Got my Suzuki Swift Diesel serviced two months back, and all of a sudden the mileage dropped from 20kmpl to 12kmpl and that is confusing! After i returned to Service shop, the guy said that there is nothing he could do about it because there is no such thing which can cause a mileage to drop other than the driving habits.
I personally dug and found that a choked air filter can cause car performance and mileage to drop (15% decrease) as for each gallon of fuel the engine burns, it requires upto 10,000 gallons of clean air. Therefore I cleaned it up (at the time of service the air filter was changed) and tried again to check if things get back to normal. Unfortunately, it didn't help!

So my question is, WHAT might be the causing this as the fuel injectors and everything else I know of is working fine? How can I resolve this?
What other attributes are to be checked to make sure if everything is really Okay?

EDIT: Black smoke does actually is spitted out of the silencer usually when I push the accelerator (NOT too hard though).

  • Have you checked the entire intake tract for obstructions? A misplace rag or plastic bag could be doing the same thing as a clogged air filter. When I say "entire", I mean from intake point to the throttle body. Commented Jul 10, 2014 at 12:42
  • Could you clarify what was done as part of the service?
    – Zaid
    Commented Jul 10, 2014 at 14:21
  • If a sevice shop employee told you that I would never go back to that service shop again, there are many faults that could hurt your mileage. Commented Dec 7, 2015 at 13:06
  • I have the same problem with a 2014 Honda CRV (AWD): highway mileage went from 30 to 22 mpg all of a sudden.
    – sds
    Commented Jan 12, 2016 at 14:58

8 Answers 8


A 'sudden' drop in mileage means a component has failed, especially if it was some time after the service. The first thing to do in this sort of event is to do a leak-off test on the injectors. If an injector has failed this will show it. A faulty injector will allow a lot more fuel through, usually accompanied with black smoke from the exhaust. Second thing, or at the same time, check the electronics for correct operation and presence. Both these proceedures would be best done in a workshop. A further consideration would be the exhaust. Check for damage, or if fitted with a DPF, does it require a Re-Gen?

  • I recently checked on the exhaust, it isn't damaged at all. Instead it does push out black smoke just as I accelerate the car into motion, quite a lot of it too. Is it the injectors?
    – Sandman
    Commented Aug 22, 2014 at 10:35
  • Black smoke means too much fuel. All of the air hoses on the turbo and intercooler should be checked for leakage as an easy first step, as well as any damage to the intercooler itself. Commented Aug 22, 2014 at 18:49

If the drop is sudden and temporary then it is probably due to a active regeneration of the DPF where extra fuel is injected into the cylinders, not part of the power stroke, to create extra heat in the DPF which in turn burns off the particulate mater. The ecu detects a build up of particulate and initiates the process which can last up to twenty minutes. If this happens try not to stop before the process is complete as in some cars this can cause a build up of diesel fuel in the sump oil. This dilutes the oil and the oil level increases. If the oil level increase is not detected then self ignition can cause a runaway engine


Mileage drop from 20 to 12 kmpl is certainly not your fault. And yes, driving habits really reflect your driving style, but 20 kmpl to 12?? I really don't think so unless you are a rally professional. We can start with the following steps. Needless to say, most of them. You might have already done most of it yourself. Hopefully this fixes your issue. Cleaning the air filter would have been my first suggestion which you already have done.

  • Check tire pressure.
  • Check Wheel alignment at a decent place. (wheel alignment can cause a lot of difference)
  • Try another petrol pump for instance and see if the problem still exists.
  • Check if too much smoke is coming from the car.
  • Check the coolant level.
  • Make sure your radiator fan is working. (Just put your car in idle for like 10 or 15 minutes and the fan should start).
  • Make sure the solenoid switch or the turbo assembly is fine.
  • Check if the inter-cooler is fine. (you might need a professional to check this.)
  • Really old cars might have fried clutch pad, which in your case is/ should be absolutely fine because its not old or not so old.

Even after you have done all this the mileage hasn't improved a bit, you can start to doubt the service person's expertise. Make sure you give him a nice shouting. Also try to scare of a few customers who come to the showroom looking to buy a new car by telling them that the service you are gonna get is terrible and might end up with a broken car. Don't underestimate the power of an angry customer. Good Luck.. Hope this helps in some way. And Drive Safe.. :) Please comment if there's any improvement.


Did you know that changing your Oxygen (o2) Sensor could improve your gas mileage? Poor gas mileage could be an indication that you will need to check on your o2 sensor soon. And don't forget the faithful periodic "Tune Up"..W/ a new fuel filter installation.


I have a Hyundai Eon which used to give a mileage of 22 - 24 kmpl. However some time back the mileage dropped drastically to 16 kmpl. The service centre told me exactly the same thing that it is due to my driving habits. However I have been driving from last 9 years and I knew it couldn't be my fault.

Then I visited a trusted local garage. The mechanic drove my car for 5 minutes and told me its a problem with the spark plug. He changed the spark plug which costed me Rs 150 only. And he charged labor fee of Rs 100.

Now the mileage has been restored to 22 kmpl.


Picking up a really old question, but will attempt an answer as I faced a similar issue in my Swift Diesel.


The black smoke under moderate acceleration is likely the key here.


There is engine oil being burnt in the cylinders.

Root Cause:

The Swift diesel DDiS engine needs exactly 3.1 liters of engine oil to be filled in. The service manual calls for oil filter to be changed in alternate services. The oil filter typically retains ~100 ml of oil when taken out, this causes the engine to have 3.1 + 0.1 liters of oil every alternate service. The extra 0.1 liters makes its way into the intake system, gets into the intercooler and gets into the cylinders, causing engine oil to burn and thus the black smoke and low mileage.


  1. Drain oil.
  2. Clean intercooler.
  3. Clean injectors.
  4. Fill in exactly 3.0 liters of engine oil (The oil filter still has ~100 ml in it).
  5. Clean EGR valve.
  6. Reassemble.
  • How does the "extra" oil get into the intake system? How does it know? Do you have any information that suggests this is a possibility?
    – SteveRacer
    Commented Jan 30, 2018 at 5:59
  • @SteveRacer The extra oil likely gets in via crank splash. The above post is based on personal experience in my Swift diesel ~8 years ago. Around the time diesel engine Swift was introduced in India, Suzuki had subsequently sent a correction mail to all service centers, after which the issue was rectified. Commented Jan 30, 2018 at 7:15
  • "Crank splash" would never put oil into the intake unless the rings have failed miserably. Even then, the oil does not end up in the intake. Unless you are speaking of a failed crankcase ventilation system, which is a cheap and easy fix. 100ml "extra" doesn't intake oil make.
    – SteveRacer
    Commented Jan 30, 2018 at 8:16

This may be irrelevant to your issue but your T.P.S. maybe just slightly off, percentage might be fluctuating even if the CEL (Check Engine Light) is not illuminating; T.P.S. maybe the culprit. The T.P.S. is very sensitive, messing with it could result worse mileage for your fuel. I would recommend to have your vehicle diagnose with a different person. There could be nearly any countless things what cause your drop in fuel mileage. For you others out there, this can happen to almost any vehicle. Sorry is all I can give to you.


I think it may be due bad fuel injectors. The problem is in the injectors. You should clean the injectors. Hope this will work for you.

  • 1
    A long shot, I'd say. It could also be at least 5 other things. Commented Dec 7, 2015 at 13:06
  • As a long shot, I'd say it could be at least 5 other things but never injectors. It's not even a long shot. I'm taking the over.
    – SteveRacer
    Commented Jan 30, 2018 at 8:17

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