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Fuel Economy



I own a '77 Corvette. It's base 350 hp engine is more or less stock - everything is correct but some parts may come from older or newer Vettes. For example, the four barrel carb is from an '81 Vette. She was rebuilt eight years ago, and that includes a new catalitic converter and muffler system.

I'm currently getting between 5 and 7 mpg doing city driving; just under 10 doing highway driving. That's well below the average 15 posted for this car.

How can I improve my Corvette's gas mileage? Is something not right with its engine?

Welcome To The Corvette Action Center davidosus!

I can't help you with this one, I just wanted to welcome you to a cool place! :upthumbs

There are plenty of Sharks around, you'll get the help you seek. ;)
Welcome David,
If you have a stock engine you have 350ci/180 or 210 hp. When was the last time you did a tune up on the car??? Changed the spark plugs,wires,timing and adjusted the carb etc.. Also check for gas leaks by the fuel pump ,lines and tank..Hope this helps..
I tuned up my Vette a month ago - spark plugs, air filter, oil filter, oil change, timing, distributer cap, rotor, etc.

Now that you mention it, the inside of my car does smell strongly of gas. I thought it was normal for the engine. I don't notice any puddles underneath my car when I have it parked (could fuel evaporate that quickly?).

Could my problem be a simple gas leak?

davidosus said:
Now that you mention it, the inside of my car does smell strongly of gas. I thought it was normal for the engine. I don't notice any puddles underneath my car when I have it parked (could fuel evaporate that quickly?). Could my problem be a simple gas leak? :confused

First of all, welcome to the CAC - great cars, great people, tons of info and not just in the forums so be sure to check out the rest of the site.:upthumbs

As for your problem, I too had the gas smell in the car and could not locate the leak anywhere for a couple of months. Then on a Sunday morning I stepped into the garage to find that gas fumes had filled the garage. Being paranoid I yanked the plug on the opener and pulled the bay door up manually. When I looked under the car, still no puddles anywhere so opened the hood shining a maglight everywhere and saw nothing:confused. I sniffed the grilles on the rear deck and the smell got stronger so I crawled under the backend and noticed a dried spot of something(?) under the right muffler. The spot was dry but it wreaked of gas. Above that spot the muffler was discolored and, you guessed it, the smell was still stronger. I ran my hand up the tailpipe and finally I hit paydirt - it was wet with gas:eek. I jacked the back end up and dropped the spare tire and removed the entire spare tire tub (top & bottom). Once the tub was removed, the source was staring me in the face; the gas tank had been replaced by a previous owner and no pads were installed. This caused the tank to rub on the straps, resulting in a pinhole leak. At this point I could see the gas dripping very slowly, so slow that most of it was drying before it hit the ground.

The discoloration of the muffler and tailpipe were caused by dripping while the exhaust was hot, causing the gas to evaporate quicker. Though my problem may not be the same as yours, you should do everything possible to find the source of the smell because it isn't normal and fiberglass burns really quick. If everything on the topside of the engine is good you need to go to the gas line and tank. Trace all of the gas line you can get at, running your finger along it. The outside of the gas line should NOT smell like gas; if it does, there's your problem. If the tank has never been replaced, it may be the seal around your sending unit on the tank. Another common problem in the tank area is the rubber hose connecting the sending unit to the frame line as it can rot over the years.

The temp of the car is key to this as a hot car is going to evap any trace of all but the worst leaks. It's best to do this when the car is cold so there is less evaporation. This is a catch 22 as well, because if it's a small enough leak, the lower line pressure may hide the leak. Gas, however, does tend to find a way out of any hole no matter how small or what the pressure is. If you don't find anything, start the car to get pressure in the line built up and shut it off and try again. I looked at a 75 with a friend last year that had a puddle on the intake manifold and bubba owner said not to worry because it would go away once the engine was hot -not the best advice.

If you do find a leak in a gas line DO NOT use the bubba approach using copper line to clamp in a fix. I've heard of several cases where this lead to a total loss claim for an insurance company, since when copper bends, it weakens much quicker than steel. Your best and, most importantly, safest bet is to replace the entire line.

If your problem is the tank or sending unit, there are generally two ways to keep gas from going everywhere. One is to detach the rubber hose from the frame rail and supply all of your neighbors with high octane lawnmower gas or clamp down the rubber hose really tight (if the leak is beyond the rubber line. This may go without saying but I'll say it anyhow, if you have to drain the tank, the gas flows a lot faster if you open the filler cap but have someone else do that after you get the lone in the gas can. I was bathed in gas when I drained mine.

Though tanks can be repaired, many frown:( on this. I had mine repaired but only because the tank was relatively new and had just been installed improperly. A kit is sold to seal the tank yourself from the inside but I, for one, do not recommend this. Though the kits work when done correctly, it isn't very expensive to get the tank repaired professionally. Most radiator shops also repair gas tanks.

Good luck and, again, be careful! Let us know how it goes.
You may have several problems going on at once. You have a 77 with a 81 Carb--Can't remember for sure, but I thought the 81 had the computer operated (electronic) carb. It will not run right when not getting the proper input and signals from a computer that is not there.

Second, check the distributor. The 81 (electronic) would have an electronic advance, not vacuum. If I am wrong, I am sure the 81 owners out there will chime in. That means that you may not have your vacuum advance hooked up (77), or you also have the 81 distributor which means you have no vacuum advance (unless they added the computer as well)

If this is mismatched you will have a serious performance/fuel economy problem.

So, while I can't offer you any solutions, I would at least point you in some important directions, as well as that fuel smell/leak to get it fixed.

If you are only getting single digit fuel economy the next time we hear from you, please let it be because you have installed a blower and are putting out 800 hsp!!!!!:cool
Chris McDonald said:
If you are only getting single digit fuel economy the next time we hear from you, please let it be because you have installed a blower and are putting out 800 hsp!!!!!:cool
:upthumbs I'm with Chris!

You are right about that Computer Comand Control standard equipment on 81s .The system automatically adjusted ignition timing and air-fuel mixture...David looks like you will have to switch out that carb. too.

My brother-in-law has a bone-stock '77 and he said he conservatively gets 13-14 in town and 17+ on the hwy. Something is definitely not right. I'm also surprised you have an '81 carb. Chris is right, I have an '81 and the carb is special for the computer control and electronic advance. On my '81 I get 15-16 in town and 21-22 on the hwy. Honest. The headers and 2-stage Cat help a bit I'm sure.

........ Nut
Thanks for the letters, guys. I really do appreciate the time you've taken to write me. Your advice has been invaluable.

I feel rather stupid - I ought to have gotten my facts straight. Upon hearing that I could have the wrong carb I went this morning to confirm just what was under the hood. It's a GM Rochester Q-jet #17080204. According to my Black Book, that means it's from an '80 Vette. This shouldn't be a problem in a '77 Vette...

Today I'll spend some time with my car to try and locate the problem. I think my troubles are under the hood though the low gas indicator doesn't work (it's not the light) - something could be wrong in the back end, too. This problem has gone on far too long. Time to deal with it. I'll let you know how things turn out.

Hate to tell u this, I have a 1976 Stingray. 90,000 miles on it, then decided to throw a cam with 480 lift in it. I also put on dual quads. I am running off both cabs at once, not progressive. I am getting 16mpg on the expressway =)
gas smell

I too had a strong gas smell and traced it to a faulty EVAP system.
If is not purging properly the charcoal in the canister becomes saturated and cannot absorb any more. Don't get a new canister. After the system is working properly it will "recover".
Also check your gas cap and make sure it is not a vented cap or not sealing well.
I relocated and modified the EVAP canister to the drivers side rear behind the wheel well and got a new vacuum operated inline purge valve from Delco.
I have a line running from the tank to the canister from the canister to the valve and from the valve to a vacuum port on my intake manifold. If the pressure in the gas tank increases the valve opens and allows fumes to enter the canister where the charcoal absorbs the fumes. If a partial vacuum is in effect in the tank the valve again opens and draws fresh air into the canister and purges the fumes back into the tank. When the car is running the vacuum opens the valve and sucks fresh air through the canister and purges the fumes into the manifold.
I no longer have any fumes or partial vacuum or excess pressure in the tank.
My mechanic looked into why my Vette was going through so much fuel. There were no visible leaks so he rebuilt the carb.

There were some surprises. A previous mechanic had blocked off several tubes in the lower plate of the carb with silicon. These were removed as best he could. The float level had been set too high and the throttle mixture screws were way out (between 8 and 9 turns).

I've driven my car for a while now and I'm still getting the same crappy gas mileage.:crazy All that's improved is my cars acceleration - it goes like a bat out of hell.

Any ideas guys? Just why the heck is my Vette going through twice as much gas as it should?

By the way, does anyone know how to determine which egr valve to use? My car is a L-48 but has some L-82 parts. The one on my car doesn't match either.

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