In getting about 14 mpg right now, but a few weeks ago I was getting 17-18, and I haven't been driving any differently than normal, I have a 305 tbi which is all stock except and open element and aftermarket muffler with 2.5" tailpipes, so neither of those should affect mpg, especially not in a bad way, so do you think there's something wrong? Or was it just thirstier for gas this week?

 

was the gas cap on tightly? did you fill up at the same station you normally do?

 

Yes the gas cap is always on tight because I know that just loosening the cap releases the fuel pressure through the whole tbi system, and yes, I work at a gas station so I always fill up before work when I need to

 

hmmm idk id go one more fill-up and see what it does before i started looking for something wrong

 

I'm going to do that, I've been planning on a full tune up in the spring, hopefully that's all it will need, I don't think it's bad enough to say it could be the o2 sensor

 

ok well my gas tank had a crack in it for years and it didnt affect my gas mileage so that would be the same as the gas cap not sealing. so that is not your problem.

Either your in need of a tuneup. your not at a full misfire yet but the engine is just going thru more fuel.

your oxygen sensor is bad, or your Coolant temp sensor is bad. telling the engine it needs more fuel so your getting overfueled.

your catalytic converter is plugged up, makign exhaust gases back up into the engine, heating up the combustion chamber and burning more fuel.

 

Any easy way to test an o2 sensor? And would any of the cause me to fail emissions? Because I just passed that a few weeks ago

 

best way to test o2 sensor is to change it. its only 20 bucks from your local autoparts store. althought i would spring for the ac delco 1 wire which is a little bit more

 

I was going to change that when I did a tune up because they aren't too expensive, it might have to be before spring if this gets any worse, I feel like the problem would be worse if the o2 sensor was bad, but I don't know much haha

 

Is the car running cooler? If the thermostat is stuck open you could be running too cold, causing the engine to stay in open loop, therefore using more fuel.

Same basic deffinition if your coolant temp. sensor is not working properly

 

I'd suggest using seafoam to treat the engine and gas tank. As far IMPROOVING MPG, better exhaust and spark plugs are a good Idea.

 

Nothing that caused a 25% drop in gas millage in one tank will be fixed by seafoam....

 

Correct. But it certainly wont do any harm.

 

Could check for exhaust leak before the 02 sensor, too.

 

Well I know there aren't any leaks from the muffler back, and I plan on spark plugs and full tune up next spring, hopefully there's nothing wrong because my budget is tight and time is limited due to school and work

 

Very odd. These cars get great MPG. Not fuel eficiant, just restrictive. I don't know why you would experiance such a drop.

 

It could be anything. you could have sprung a vacuum leak anywhere so small. its really hard to say. Are you sure your getting into OD and lockup? a lot of variables here..

 

Check for vacuum leaks, esp the hose going to the map sensor. Put a vacuum gauge on it, tee'd in the map hose. It should be 18"or more at idle w/ a smooth needle. If it is low check for leaks, check the ignition timing too. Slow timing or no advance will cause a low vacuum condition which will make it run rich.
If you haven't put a t-stat in it in a while, go ahead & put a 195* stat in it. Those are the best for mpg.
Check the ecm grounds, bad grounds will make 'em run funny. They should be on the back of the heads or near the t-stat housing.

 

check for vacuum leaks first. look for lines that may be cracked or have rotted. mine was the coolant temp sensor for the ecm. also check the vacuum on the engine. borrow a timing light and check the injectors pattern. if there is no vacuum leak and the vacuum shows a good smooth readint then it will either be the map of the coolant temp sensor. the coolant temp sensor is the cheapest.

 

What's lock up? And will driving to school with three other people in my car cause this big of a drop? It's only a mile away

 

Its hard to get good mpg on short trips.

 

Lockup means torque converter clutch lockup. When you go into lockup the engines rpm drops a couple hundred more RPMs without that drop you'd be burning a little more fuel. when you get into overdrive and again you cant be really accelerating but more of the cruise, you should see the rpms drop once again after the overdrive shift, that is lockup.

I agree. If your going 1 mile with a fully loaded car your going to lose gas mileage. Right now I am averaging 19 mpg highway and around 13 in town or so.

 

Well I've only had short trips in my car, 3 miles to work, 4 miles to my girlfriends, and 1 mile to school, and I dont really go anywhere else, but my mpg has never been like this even tho I only drive short trips

 

smh

 

seafoam could. My friend ran it on his carbed car the seafoam killed all the rubber in the carb and almost caused a fire. Fuel leaked out over a hot engine

 

yeah, thats why when you have seafoam you do as the instuctions say not to lol! Ifyou have a carbed setup, if you really want to clean the engine out, POUR IT DOWN THE CARB!!! I did it with my 1990 silverado, white smoke all over the o'reilley parking lot... it sigificantly helped! mpg's didnt get any better, but it sure ran much smoother. I drove the **** out of that truck, and wished I kept the TBI 350. It had all the emissions delete stuff, (which supposedly means it came from canada) Good luck my combined mpg in my 88 RS 305tbi-T5 is 15mpg... 14 isnt bad. I got 8-10mpg out of the 350TBI in my truck

 

Well I'm down to 12 now

 

You might have a leak.

 

have put 404 miles on my car this month. after changing the coolant temp sensor and bleeding the cooling system one more time i got mine doing well. went from 14mpg back up to 21 to 24 depending on my mood. the first 200 mile trip was 21mpg doing 80 all the way. this time i only rolled the windows down about half way and ran 70 the whole way. 24.5mpg. might give it a try. i also only use ac delco sensors.

 

About 2 months ago I needed a new radiator so I got new hoses too and flushed the system and put new coolant in, so I dont think doing that again will help, I'll probably get some new sensors when I do a tune up in spring though

 

dang i been having the same problem with my ford ranger, its got the 2.3 and im down to 15mpg, and every sensor is new, new distb/module/coil/wires/plugs no cat egr blocked off, fuel pressure is good, compression is good..... im down to the fuel injectors on it but its stupid that i get 18-21 in my maro and only 15 in the ranger..... i got to get that thing fixed

 

That's because it's a ford haha but there's deffinetly something wrong if you're getting 15 mpg

 

Maybe your not burning fuel well. Are you running rich or having to give it more gas to get up to speed?

 

im running lean according to the plugs and the fact there is no trace of any black "soot" in the exhaust ...... i was hopeing that me and the O.P.may have the same problem, but it looks like he has some sensors and all to try and maybe even a tune up, and i have done all of that ...... got to love it

 

suppose this would all be relivent to me with a 87 firebird 305 w/4bbl quadrajet? Gas mileage seems way worse than it should, i also have black soot coming from exhaust, running to rich? First f-body, and new at working on cars, so its a learning process for me

 

sounds like it could be rich, if you are new to cars you sure dont want to start by rebuilding your q-jet .......... try making a new post in the general engine section

https://www.thirdgen.org/forums/tech-general-engine/

 

alright i will do that, I was told b4 that it was probably "running rich" i have to go to work now so I will post where u said too, is it as easy as tuning the carb so it less rich?

 

Definitely running rich and just wondering but why do you have a 4bbl on a 305?

And 89RS mine might not be sensors, it might be the short trips to school with an extra 450 pounds in the car, plus all my tools for work

 

Wow. Your really putting on weight.
But seriously, what do you have in your car?

 

3 brothers and tools for work

 

That'll do it.

 

Between the last 5 fill-ups ive dropped from 18-15 and i only do highway miles for the most part.

 

Colder air temps & winter blend fuels really hurt mpgs.