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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).

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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. – Pᴀᴜʟsᴛᴇʀ2 Jul 10 '14 at 12:42
Could you clarify what was done as part of the service? – Zaid Jul 10 '14 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. – I have no idea what I'm doing Dec 7 '15 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 Jan 12 at 14:58

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?

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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 Aug 22 '14 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. – Allan Osborne Aug 22 '14 at 18:49

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.

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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.

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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.

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A long shot, I'd say. It could also be at least 5 other things. – I have no idea what I'm doing Dec 7 '15 at 13:06

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