there was anything changed on them since that car left the factory. And the gearing is of course all the same.

It has to do some learning, as the adaptations just get it back to a zero point calibration, but it is not a new transmission and clutch assembly anymore, so it has to learn some values over again to fine tune itself. Which would not have happened in a few hundred and maybe not even a few thousand miles.

Do you notice it shifting differently?

Thanks Oilhammer,
I'll check out the Economy link...I'm always open to new ideas. I have achieved some astounding results from other vehicles in the past. The Jetta is my wife's vehicle so I rarely have the opportunity to experiment with her car.

You are correct, 3 tank fulls is hardly a reliable trend, as a former machinist and someone who used statistical process controls, I can confirm that. If I had patience I would sample 100 tanks of fuel instead of assuming that the car is incapable of the desired result.

It does down shift differently on occasion. I would assume this is part of "the learning process." VW did not instruct as to what to do after "the adaptation process." I am going to assume that they did the "basic setting engine adaptation." I have heard that there are some DSG's that do not allow a "kickdown" test as part of calibration maybe that is why it is still learning when it downshifts? I do not remember where I read or heard this, it wasn't from VW. They cannot confirm anything but the calibration test and I am in no way a VW tech. That being said, the new M-unit does occasionally downshift roughly compared to a decade of prior use. I have driven it in tiptronic mode keeping the engine in a 1500-3000 rpm range. I have also tested the M-unit by braking hard and then going full throttle. The trans shifts great...it does occasionally downshift harshly in the lower gears. I don't like using the word harsh...I can only compare it to my reverse shift pattern race trans. It feels solid and secure in the downshift...maybe that is a better analogy, which is different than before, but not necessarilly better.

I'll keep calculating fuel economy. The timing is inexcapable though. Although, I did experience an alternator clutch failure within days of complete DMF failure...talk about timing.

Thanks for the suggestions...much appreciated.

Possible Reason...

The following is a partial quote from another VW website, "I had done a few of these adaption changes... to try to help the customers save or prolong some future expense. In keeping with the old axiom "no good deed goes unpunished" several did notice loss of fuel economy so I set them to original spec and have discontinued changing them." this is in regards to the pros and cons of VCDS EGR recalibration. After finding this I cannot help but wonder if this is possibly what has happened. Question is; is this the same adaptation process as the M-unit or something different? Where is the port they need to connect?



If it is under the hood, I am thinking that VW noticed my EGR delete. It is VERY noticeable that two large chemically resistant hoses run to a coalescing filter housing that is bolted to my oil filter housing.



The car continues to be consistent at 38mpg.

Twenty more tanks of fuel and the new norm for the 1.9 is 38-40mpg. Aggravating, but nothing I can do. I'm about 100% certain that the programming for the M-unit I replaced is responsible for this. I have no other explanation.



I am certain people will disagree, but the results remain the same. I do want to thank everyone that responded. I know you all have lives outside of a computer keyboard and I appreciate the information you have given freely!


Thanks!