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!
