rev up your engines, today I'm gonna help answer a very common question
Scotty all of a sudden my car's getting worse gas mileage what can be doing that
the first thing to check of course is the air filter, I don't know how many customers
I have over, their car was running sluggish getting bad gas mileage, first
thing I did was take the air filter box out and find out it's all clogged with
dirt, check it every once in a while, I know people don't look at stuff much
these days but check the air filter, they get clogged up you're gonna get
horrible gas mileage, and don't fall for any of those foolish claims that some
companies had that say, Oh our air filter will get you better gas much it's a
bunch of nonsense, the filter that comes with the car was designed for the car
and a lot of times if you don't get that OEM filter but you get a cheaper
aftermarket one, it's worse for the car because they don't have as much
filtration area in them, and then they clog up faster so you'll get worst gas
mileage now the next thing about worst gas mileage is, you want to keep clean
oil and you want the right weight, whatever your car calls for on the oil
cap use that because of course the lighter the viscosity, the less friction
the better the gas mileage, I've seen people put in a heavier weight oil than
the car was designed for, sometimes these things you'll get 8-10% worst gas
mileage, especially in a modern vehicle that might use like a 0w 20 oil, that's a
really lightweight oil, it calls for that use that oil, use the correct oil you
find you had your oil changed and your gas mileage went down, probably got the
wrong oil in there, now another common thing that give you bad gas mileage is a
dirty or bad mass airflow sensor, this is the mass airflow sensor from a GM, if you
look inside you can see there's tiny little wires those things can get dirty
then they give a false reading, I have a whole video on how to use mass airflow
sensors to clean cars, it's called make your car run better with a little spray
cleaner and you might do that every once in a while to keep the crud from
building up on them, because if these mass airflow sensors get bad data to the
computer, you can get really bad gas mileage, anybody who
knows how to use a scan tool you can plug the scan tool in and and you can read
the mass airflow data, if you're really into cars hey you can even get a thirty
forty dollar scan tool that'll read mass flow and you might just want to put it
on your car when it's running normally, write down the data of what it's like
when it's warmed up and it's idling in drive and see what the data is, you start
getting worse gas mileage just plug in your scan tool see what the mass airflow
data is and if it's way off, you can try cleaning if that doesn't fix it you can
just replace them, they just bolt on and off they're very easy to change on most
cars, another reason you can get bad gas mileage especially in a late-model car
you could just have a weak battery, absolutely everything is run by computer
in modern cars, if the battery doesn't have enough voltage in it the systems
can go haywire, now it may sound crazy but I've even had customers, one had a
Mercedes that the engine would crank fine, but the engine
wouldn't start, and I do have to admit I spent a reasonable amount of time
checking all kinds of stuff to see why the vehicle wouldn't start, but I found
out that when I put my battery tester on the battery showed that it was no good
and it needed replacing, even though it was cranking the engine pretty fast, now
that would have never happened when I was a
young mechanic, if the battery went that was good enough to get a
old carbureted car going but these modern fuel injected computer run things,
hey it had enough voltage to spin the engine over but there wasn't enough to
get the fuel injection system, and the electronic ignition system to fire up,
soon as I put a new battery in start it up ran like a champ, so as crazy as it
might sound, get your car battery checked if
you're getting worse gas mileage, and while you're at it these machines also check
the alternators and the alternator and the batteries work in conjunction so you
just want a test of both, the whole thing takes like four minutes with one of
these machines, now the next common reason I see that cars get worse gas
mileage has nothing to do with the engine itself, has to do with the transmission
the transmission does exactly what it says, it transmits the power of the
engine to the wheels to make them spin, if it's not shifting into all gear
smoothly you're gonna get worst gas mileage, and if it's slipped you're gonna
get worst gas mileage, the transmission has a lot to do with your gas mileage
and with modern computerized automatic transmission a lot of times it's a good idea to pay a
mechanic like me to run a transmission scan with our fancy scan tools, I've seen
cars like this Mercedes behind me pop up four or five different transmission
codes when you hook that up, and then I explain to the customer, that's why your getting
bad gas mileage your transmission isn't working right, now the
next big reason you can get bad gas mileage is because the problem in your
braking system, the brakes are supposed to just do nothing while you're driving
down the road, then when you step on them they're supposed to squeeze the rotors
or the drums to make all the energy of the car turn into heat and stop the car,
which is great when they work, but they get old a lot of times the brakes will stick
on them, all you got to do is jack the car up
in the air spin all four wheels, if you see some of
them they drag or you can hardly spin them at all, your brakes are drag and it would
be like driving with one foot on a brake and one foot on the gas, now years
ago I had a customer who's getting bad gas mileage in a BMW, I checked all kinds
of stuff out and I really couldn't find
anything wrong with the car so I said, okay let's go for a road trip
you drive, so he's driving his bmw and I watched him, and he had one foot on the
gas and one foot on the brake, and I said why do you have you foot on the brake,
he said well they taught us that way, one foot
on the gas one foot on the brake, so I made up some flimsy excuse to him and I said
well yeah let me go behind you and I'll follow you and see if your car looks
weird, well the only thing I was looking for to see if his brake lights are on
and the whole time he was driving down the road he had the brake lights on
which means he was driving down the road with one foot on the brake and one foot
on the gas, and of course he was getting bad gas mileage there was absolutely
nothing wrong with the car, I couldn't find anything wrong with that at all
it was cause he had one foot on the brake and one on the gas, and sticking
brakes will do the same thing of course, now the last common thing for getting
bad gas mileage is using the wrong fuel in your car, most modern cars are made to
run on plain ol regular gas and they run fine, but let's say you've got a car that
needs the super octane gasoline, if you put in regular gasoline at least in the
modern ones, it'll run okay but it will get worse gas mileage, it won't be as
efficient, and vice versa as crazy as it might sound,
if you put high-octane gas on a car that's made for normal gas, you can
actually get worse gas mileage, the thing about
high-octane gasoline is, it burns slower it can take more pressure too before it
explodes, a high compression engine has to have a fuel that won't ignite too
early, where a low compression engine doesn't have to worry about that, you put
high-octane gas on a low octane engine, it's not gonna destroy anything it's not
gonna make it ruin any better either, but you could actually get worse gas
mileage doing that, hey the engineers design these things to run as well as
they can on whatever fuel they're supposed to run on, don't mess with the
way they designed it, they know what they're talking about when it comes to
that, they have to work their butts off to try to get these cars get the best
gas mileage they can, and if you go against the way they designed it that's
a foolhardy move, now I bet somebody's gonna say, oh Scotty doesn't know what
he's talking about, I got a really old car it was made for regular gas and I
put high test gasoline on it, and it actually gets better gas much, yes that
can happen but here's why it can happen, it can happen because your old engine
hasn't been taken care of and it's building up with carbon inside, and as
the carbon builds up, carbon takes up space, so if it takes up space inside the
engine that increases the compression ratio and makes your engine to be a
higher compression engine now, the more carbon that's inside there
the less space so when it gets compressed by the piston, the pressure is
higher, so you're actually driving a higher compression engine in which case
the higher test gasoline will make it run better, but if you took such an old
engine apart if you wanted to do a physics test, clean all the carbon off
made it whistle clean and put it back together, you'd find that it would get
the same gas mileage that it always did using regular gas, and if you put high
test in, it wouldn't make it run any better that's just kind of bizarre
consequence of a carbon up engine, things are not as cut and dry as people often think
they might be, you gotta understand the whole picture if you're gonna talk about
an aged engine all the things that can happen inside them, but for a normal car
modern one you've taken care of, it says use regular gas, use a regular gas
you'll get the best gas mileage using that, so now you know reasons that your
car can get worse gas mileage, and what you can do about it, so if you never want to
miss another one of my new car repair videos, remember to ring that Bell!