I still have a couple of problems with my car. I can't seem to get more than 150 miles per full tank of fuel... My car is a 1988 Pontiac Trans Am GTA with a 305 TPI, 4 speeds automatic and 3.27 rear. The car have 115 500 original miles and never rebuild.

I wan't to know EVERYTHING that can have a bad impact on MPG for our cars. I would like to get the same MPG as announced back in the days when it was new.

So far I changed air/fuel filter, spark plugs, spark plugs wires, cap, rotor, PCV valve and I put a bottle of fuel system cleaner from Gunk.

With all this I still have an hard time to get 150 miles (mixed driving) per full tank.

So what else should I do next?

When I start the car and it sat at least 1 day, it run rough at the begining. I turn the key and it starts right up, but a second after it's like it wan't to stall... 4-5 seconds later it idle higher and seem to run fine after 25 seconds or so. After a couple of minutes the idle is normal.

Also, when I take off and the engine is cold, it hesitate a little but when the engine warms up, no more hesitation at all.

I did a 15.56s 1/4. Normal for a 305 TPI with auto trans right?

Help me get everything right so I can save on gas!

Thanks!

 

Make sure the alignment is set correctly, especially the toe-in. If the toe is out it can eat gas like crazy. Also to get the EPA estimate numbers you pretty much have to have a light throttle foot. Another thing that can help is to send the injectors off to have them cleaned and flow checked to make sure they are atomizing the fuel correctly.

 

higher temps = better combustion (at least that's what GM told us all). Make sure you've got the 195 thermo in there.

Also - a new o2 sensor never hurt anything.

 

The car been aligned last year and I haven't drove the car for more than 2000 miles since. I was pretty light on the throttle too on the last tank of fuel.

So far I have 3 things I can check/change:

-Change the O2 sensor
-Change the thermostat
-Clean the injectors and check the flow

Beside what I did and these 3, is their anything else that can affect gas mileage that I should check/change?

Thanks!

 

Those 3 things are about it...besides how you drive it.

If you are easy on the gas pedal it makes a big difference...but I would change those 3 things first...and document your mileage.

 

Your issues in the morning do suggest a good hard look at the injectors like mentioned prior.

Wouldnt be a bad idea to get it on the scanner in the am when u start it and alos after hot while driving.

U mentioned u replaced the 02. If u used anything other than a Delco 02 I would serioulsy suggest replacing it with a new Delco sensor. Long story short aftermarkets are close to, but do not exactly math the factory specs. This has been covered in earnest by people far smarter than I.

Check to see if your car is indeed running around 200ish on the highway according to a scanner. Also see if your MAT and coolant temp sensors are in synch when the car is cold for a reference. Basically lok for things that dont make sense.

Also any old vacuum lines are sourves of leaks as well. Many little things can add up to a lot when they all compound each other.

Another trick is to bump your tires to 35 psi vs 30 from the factory. I tend to do this on road trips and then drop it down if I am racing or driving on the regular, non hihgway streets

later
Jeremy

 

I DIDN'T changed the O2 sensor yet. Maybe I should look into that first of all? But will it help with the problem I get when I start the car after a while?

I don't know if their's a place around me that can check my injectors, I will have to call a couple of places first.

Well thanks for the tips so far!

 

Quebec is not too far from Big Rapids, Michigan. There is a place there that performs excellent injector service: http://www.cruzinperformance.com/

Beyond "Those 3 things are about it...besides how you drive it there are a few others:

Set base timing correctly;

Use the lowest octane fuel you can safely use without detonation. 89 works fine for me;

Keep tires properly inflated at the maximum safe pressure;

Balance the tires;

Inspect brakes for unusual wear and excessive drag;

Maintain your front wheel bearings;

Change the rear axle oil to a synthetic formula;

Use synthetic engine lubricant instead of oil, and use a viscosity that is just adequate to maintain proper pressure when the engine is fully warmed;

Check your EVAP system - in 85°F temperatures, 11% of the fuel can evaporate from an unsealed container in just 10 days. The tank filler cap, pressure control valve, and EVAP purge valve must be in good condition;

Reprogram the ECM to enable Highway Fuel and Spark Modes. The programming is probably there, but the factory didn't enable it;

Be certain there are no exhaust restrictions;

Aviid short trips unless they are grouped together so the engine stays warm between trips.;

Maintain the EGR system so that the EST can advance the timing as much as possible at part throttle cruise;

Maintain the cooling system, and use a thermostat that will allow 190-200°F or higher normal temperature;

Maintain the Air Injection system so the catalytic converter and oxygen sensor achieve operating temperaure sooner;

Avoid using the air conditioning on short trips;

Learn efficient driving techniques.

 

Vader,

I have two questions.

Can a malfunctioning EVAP canister ( mainly pressure valves) have that much of an impact on mileage? I'm missing the vacum reservior ball that would supply one of the pressure valves.

Also, I have my original air diverter solenoid, could that be causing me to loose mileage?

Ok, three questions - I think I know the answer is no to this but, can a faulty sending unit in the gas tank have any impact on fuel consumption? ( Even if the Fuel pressure is ok?)

I've been on this board for years ( maybe since 99') and never heard a really good answer on these questions. I hear of people with modded cars getting 20+mpg, I never get more than 14city 15/16 hwy on a good day - is it the crappy oxygenated/methanol fortified california gas?


Looking for the mechanical key to mileage, everything else on my car is in good working condition except those things mentioned.

Vader looking for you're expertise with thirdgen's

Regards....

 

oh yah, the pressure valves are connected to an engine vacum source, no lines are left open ( I thought I might of given the impressiont that something was left unhooked) sorry..

 

The EVAP system is designed to capture volatile vapors that may evaporate from the fuel tank, then allow them to be purged from the canister and burned during cruise conditions. If the tank pressure is not controlled so that evaporating fuel is routed to the canister, that fuel can be lost to the atmosphere. Coincidentally, if the tank has other leaks (from a loose or damaged fuel filler cap, failing tank pressure safety valve, or damaged vapor hose) the evaporating fuel will escape. There is information that indicates up to 11% of liquid fuel can simply evaporate from teh tank if it is not sealed. That's like voluntarily paying 11% more for fuel.

The vacuum reservoir is not used to maintain the valve position once the engine is turned off.


If the diverter valve is leaking internally, it is possible that some injected air could leak into the exhaust manifolds after the ECM has entered Closed Loop Mode. If that were the case, the oxygen sensor would tend to indicate an artificially lean condition, and the ECM would tend to enrich the mixture to compensate, effectively lowering fuel mileage.


No. Mileage is the distance traveled ÷ the volume of fuel used. The operation of the level sensor doesn't affect the rate of consumption.

 

Fuel filter. I changed mine and noticed easier starts and I just drove 150 miles on about 6 gallons of gas (yeah, awesome!).

 

Unless I missed it above, why shoot for factory ilage? Free up the inake and exhaust, get=ito the engine...so much more can be done, if you get "outside" the factory box...that said, you do have to cnsider how long it will take you to recoup 2K in parts, vs.fuel cost......but hey, I'd have to give anyone who drives a V8 and is concerned with fuel milage a big...all said and done, I'll let you figure out how much savings is there between averaging 18MPG and 22MPG......not much when you can buy a toss-a-way KIA for less than a grand, and get two/three/four year of running time at 40MPG's.

Hmmn, anyone tell me how to get my LS5 in my '57 into the 20's?

I don't like the power...just wanna cruise some...

Ya, Ya...forget it..don't need an answer.

 

tire pressure

 

Fuel mileage is pretty easy to take care of. Do as the above mentioned and drive it easily and take results, make sure you have distributor set correctly too. Next get into ECM tuning. With a rebuilt 305 with many aftermarket parts or ported originals I am getting 25 in town and close to 30 on the highway with a t5 and 3.45 gears.

 

hah!!
Yeah, make sure you inflate your tires to the tire pressure number printed on the side of the tire. That gives you the best treadlife and millage!!!

I had a guy continue to tell me that even after I proved him wrong by showing him the car manafacturers required tire PSI on his door sticker. He said, "yeah, but the tires im using say 44 PSI on the side of the tire!!!" I guess he didnt understand the MAX part of that.

 

Would overinflating the tire a little actually hurt mileage though?

 

a few psi, no
14 extra psi like this guy had, YES!!

 

Explain how too much air in the tires can cause a decrease in fuel milage.

 

Hmmnnn...kinda what I figuered...

 

Hmmm, don't really know myself! The tighter the tire is pumped up, the less rolling resistance. Of course you would be buying tires more often cause going too far overboard will make you lose the flat "traction" patch. You will be riding on the center part of the tire, the opposite of underinflation where the tire is cupped.

Also, the EFI is in closed loop mode until the engine is warmed up some, meaning the O2 sensor does NOT come into play during startup. But I would still replace it!

As for injectors, it's really almost cheaper to buy new ones, even if you go SVO. Sometimes they sell takeoffs for ~100 for the 19lb./hr. 305 ones

BTW nice 1/4 mile times for a 305, especially with an automatic tranny! These cars never cease to amaze me! Sounds like yours is still healthy.

 

Someone think that the IAC could be the problem when starting after a day or more?

And to save on gas, what should I do next. In my first post I said everything I did to my car so far so please check up if you didn't read.

Do you think the injectors and O2 will bring everything up to normal when I replace them?

 

well if you must ask.........
As the wheel of a properly inflated tire tilts during a turn, the tires sidewall flexes instead of pulling the tread from the road. Thus, the soft radial tire sidewall flexes more readily over various road conditions. The tires flexing sidewall results in a smoother ride, better fuel milage and less rolling resistance. Lower rolling resistance = increased fuel mileage. A over inflated tire will suffer more from loss of traction and premature uneven wear than it will from poor fuel mileage. Maybe not a major loss, but its there.

Mike, I had to cheat and copy some text from my old repair textbooks to figure this one out. At first, I couldnt think of where I had heard that. Im not talking monkeyspank either, its right out of a automotive service book.

 

things that help MPGs:
proper tune up
timing advance (tuned to the gas you are using)
synthetic oil in the rear diff can help
roll on the throttle
avoid jack rabbit starts
coast when a red light is in sight
shift kit in the tranny (properly adjusted/shifting tranny)
aluminum drive shaft
waxing the car (helps wind resistance)
properly inflated tires, a few psi over stock helps but becareful with that
sometimes using the ac helps, sometimes it hurts
good shocks/suspension/alignment/brakes

some of this is extreme, but it all adds up

also try indexing the spark plugs
in some cases that might help

also an msd inginition box (i've personally noticed a few more miles per tank on all of the cars i have installed it on)
perhaps a high output coil too
cleaned TBI/TPI/CARB

I'm actually trying to see how many miles we get to the tank with her formula 350, it's an aftermarket tanke that for some reason only accepts about 16 gallons.
she constantly uses the air
so i'm waiitng to see
and she's a lead footer too
so if we get 150 i'll be impressed

 

Make sure you don't have any exhaust leaks upstream of the O2 sensor, and that you have a good O2 sensor.

Another thing to check is if the diaphram in your fuel pressure regulator has ripped and leaking gas into the vacuum line.