Sudden massive drop in fuel efficiency in my 2014 Honda Brio

Historically, I have been getting around 15-16 kmpl in city drive and on highways, it used to be around 18+ kmpl. However, since the past few months, I have seen that the mileage has dropped significantly to <10 kmpl (tankful to tankful) for no apparent reason.

BHPian palsaumik recently shared this with other enthusiasts.

I own a Honda Brio i-Vtech Petrol EX MT manual variant (2014 model) and has been running flawlessly all these years. Currently, it has clocked around 58,000-odd kms on the odo.

Historically, I have been getting around 15-16 kmpl in city drive and on highways, it used to be around 18+ kmpl. However, since the past few months, I have seen that the mileage has dropped significantly to <10 kmpl (tankful to tankful) for no apparent reason. Last weekend, I dropped off my car to Whitefield Honda in Bengaluru highlighting this issue and asked them to specifically:

1. Check and clean the fuel injectors and throttle body
2. Replace all the spark plugs
3. Replace existing synthetic engine oil with new synthetic oil (part of usual service schedule though)
4. Check clutch plates (if it needs replacement)
5. Clean fuel tank of any debris / deposits
6. Check fuel pump
7. Do wheel rotation and alignment
8. Check for anything / anywhere else that could be a potential problem area leading to low mileage

The car came out totally clean - there were no injector clogs, throttle body was quite clean, clutch plates are still good to run with for many more kms, etc. They also checked the ECU / ECM for any error logs but couldnt find any. Basically, they declared that the car is healthy and were baffled by the low mileage. Net-net, they couldnt pinpoint any other area which could lead to this issue.

I then went for another tankful-to-tankful test and I got 8.33 kmpl - indicating a further drop in mileage .

As a last resort, I reached out to a local mechanic and they also couldnt effectively pinpoint to any specific issue once I told them the above items have already been checked.

Just for everyone's information:

• There are no engine knocks - in cold or hot engine conditions
• There is no black smoke on hard acceleration / revving
• However, there is a considerable lag in acceleration in 2nd and 3rd gears since this issue came to my notice

Can the more experienced forum members here help in terms of where else I should look at for potential problem area(s)? Any advice to resolve this issue will be of great help.

Here's what BHPian vishy76 had to say on the matter:

You seem to have crossed everything out on the fuel and spark front. But have you checked the MAF sensor and the O2 sensor?

A bad MAF sensor can cause the ECM to think there isn't enough air and dump fuel into the combustion chamber. A bad O2 sensor giving off erratic readings can also do the same. These two are very common reasons for bad fuel economy.

Take the car to a mechanic, hook up a scanner and access something called live data. As the name suggests, this will give you an option to monitor hundreds of engine parameters live including the MAF and O2 sensor readings and even the fuel trim data, indicating how the ECM is controlling the air fuel ratio. You can diagnose what exactly is causing extra fuel to make its way into the combustion chamber based on this data.

Here's what BHPian Indian2003 had to say on the matter:

It could be a dirty air filter or binding brakes. It is not up to you to tell them what to do. The shop should look for the fault and fix it for you. There is also a temperature sensor in you intake manifold that regulates the amount of fuel for cold starts.

Just a few months ago I noticed my car drinking diesel and loosing power after about 20-30 kilometers of driving. Everything was fine when I started the car but after a few kilometers, the car would loose power and brake by itself at traffic lights. The car would not even roll down a hill.

The fault was with the front right caliper that got stuck when the brakes got hot. The brake rotor was blue on the outer edge.. I replaced both calipers and the problem was gone.

Here's what BHPian longhorn had to say on the matter:

Check if there is a leak in the fuel lines. Being petrol, it evaporates very quickly. Check for cuts or fraying on the hoses. It could be on the engine side or the tank side. Have both the ends checked.

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