Hello. I've been lurking for a long time and have found some great info on the forum. Thanks to all.

I haven't had much of a reason to post until now and have been thoroughly enjoying the car.

I've recently seen a >10% drop in fuel economy from a pretty steady from 32-33 mpg to 28-29 mpg (calculated, not OBC). I have a one-way 25 mile/day commute that is mostly highway. The change came after my last service in Nov. As performance did not really change, I wanted to get a few tanks in before confirming the issue.

The work done in Nov was fairly major and included the following:
- Inspection and oil change
- Timing tensioner and timing chain replacement due to cold start chatter
- Removal of carbon build up
- Replace warped hood scoop
(current mileage is ~39.5k)

I've talked with the service manager at the dealer to get a theory before bringing it in. At this point, they are at a bit of a loss as to what could be going on as there are no warning lights and no noticeable drop in performance.

The current theory they proposed is that during the service they upgraded the engine management software to help deal with the carbon build up. As part of the upgrade, the fuel injection pressure is increased and they think that this may have an impact on fuel economy.

Does this sound reasonable? Would it drop fuel economy by that much? Has anyone else noticed this after a software "upgrade".

Any thoughts would be great.

Thanks

 

Fuel is metered at 14.x to 1 basically. That's what the oxygen (lambda) sensor fundamentally does. Thus, if pressure is different, the engine computer will just back off the injector duty (on) cycle to get back to the right fuel does so the oxygen sensor then sees the right exhaust gas make up.

From the work you describe and that you are saying your "before" and "after" driving patterns are similar, I would give you two shots in the dark:

1. They got the engine timing wrong on the timing chain replacement. That could definitely mess with fuel economy (and overall power). Or, I suppose it would have theoretically slipped before the service, throwing it into a lower power, but use less gas mode. That seems very unlikely though, so I would speculate more on a mistake in aligning the cam to the crank correctly when putting the chain and tensioner back.

2. Carbon build up had generally de powered the engine, and now the power (and the lower gas mileage) is back, if you put your foot in it both before and after. That one doesn't seem all that likely, but just using the data you mentioned.

Other than those, I would be looking at ECU faults (whether from bad programming or something else), clogged air filter and worn plugs. You didn't mention tune up items generally, and of course if a plug is outright bad it should generate a CEL.

 

Could also just be as simple as winter fuel formulations, which tend to decrease fuel economy; or the tendency for engines to run richer (and therefore consume more fuel) as the duration of their warm-up periods increase during cold winter months...

 

I think timfitz got it exactly right.

Dave

 

Im currently fighting with MINI about the same thing and have a similar post to yours. But Im only getting 19-21 city, a drop from the 28-29 i was seeing pre-one year service. They are trying to say its normal.......

 

Yeah, that much of a drop seems a bit excessive to attribute to fuel formulations or longer warm-up times...

 

Thanks for the comments and thoughts. I am leaning towards something during the timing chain work per MP1.6T's comments.

If I lived further north and the drop didn't coincide with a service, I may go with the fuel formulation change. I am in California and have 3 years worth of data on my MPG (I've gotten into the habit of logging my fill ups on fuelly.com) and have not seen any seasonal change in fuel economy. In fact, until the service, the lowest fuel economy I saw in close to 3 years was over 30mpg with the most of the fills in a pretty tight 32-33 mpg range regardless of how I drive. Now every fill since the work was done is under 29... We'll see if the having the data logged helps in the discussion with MINI.

I've asked the dealer that serviced the car to look into this a bit more to see if the drop is typical after the software change, but have not heard back. I've also scheduled it in at another dealer to have it looked at it in about a week. My confidence in the dealer that did the work is rather low due to some issues with quality on that and prior services.

I'll post more if I get any clarity from MINI.

Finally, I don't think I included the info that I'm driving a 2008 MCS CR/B.

 

Well, don't be too quick to dismiss fuel formulation, even in sunny California. The EPA approved the use of E15 ethanol last October, and it wouldn't surprise me if California was leading the charge to switch from E10... Higher blends of ethanol will decrease fuel economy; check the pump labels wherever you normally buy your fuel to see if they're still selling E10, or have switched to E15.

It's good you have historical data with which to establish a credible trend in your fuel economy. I guess all I'm saying is: until you've firmly established the root cause of the drop in fuel economy, don't overlook other plausible explanations.

 

A big +1 for this thought!

Just wait until the E-15 (ethanol)mandate from the government hits us this year and our engine warranties go belly-up as described by our Service Manager yesterday!

S.

 

Thought I would post an update after taking it in to the dealer (a different dealer).

First of all, the fuel economy is trending back up as per the attached chart. You can see the drop I noticed between 10/21 and 12/16. The service was done in November.

Also, I did check the pumps and the "may contain up to 10% ethanol" sticker is there as it has been for as long as I remember. I'm using the same Shell station as I have for the past few years.

Prior to running some tests, the service manager mentioned that of all the things done, the timing is the only thing that he could think of that would cause this. After running a battery of tests including timing, idle, misfire checks, waste gate, etc they concluded that "with replacement of timing chain engine may need to be broken in again". Given the trend in fuel economy that seems to be a reasonable assessment. I've had timing belts/chains replaced on other cars and not seen a difference, but this may be different with the tensioner issues MINI has been dealing with.

I feel a bit better that a different dealer checked over the car (one that doesn't leave tools in the engine bay) and that the trend is in the right direction. If it wasn't getting better, I'd be more concerned.

Thanks for the opinions, it's always good to here what others think.

 

I had the same thing happen when they changed my timing chain. I asked if it had been reflashed and he didnt think it had been. My car ran extremely well and had great mileage. He told me the chin had slipped one cog. Now my power and mileage is not as good.

 

Yeah, that seems a bit of an odd explanation, but I guess stranger things have happened. What with adaptive automatic transmissions out there, I guess it's not too far-fetched to believe the ECU programming is adaptive as well...