All over my long driver career, I had 4 cars (5 years).

I always compare my fuel consumption with other people who have the same car. 3 times out of 4, I found out that my cars were consuming more fuel then others (I like buying car in poor condition since they cost nothing and I repair them).

So I think it might be interesting to list here what can cause a car to consume more fuel (not specific to a brand) as I'm pretty sure I'm not the only one who wonder about that.

In my case, I actually drive a Saturn SL1 1998 which consume 12 liters per 100km which I find a lot for a 1.9 liters 4 cylinders...

I tought it might be :

  • Fuel injector leak
  • Old tire
  • Not enought motor oil
  • fuel line leak

But I checked all of this and it seems fine. I also changed my tire for winter's one and still the same consumption, any other idea ?

Also, it would be interesting when listing an Idea to describe how it can influence the fuel consumption and also if this could make a "noticeable" difference or not ?

  • 13
    If 75% of the time, your vehicle is consuming more fuel than its peers, maybe your driving habits are part of the problem? Commented Oct 28, 2014 at 19:04
  • Indeed. Since it happens to multiple cars. It could be as simple as almost only driver in towns rather than the average between 'in town driving' and highway driving. Or mostly being stuck in traffic jams on a highway.
    – Hennes
    Commented Oct 28, 2014 at 19:06
  • I doubt my driving habits are part of the problem (or at least it would not do that much of a difference).. But anyway this is not the point of my question as my goal is to list here as much possibilities as possible for further references. As I wrote on my post, I always bought cars in poor conditions (around 500$) so there is a lot of possibilities.. Commented Oct 28, 2014 at 19:08
  • Funny enough. Having non-alloy wheels apparantly adds more weight to the tyres and cause MPG to go down (probably not significantly). The other thing I can think about is the surfing board/bike/load carrier racks mounted on rooftop with/without anything, underinflated tyres (unnecessarily), worn/very old gearbox
    – ha9u63a7
    Commented Oct 29, 2014 at 10:53
  • 1
    Not putting this as an answer, because it doesn't relate to problems associated with vehicles in poor condition, and because the impact might not be "noticeable", but: since all electrical consumption adds load to the engine, things like turning off the radio and the cabin fan, and even (assuming these systems use a bit of electricity even when not actively engaged) pulling the fuses for some comfort systems like cruise control or the rear window defroster, may (marginally) improve your fuel economy. These should all be very small compared to anything related to moving the car, though. Commented Mar 16, 2016 at 16:06

8 Answers 8


The most common causes (aside from poor driving habits) are

  • bad timing
  • bad sparkplugs
  • low octane fuel*
  • malfunctioning coolant temperature sensor
  • blocked catalytic converter(s)
  • malfunctioning MAP/MAF sensors
  • driving with the parking brake engaged**
  • having aftermarket wings on the car.
  • tyres not inflated to the recommended pressure
  • and a few others I can't remember

*Cars designed for low octane fuel won't benefit from high octane fuel, but cars designed for high octane fuel will perform poorly on lower octanes.

**Usually not because you forgot to release the parking brake, but because it may become stuck. Especially on cars with rear drum brakes.

  • 1
    I had a faulty alternator recently that was placing a significant load on engine - enough to cause my fuel consumption to go from 12 L/100km to 16 L/100km (as reported by trip computer). Commented Oct 30, 2014 at 5:55
  • Faulty oxygen sensor will do this as well. They don't always just stop working, they can give increasing bogus readings until the engine management software decides that the reading is too far off to be correct. in the meantime the bogus readings will throw off the fuel mixture. Commented Mar 16, 2016 at 14:53
  • Very old cars may lack knock sensors, and thus, if designed for high-octane fuel, a low-octane fuel can actually cause real damage.
    – juhist
    Commented Feb 4, 2017 at 9:46

You are driving a Saturn S-Series (My car for the last 8 years, which I am very happy to have.) You should definitely have much higher fuel economy. (19.6mpg is what you have. 33+mpg is easily achieved.)

I would hardly be surprised if it is the ECTS (engine coolant temperature sensor.) These plastic temperature gauges fail with about 100% certainty. This will lower your fuel economy because the engine computer does not think the engine has warmed up, leaving the computer running in safe fuel-rich "open-loop" mode, despite the engine being warm. This is usually the first issue checked on these cars. I had the same problem. The new brass ECTS is about $12 from your local auto parts dealer.

Beyond the ECTS reading wrong, your thermostat may have failed open, leading the engine to not computer warm up either. This does not explain 19.6 as would the ECTS. The thermostat failed open would cause you to lose about 1-2mpg. The fuel loss due to an open thermostat is due to poor combustion efficiency. (i.e. Carnot's theorem: efficiency = 1-Tcold/Thot.)

There of course can be many other issues, including your driving style, but I would start with the cooling system. I highly recommend you join the SaturnFans.com message board. All major issues with this vehicle are heavily documented, including a series of Youtube videos. These cars are very easy to work on. In fact, I will point you towards the New and Returning users checklist.

Edit: Always check the oil weekly or when you add fuel! These engines are notorious for burning oil, due to a bad oil control ring design.

  • 1
    +1 for WEEKLY oil checks. I owned a Saturn and went through oil like nobody's business. It's a shame, the 3 door coupes were cool looking cars, but the engines were SUPER high maintenance for what they were. Also, if I could +1 twice for a reference to Saturn Fans I would. That forum is excellent for S-Series owners.
    – Sidney
    Commented Oct 28, 2014 at 17:58
  • I learned to wrench by reading SaturnFans. It's a very supportive community. Commented Oct 28, 2014 at 18:01

Where you drive your car has a significant impact on the fuel economy. If you drive mostly on highways and other roads with high speed limits and no stop signs or stop lights you will have a high fuel economy, while if you mostly engage in "city driving", that is, roads with lots of stop signs, stop lights, and heavy traffic, you will have a lower fuel economy. My 2010 Jetta used to get 30 mpg where I used to live (where it was primarily driven on highways and wide, clear surface streets) but I now live in a place that has notoriously poorly designed roads and even the highways have traffic lights and I am getting 20 mpg or less.

  • High speed limits will have the opposite effect. For example, just a few weeks ago I had two similar length trips, both highways, on but one one route I had 90-100 km/h average, and the second route had a nice highway with 130 km/h speed limit, which was nice but cost something like 25+% more fuel for the same distance.
    – Peteris
    Commented Oct 29, 2014 at 17:08
  • @Peteris, that certainly is true, but I think that the effects you describe are car-specific. For a similar situation as you describe I only find myself losing about 15% fuel efficiency. But high speed limits can also have a negative effect on fuel economy. What I hear most often is that fuel economy peaks at 100 km/h (60 mph) Commented Oct 30, 2014 at 1:39

In addition to all of the other good answers here, especially bad coolant temp sensor or thermostat, obstructed emissions, ignition related (anything from coil(s) to plugs, timing, MAP sensor, brake problems, etc., here are some other common problems:

Bad EGR valve, oxygen sensor(s), fuel pressure problem or bad fuel pressure regulator, dirty or defective IAC valve, vacuum leak(s) including a bad pcv valve (if you pull it out of the valve cover and shake it, you should hear it rattle- if not it's bad), very clogged air filter and sometimes even a bad ecm. Low compression can cause a lot of fuel consumption, too. This can be caused by a number of issues. e.g.- change your oil+filter as required people! It's so easy and prevents BIG problems!

Vacuum leaks can easily be identified by a smoke machine or even just blowing cigar smoke into a vacuum line (engine off). Watch to see where smoke escapes and if you need to refine the location further, spray some carb cleaner in the area to see if engine idle changes or stalls altogether (engine on and a FIRE EXTINGUISHER READY just in case). That's your leak. Check around intake manifold gasket while you're at it.

Unless there are other obvious accompanying symptoms that may suggest otherwise (like hearing ceramic chunks rattling in the cat or an obvious vacuum leak howl or high-pitched screech), you should always check fuel pressure first. Low pressure could be the pump relay or pump itself, clogged lines, or clogged filter.

Make sure that, if you're in a location with consistent 0-degree-or-below weather, you use at least 5w30 or maybe even 0w30. Best to check your manual if available for that info. Even weather aside, newer vehicles may require lower viscosity oil anyway so check manuals!

Believe it or not but if your tranny is slipping then that can cause more-than-normal fuel consumption, too. I won't go into how unless someone asks, but it should be kind of obvious anyway.

Lastly, failing struts and steering problems can cause excessive fuel consumption for similar reasons as tranny slippage.



Other common thing that causes high fuel consumption is imperfect work of brake system. Brake pistons and pads should travel freely... Just take your lifting jack and rotate the wheels right after applying brake: wheel should be fully released with no delay.

UPD I suppose every sticking brake caliper adds 1 liter per 100 km.

  • This is also a good point. I had reduced fuel economy due to the front rotors dragging. You can often tell if it's a problem by feeling the temperature of the brake caliper. Another symptom I was having was brake pulsation. Commented Oct 29, 2014 at 12:23

OK, I have no knowledge of this specific vehicle.

However, assuming faults fixed, the following I have found do work (on many vehicles over 40 years):

  1. Friction reducers in the oil. PTFE ones gives about 10% improvement; takes a few 1,000 miles to coat the engine internals. Brands: Greased Lightning, Slick 50. Note: as losses in the engine drop, you get more power to the wheels....

You can get these for gearboxes too, but uplift minimal. Good though to help extend the life of a troubled gearbox.

  1. +10% tyre pressure; gives another about 10% but slightly reduces grip and may cause tyre balding in middle. Use with caution.

You could go for low rolling resistance tyres, but they struggle to get +2% in my experience.

  1. On carburettor engines: a powerful magnet (e.g. 'Eco-flow' brand) just before the fuel intake on the carb. Instant effect; however injected engines don't really benefit. This works with all moving fluids (gasses, liquids) which exist as ions (=most). Works well with gas or oil into domestic heating boilers / furnaces.

Improvement varies by design of burners; expect anything from 5% to 20%. Old burners get a decent improvement, new systems less.

** Magnet needs to be clamped onto a metal pipe which goes all the way to the burners ** plastic removes the voltage potential necessary to maintain ion separation (the magnet polarises the moving fluid electro statically).

  1. Lighten the vehicle; don't go mad, just don't carry heavy junk or spoil the vehicle airflow.

  2. Be conservative using / changing vehicle momentum e.g. don't accelerate hard then break hard.

Ultimately, Don't Use ICE !! intrinsically they are c. 20% efficient. When you can, go electric...

One day!


  • Thanks for the answer, but can you be more specific on what you mean by "Don't use ICE" please ? Commented Oct 29, 2014 at 15:03
  • 1
    He means Internal Combustion Engine. Commented Oct 29, 2014 at 15:47

I was also having a similar sort of question few months back, I have a diesel car and I am from India where diesel costs to 1$ [around 62 rs ] per litre.

I made few changes to my habbit of driving, dont overspeed the vehicle, drive at proper RPM, switch off the car where you are about to stop more than a minute, especially at signals and traffic jams.

Proper maintenance of car, servicing, good tyres.

And more important is keep your car clean and respect your car.

It helped me manage my fuel expenses :)


I want to share my fuel consumption story....I have a second hand Suzuki Ignis 2002 1300cc. When I first got it in 90.000km the gasoline consumption was 7.2lit/100km. Suddenly the consumption raised to 11lit/100km. Also I had a lot of emissions. No strange lamps on my dashboard... Everything normal except the consumption and emissions. My mechanic, serviced the car, cleaned the injectors cleaned the EGR and pipes, checked the valve gaps. Nothing. Then exchanged and tested with used: the spark plugs, both lamda sensors, ignition coils. Nothing, emissions continued. So the final suggestion with no guarantee to success was to change the catalyst.The exhaust guy found no trace of catalyst in the exhaust. Debris has filled the silencer. So a new catalyst was installed and the silencer was cleaned. Car runs quieter, Gas consumption lowered to 6.3lt/100km SUCCESS... No more emissions, no more high fuel consumption..

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