what could cause lower than normal avg mpg?

[Tim Bevan]

what could cause lower than normal avg mpg on any car (mine is a 1990 cadillac fleetwood) the digital number will jump around but usually fall around 9.5 i have filled up my tank completely and reset the avg mpg but it came back to around 9. someone told me that 1- my fuel injectors could be partially clogged. 2- that cadillac sell (only at the dealership) some can of fuel cleaner, like the cheap stuff at autozone. to preform the same task. is this true? also could things like map sensor/fuel filter cause me to lose mpg?

Any help to a problem even a little bit related to this helps i gain most of my knowledge buy talking/exchanging ideas from people who have years of experience.

[winterset]

Only cleaner i would use in the tank is techron. Have you looked or listened for a vaccume leak, and are your tires properly inflated.

Other than that, a heavy foot, or excess cargo in the trunk. Remember on 10/1, a 10% mix of ethanol is added to the gasoline at the refinary. That cuts into fuel economy.

Costco has techron on sale now for just a few more days.

[KHE]

9.5 MPG???? Geeze - it should be at least double that - at least into the low 20's.

You may have a restricted catalytic converter, bad injectors or other issues. It might be time to do a compression test.

[Tim Bevan]

it displays 9.5(jumps around) so i dont think it would be tire pressure, light foot but thanks i know that affects it but somethings got to be bad. and cat i never thought of that. a pressure test i thought would only test for warped/ cracked cylinder head what would i look for?

[rockfangd]

the question is, what kind of driving do you typically do?

I do mostly highway with my Eldo and get right around 400 miles to a tank. If I do just highway I can get over 500.

Check your suspension. If the rear is sagging your mileage will suffer

Dragging brake

bad spark plugs

low tire(s)

Bad gas. I never use ethanol fuel in anything. Only non ethanol minimum 91Octane

Possible dirty injectors or air filter

Vacumn leak.

Is the Check engine light on?

o2 sensor can definately cause a drastic drop in MPGs.

Hope all this helps

[Tim Bevan]

thank you i will tweak everything you listed. i usually just drive to work about 20 mins sometimes stop and go, and if the suspension was bad what would cause that shocks i dont think mine is but it that what would let it sag? also i was so worried about not putting high octane in it, i remember seening a news report about the gas stations selling the lower octane for the higher price annyway... so how could i really tell im getting my moneys worth

[Tim Bevan]

okay i still have to look at a few components but yesterday i replaced o2 sensor and filled up my tank. my digital avg mpg reader used to display 9.5 (varied a little bit slowly). now it displays 0 at a stop and increases rapidly when not on gas down hill up to 32 avg mpg, and rapidly up to about 13 on the gas on a flat surface. why would this change so drastically with a new part? could it be displaying a ritch condition? or could i have gotten a bad part? my check engine light (since i cleared codes) is not on but i will check if anny codes get displayed at all, i will also test to see if my o2 sensor is good along with the connector..

[cdgrinci]

What you are describing is how the instant fuel mileage setting works. It doesn't appear that you have your display set to average.

[Tim Bevan]

thank you! sorry about that this is my first of my own cadillac, still getting use to it. im about to go out and flip it to average mpg in a couple mins. and see what it says. but what would the purpose for a instant fuel mileage setting serve?

[Tim Bevan]

I made sure it was on avg mpg and when i started then drove it said 7.5, the after driving a couple block to see if it would change i reset it and it started displaying 5.5-6.5 slowly? does this sould like my (new) o2 sensor could be faulty? i belive it could be some other issue as to why i am getting low mpg. but there is a very noticeable change in the avg mpg even tho it started out bad. and only happed after the new 02 sensor was installed..

[KHE]

thank you! sorry about that this is my first of my own cadillac, still getting use to it. im about to go out and flip it to average mpg in a couple mins. and see what it says. but what would the purpose for a instant fuel mileage setting serve?

It lets the driver know what fuel mileage they're getting at the current monment. I leave all my cars (and truck...) on the "Instant Economy" setting. That setting will teach you to drive with a light foot - although it hasn't taught me though.....LOL.

[rockfangd]

you sure about that? lol. those v8s beg to differ.

I would give it some time and keep track of your miles per tank

[Z15]

You got a drive it for a while for it to be able to calculate average mpg. Going a block or two is not going to give you accurate reading, go 1000 miles or more.

Edited November 23, 2014 by Z15

[Tim Bevan]

will do ill fill up the tank and go for a long drive

[Cadillac Jim]

FPR

[Tim Bevan]

FPR?

[airmike]

that would be fuel pressure regulator

[Cadillac Jim]

Yeah, fuel pressure regulator - that's what I said!

Seriously, a leaking FPR can cause the car to run rich. You may get an OBD code but not necessarily, and in fact usually an FPR won't throw a code. If the car is a bit hard to start when hot, that's another symptom of a leaking FPR.