Hello all - I could use some technical help. Own a 2017 CX-9. Has 50,000 miles. Last year (7/20) dealership said the engine block was cracked and needed replaced - which they did. Since then I have noticed (as per mpg computer readings on dashboard) that my mpg has gone from 23-24 to max 18-19. The loss of fuel efficiency is real (not a computer or software issue): I have been driving the same routes weekly, at a max of 2,000 rpm (approx max 69 or 65 mph) and when I used to refuel every 12-13 days, I do so now every 7-8 days. Measured on identical routes.

Dealership did acknowledge that the engine block replacement might have to do with this, but declined to go further. Offered a software update which did not resolve the issue.

Fuel injection clean up was performed in summer of 2021. No improvement.

Any suggestion welcome! Thank you!

 

Was this engine block replacement under warranty due to some kind of defect? If so. I'd press the dealership to get your vehicle back to its original condition.....which includes fixing whatever is causing your drop in mileage issue. If they balk, I'd escalate to Mazda division/corporate.

 

Thank you gphin - yes the engine block replacement was under warranty. I certainly will continue to lush the dealership, but I wanted to get feedback from Mazda forum experts as I am not sure the engine block replacement is the root of the issue. It certainly seems that it has had an impact.

 

Have you manually calculated your MPG(miles driven/gallons) ? The computer calculations are often inaccurate. I would run a few tanks to get a baseline.

 

No mention of when you last changed your spark plugs. Also the condition of the tires and what your air pressure setting is. You might also check what fuel you are using and if it is not a top brand then it will effect your MPG.
You reading are in line sorta with seasonal fuel changes in many states for emission standards. I live in CA and they are the worst! With winter formulas I am lucky in my 2018 2.5 to get in the high 20's in the town and seldom in the upper 30's on long highway trips.

Oh and the MAZDA MPG is very, very ,very ,accurate. I do both a carefully done old school with calculator and compare to the dash. And at least with my MAZDA they are within less then 1 mile per gallon different. In fact I filled up this morning hard math was 30.04 and the dash was 29.65 . But the trip to the gas station was less then 1 mile and I was still on the warm up cycle so MPG was 12.85. LOL

 

I have not. Great idea!
What is perplexing is that I had no qualms with the numbers prior to Summer 2020 - 22-24 mpg street, 27-30 mpg freeway - using '93 gas. Now I am stuck at 19.4 mpg max (freeway) and I am happy if I reach 16-17 mpg street.

 

Thank you Callisto,
I have consistently used '93 gas since I got the car in 2018. I do not think I changed the spark plugs, so this would be a very worthwhile avenue to consider. I monitor my tire pressure weekly, I have not noticed any significant changes, and seasonality has not been a factor since the drop in mpg has been consistent since June 2020. I used to average about 22-24 mpg on a full tank on my routine commute (40% surface, 60% freeway) and my calcs yield 17.6 mpg (computer says 18.0).

The mpg numbers you cite are what I was used to prior to the engine block replacement.

 

You say engine block I assume you mean the short block assembly? Never the less this would not really effect the MPG unless you have a very low and uneven between cylinder compression. You would most readily have noticed this with less power then before the engine changes?

Spark plugs are cheap and easy to change for most. You could pull the easiest one out and take a picture of it and post it? All though by that effort you may as well change them out? HA!. But still post pictures of them?

Beyond this without doing a compression and leak-down test and also a data log of a few ECU operating area's i.e. Timing, fueling, and maybe also a sniff test of the exhaust it is hard to pin point the exact reduction cause in your MPG.

 

Hello Callisto - thank you for your input - it is very valuable to me! I can confirm that I never changed the spark plugs on this vehicle. It has 50,900 miles on the ODO and it lools like I should have replaced (or at least cleaned) the plugs after the first 30k-40k! I am now pretty sure this is the major cause for loss in mpg performance. Embarrassing.

I will update this post once my car runs on new plugs! Thank you!

 

Well I hope we all help solve your loss in MPG???? Look forward to reading what you gained!!



Many people seem to think that spark plugs can run a reticules amount of miles? SURE! But that longevity is at the expense of performance in both power and in MPG. That is why I often suggest that changing spark plugs at about 15-30k miles is better. 50k miles is really pushing the limit imho. This is also very *easy to see on any engine using any brand or type of spark plug on a oscilloscope. Spark plugs are the cheapest way to maintain a good ruining engine. That and regular oil and filter changes.

Now remember there is always people that are going to post they got 50-100kplus miles on a set of plugs and have never loss performance drivability , engine power or MPG. *Its bogus...... LOL