Culdee

Greetings all...

This is my first post on these message boards. I purchased a 1982 Trans Am about a month ago for $2,800. I'm a new college grad and it's my first car, so I'm excited about fixing it up. I've so far been doing great fixing/replacing things like coolant temperature sender and sensor, heater core, etc...but one thing that I'm not sure is normal or not is the car only gets about 12mpg city, 16mpg highway. Here are the appropriate specs:

1982 Trans Am
LG4 V8 engine
4BBL carburetor
Automatic 3-speed transmission
Catalytic converter was removed
The engine was rebuilt 40k miles ago, but not modded as far as I know

I noticed an older thread from last year (July 2013) that involves another 1982 Trans Am with a similar fuel mileage issue, located here.

I feel 12mpg city/16 highway is low based on that previous thread, where the rough consensus was 15+ mpg city and 20+ mpg highway, "easily". It also seems low considering I drive the car granny style with only a very rare hard acceleration.

Also, a commenter said in post #8 of that thread that a distributor with a mechanical/vacuum advanced timing would provide a significant improvement to fuel economy. I can confirm that my vehicle does *not* have a mechanical/vacuum-type distributor. I believe it still has the factory concept High Energy Ignition (H.E.I.) distributor with Electronic Spark Timing (EST).

Would a mechanical/vacuum distributor help improve fuel economy? If so, would it be a difficult upgrade since the current distributor does not receive vacuum?

Another suggestion I read in that old thread concerned the base timing adjustment on the distributor. But according to the factory service manual, I need a special tool to access it. True or false? A little insight into this would also be helpful.

Anything I'm missing?

Thanks in advance!

OrangeBird

iTrader: (1)

If I was getting 12 MPG city with a basically stock 305 I would be be checking into where all the fuel is going to ....

Unless , , , HOW are you driving the car ? You have given great detail and description of the car itself , but fuel mileage questions always involve the one variable seldom mentioned ;

Are ya driving it like a road racer , punching the gas from every stoplight/stopsign asking for all of it's ponies ALL the time ? Or are you being reasonable with the gas pedal and only mashing it to the boards occasionally ? Or maybe even driving it like a granny and getting only 12 MPG ? Thing is , if your major into the pedal and getting 12 there likely ain't much your gonna be able to do about it other than restrain the right foot . But grannying it and getting 12 ? That would be a problem worth looking into for certain ...

sofakingdom

iTrader: (1)

That mileage is unfortunately typical of what such a car gets these days, but not "normal".

15/20 is more like what it should do.

Most likely, besides just all the accumulated "deferred maintenance" you could do that would undoubtedly help, (change the trans & rear axle fluids, tune-up, replace fuel & air filters, get the cooling system working right, etc. etc.) your carb probably has the leak in the bottom from the dissimilar-metal electrolysis that always seems to get them.

Only "special tool" I've ever used to get at the timing clamp, was a short 9/16" combo wrench.

A different distributor would only help if you had disabled the engine computer and replaced the carb with a non-electronically-controlled (without the "E" at the end) model. That advice doesn't apply to you.

Your mileage would be a few mpg better still, hwy esp, if it had the 83-up 4-spd trans in it instead. (700-R4)

Culdee

Excellent. Thanks for asking that. I would definitely say I drive it granny style with only a very rare hard acceleration. 95% of my driving in this vehicle is commuting 3-4 miles to the parking garage where I commute on the train to Seattle. There are a few stoplights, but I accelerate from rest at a slow-to moderate rate.

Also, I recently drove 100 miles on the freeway with an average speed of ~65, and my fuel economy there was 16.6. Again, I was at a steady cruising speed without any stop and go.

Considering the above, does the mileage still seem low in your experience?

I appreciate your comments!

Culdee

Do you say it's 'typical' simply because folks defer proper maintenance, and/or other factors such as the fact that gas has 10% ethanol in it?

Wow---thanks for pointing that out! It helps to know where to start. I know the previous owner did a decent job of staying current with the maintenance since he gave me all the service records, but he may have missed a few of those things.

I appreciate you mentioned a possibly leaking carburetor. I'll have to check it since the car mostly sat in his garage for a decade before he finally decided to sell it to me. Electrolysis can do nasty things to a stationary car.

I'm really glad you mentioned that. I was starting to look at new distributors for sale on Amazon, but as you just said that would be an unnecessary expense (and they are expensive!).

So~~~~assuming the distributor is still factory tuned, is there any alternative tuning that would waste less fuel and restore horsepower? Other sources I've read murmured something about manufacturers being forced to cut horsepower and fuel efficiency to meet impending EPA regulations by 'de-tuning' things like the distributor. If so, is it possible to undo the 'de-tuning'?

I agree. It feels weird to get up to freeway speed without an additional upshift. Do you happen to know if the 1983 Trans Am's automatic transmission is compatible with my '82?

Thanks so much for your response!

Drac0nic

You never really get the engine warmed up in 4 miles. Probably a big part of the issue. Would not sweat it if you get decent highway mpg.

naf

iTrader: (7)

Pull the carb and seal up the well plugs, then come back and report.

Properly functioning with good cap/rotor/wires and plugs you should be at 20 on the highway. A leaky carb may not be your only issue but that problem should be eliminated first.

sofakingdom

iTrader: (1)

The distributor in this system isn't "tuned" at all. All it is, is 3 things: a pulse generator that produces a pulse as the engine rotates through certain points; a spark generator, that makes a spark when the computer says "spark now"; and an 8-pole switch, that sends the spark to the right cylinder.

All the "tuning" is in the ECM. As such, it hasn't changed since the car was built, and while we can all pick it apart and find ways that it's not maybe "appropriate" for whatever we wish the engine would do, it also hasn't changed in any way that would prevent the car from working like it did when new.

The reason poor mileage is "typical" of these cars is partly due to the overall complexity of the system, compared to EFI, which renders it very "fragile" if you will; coupled with the inherent tendency of the Q-Jet to leak out the bottom of the fuel bowl, which is something that carb has been plagued with since it appeared in the mid 60s. Since your car is literally an antique (32 yrs old), expect it to have all manner of old-age problems, including both "deferred maintenance" and just long-term deterioration of all manner of things. It's not like there's some one magic bullet you can shoot at it and restore it to like-new operation.

The changes in fuel, driver expectations, speed limits (gotta remember: when that car was built, there were NO roads in the US with a speed limit higher than 55, meaning, its "design envelope" ended there, quite different from today), other cars on the road which when that car was built it was among the fastest there were but nowadays a Kia minivan will kick its butt, and so on.

A 4-mile commute is about the most severe duty you can put the car to, besides deliberate abuse (racing). The choke probably doesn't even pull all the way off. That all by itself might even be a factor in your poor city mileage. But I'd tend to credit electrolysis for the bad hwy mileage, as that type of operation (high vacuum) is when it will create the worst leakage around the plugs.

If I had just bought a car like that with a history like that, the VERY FIRST THING I would do, would be to check those plugs.

The later-model transmission is almost compatible with your car. You'd need to drill 2 more holes for the crossmember (the one you'd need mounts a couple of inches rearward of your existing one), the crossmember itself, and a shorter drive shaft. All of which are very common and easy to obtain. In fact you can likely do so at a profit to yourself: people who want to put Turbo 350s in their car will pay GOOD money for the parts you have.

Culdee

Thanks so much for your response! Alas, I get about 16.6mpg on the highway.

Another commenter pointed out in post #8, driving at 65mph on a 3-speed hydra-matic that was only designed for a 55 cruising speed isn't going to yield ultra high mpg. However, my intuition tells me I should at least be getting 20mpg, not 16.

Culdee

Thanks for sharing that! I agree that I should be getting at least around 20mpg on the highway. I'll start running through the checklist you gave me and report back in a few weeks if that helped.

Thanks!

Culdee

Fantastic. Thanks for sharing that. I didn't realize the ECU was doing the tuning. The car is more advanced than I thought it was. In this case, I'll just leave the tuning alone since it isn't going to be the cause of my low mileage.

Your answer here directly addresses my question. Thank you. I'm glad to know that there is a checklist of items I can address (plugs, carburetor electrolysis, etc..) instead of a 'magic bullet'. I'll start working through that list starting with the plugs and carb, and report back on if it at least partially restores the 15city/20hwy I'm seeking.

Great comment! I am indeed pushing it beyond its design envelope when I drove it on the freeway once at 65mph. At some point I may try driving at 55 mph to see if that allows the vehicle to reach my expected mpg.

Yikes, I thought a short easy commute was gentle on the old girl. Well, I'll add the choke shut-off to my list of things to check.

I'm not sure if it's relevant, but I just put in brand-new valve cover gaskets, but the car still belches a mild to moderate burned oil/exhaust smell from the engine as I drive. Could that possibly indicate anything? My intuition is to just attribute this to it being an old engine and that's what they do. It still has good oil pressure so nothing damaging is happening.

I'm glad you added emphasis. I'll certainly take a look.

Great! Thanks for sharing that! So why is my current transmission and trans accessories worth 'good money' to some? Wouldn't it be considered a downgrade? Is it something about compatibility with the Turbo350 engine?

knight2

I know this thread is a couple of years old now but I thought I'd add my two cents.
I too have an 82 4bbl trans Am with turbo 200c.
Is the torque converter clutch working on your ride?
That too is there to avoid tranny/engine slip during cruising to aid fuel economy.
Also knowone has suggested a vacuum test. VERY easy to do and will also highlight the engines general state and condition.
Also if the engine has a mild oil smell, then try adding a quart of marvel mystery oil to the oil. It will clean up things and remove and crud that could be causing your 'issues'.


..Kev

Culdee

Hey knight2! Thanks for your reply!

I've been in Shanghai the last year and haven't had a chance to work on my T/A back in my garage.

But I'm back to the States now and starting to drive it again. It's still likes to eat gas like there's no tomorrow.

I believe the torque converter is working well. Unless I keep my foot on the break at a stoplight it's more that eager to start rolling forward.

As for the vacuum test, this is something I haven't tried yet. Forgive me for being an amature, but what are these easy methods to check that vacuum is okay? I pulled off a tube and felt is sucking hard but that just means there is some vacuum, not necessarily good vacuum.

Now it does have a little oily smell. I figured that's just what older American cars have from those days. I put on new valve cover gaskets but that didn't seem to fix the oily smell. I'll give it some of that Mystery Oil you mentioned and see what happens.

Bottom line is I really want to help mitigate this low fuel economy. Not just to get the mileage up, but because low mileage usually means stuff isn't at optimal performance.

3rdgenparts

iTrader: (8)

I have had more '82 t/a's then I can remember lol


The thing is you have a 200C 3 speed trans with NO overdrive = THEY SUCK DOWN GAS !!


If you want something economical in gas, get a 4CYL beater car


And other advice, Dump the "200C POS" and go with a built TH-350 or 700R4 to get overdrive and save on gas. Only this is you have to change some parts to adapt a 700R4 in a 82 car.


Dave