i did a search but couldnt find a straight answer so if anyone can clear it up for me id appreciate it. its a 305 TPI and its pretty stock except for tri y headers, gutted cat, K&N filter and a shift kit. this is my daily driver and i do alot of driving (about 75% of my driving is on highway). just this past week i literally went through a full tank daily averaging about 12-13 MPG like i said mostly highway at about 65-70 mph. i dont have a heavy foot and i even have my car idleing somewhere between 500-1000. its also just hit 25,000 on the odometer on a rebuilt engine. would a tune up fix it? or what could i do?

 

Are you throwing any codes? That mileage is horrible!!! I have a 1982 Suburban with a carbed 350 that gets better than that...even pulling a boat! Somethings wrong. It should be running really rich. Have you pulled a spark plug?

 

code? sry idk what that is :/. i bought the car at 15,000 miles and havent done much but oil changes and replaced all the spark plugs. just did a synthetic oil change on both engine and drivetrain. and a tranny oil change.

 

My 91 WS6 gets 11 mpg when you take the cat off you run rich. but we still shouldnt be getting that crappy mpg.

 

Does your check engine light ever appear? If so, take it to an Auto Zone and for nothing they will hook up a computer diagnostic analyzer. It will give the viewer specific codes that tell the owner the problems he needs to address. These codes will be specific numbers that coincide with a GM manual. Auto Zone can tell what codes they are. The whole point of the free analysis is to be able to sell you parts. Not a bad deal for Auto Zone or car owner.

Another good place to start is looking for vacuum leaks. They are mpg killers. Purchase a can of starter fluid. Using the little hose that attaches to the end of the nozzle, start spraying little amounts around the intake such as the union between the base and the TPI tubes, around the union between the throttle body and the plenum....just anywhere where a vacuum leak could occur. Do this while the engine is running. If the engine speed picks up, you have located your vacuum leak. Get if fixed. See...your computer has sensors like the MAF sensor telling it what adjustments to make. If you have air leaking in from sources that aren't supposed to be letting air in, your air/fuel management will be all screwed up.

Check your timing. Here's how...
https://www.thirdgen.org/settingtiming

Replace your oxygen sensor or sensors.

 

dont fell bad. my acr was getting 11mpg. but my car isnt running right. i ahve high idle at 2000rpms in park and nuetral, and engine light is on and its code 51( PROM ). before i bought the camaro i did a search for specs. for a granny driver which is what the mpg in the books is based on. it should be around 17 highway i beleive. not a 100% sure.

waht you could do is get another cat, and itll solve it a little. mayeb look into the air/fuel ratio thing. rich or lean.

 

Try going faster, around 2000+ rpm.
Or mess with gearing, with tire size or, sounds like your just out of the range for mileage. Lugging around at to low of a rpm uses more gas.

 

My check engine never is never on so I guess Ill look for a vacum leak. If it's not that what else could it be?

 

Honestly, your best bet is to purchase a scanner. Make sure it will work for your TPI. You need to find out what the computer is seeing through it's input by the various sensors. Here's a short list of what a bad sensor would relay to the computer.

A bad water or intake air temp sensor or sensor wire would make the computer think the engine is cold when it isn't.
That would keep the mixture rich.

If the engine is warm up enough to go into closed loop, any vaccum leak would put excess oxygen into the exhaust and the oxygen sensor would tell the computer to go rich.

A bad plug wire or plug would cause excess oxygen in the exhaust and do the same.

You might have a bad seal on the intake or a hose. Even a bad brake booster leak can do it.

Your throttle position switch could need adjustment.

The factory states that your car should be doing 15 city/22 highway. So obviously something is wrong. A scanner will go along way in solving the problem.

 

where could i get one of these scanners and how do the work? r they pricy?

 

Honestly, I wouldnt put a lot of money into buying a scanner... Go to autozone and let them run a FREE diagnostic test on it (This will pull up "Inactive" codes as well) When you first turn your ignition on, does your "Service engine soon" light come on for a few seconds, then go off?(I have seen a lot of them with burned out bulbs) It could also be your o2 sensor sensing a lean condition (Bad o2 sensor) Your car should get around 17 mpg...

 

alright i guess ill take it today and yea the light does come on when i turn my car on so i know the bulb is good but like u said, it might not be picking it up. BTW thanks to everyone whos contributed cause im throwing away money on gas as we speak lol. Ill be taking her out of town this weekend so i need to find a solution fast :/

 

You should be getting well over 20 MPG hwy. with a stock well tuned engine and working overdrive trans, have you checked compression? The gutted cat can be worse for performance and MPG than a good working cat as the airflow inside is turbulent.

 

i went by autozone today to get it checked out but they said they could only do '94s and up and told me i should take it to a dealer . so ill probably take it tomorrow. as for the gutted cat, i was getting same mpg before i removed it :/

 

Go to a different autozone. Or go to checker.

 

Or tell them you want to purchase a scanner and see how one for your car magically appears....LOL

 

The gutted cat on a system with only upstream o2 sensors isnt going to cause that bad MPG issue your having. Now if you had a downstream o2 sensor behind the cat yeah it could do it. If the light isnt on while your driving then I doubt you have a sensor issue but just by chance do the tri y headers have the 02 port on them that 02 sensor being plugged in and working is kinda critical. What temp is your car runing at? Is it cool all the time or does it get to operating temp? That kinda hwy mileage I would check to make sure your car is getting upto temp going from open to closed loop. If its running cold its not going to do that and it will waste alot of fuel. My carbed 350 gets near 22mpg hwy with no converter lockup the BB in the truck gets 14 when I can stop myself from smashing the gas.

 

im not sure about the tri y sensor cause the car had them when i bought it. yea it does heat up to normal running temperature