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Discussion Starter · #1 ·
So I picked up my '79 Fairmont in Maine this past January and drove it around briefly before having to shelve it for need of time/resources to get some work done on it that it really needed. I just got a good chunk of that work completed last week and got it back on the road. The last time I filled up before today was January 19 and I had gotten 16.4 mpg, though it was a small fill-up at just over five gallons. Today I filled up and had gotten 13.7 mpg over the course of 122 miles. I can guess that part of the reason for the drop in mpg has to do with idling the car while doing checks on the engine. I can also guess that another factor is finding that the electric choke assist wire had popped off and I'd driven it this way for an unknown number of the 122 miles. But I wonder if the gas may have deteriorated enough in the last five months to have had a negative effect on the mileage. I'm a bit concerned initially because I sent the carburetor to Pony Carburetors and had them re-engineer the carburetor and was expecting better mpg for my $400 (though, again, I haven't put many miles on the car since the carburetor rebuild and with the other problems it's REALLY hard to tell). But anyway, back to my question: will the gas deteriorate enough in five months to have had a negative effect?
In the meantime, I'm also going to look at anything else that I can think of that would/might affect gas mileage. Any suggestions?
 

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wait a second, what engine do you have in there again? even the 16.4 doesn't sound right, not even for the 302... sounds low
 

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What quality of gas did you put in there? (brand + octane).... my 302/5spd just got 18mpg (if my math is right) to MN and back.... BTW tuneup would help more.


(302,650cfm 4bbl and a bunch or unknown 'goodies'.... 3.08 gear ratio in back)
I think he was talking more in terms of because the fuel was sitting for a while...

which could be the case. finish running that stuff out, fill it with fresh 89ish(mid to high grade) and replace the filter, usually on the carb, or inline with it, if you haven't figured it out already. that should clear up some of the problem. actually in 5 months, you should be ok, it's when it sits for YEARS that it really has an effect on the performance. the fix i put in this post earlier will cost around $5. did you replace it between Jan, and last week?
 

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Discussion Starter · #5 ·
The engine is the 200 ci six and the transmission is a C-4 automatic. I didn't replace the gas at all between January and yesterday. I did replace the spark plugs, wires, cap, and rotor. I gapped the plugs to .050". The gas I put in was 87 octane Citgo gas. The EGR valve seems to be working right. It's closed at idle with the car warmed up. I raised the RPM to 2500 or more (had to go by ear) and held it. The EGR valve opened, though it may have taken awhile long to start opening and may have been a little sluggish. I can't really tell since I don't have a good experience base to call upon with that particular component. However, a slow or sluggish EGR valve shouldn't have a negative effect on mileage as near as I can figure. I also checked for EGR valve leakage by using an infrared thermometer. With the EGR valve closed, I got about 240 degrees farenheit on the EGR spacer right where the valve bolts to it and a bit more on the EGR tube as near to the spacer as I could shoot it. With the valve open, the values went up about 100 degrees farenheit and were still slowly climbing when I stopped. The EGR valve closes plenty quickly, by the way. Is the EGR valve leaking? It would seem that the answer is "no" unless it's only leaking a little when shut.
Anyhow, from what I gather from the responses to my question about the gas, it looks like the answer is "yes, it probably did deteriorate somewhat and did have at least some negative effect on mileage".
I'll continue to keep an eye on the mileage and looking for anything that may be hurting it and I'll keep y'all posted and ask for advice as time goes on. Thank you all for being there and for being so willing to help!

P.S.
Future plans call for a fuel-injected 5.0, but I'm unemployed and can't afford that upgrade right now. In the meantime, I'll have to do the best I can with what's currently under the hood. Bye for now!
 

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One more thing you could look at. Check your timing, as the engine wears and your timing could be off a pinch and now with the carb. all freshen, you maybe loosing some performance. Also, I would agree with the fuel going bad and cause poor mpg. Even check your tire pressure....it all can add up.
 

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if you can manage to get it timed, tuned and some fresh gas, you should be getting mid to high 20's. when i still had my 200 i was pulling around 27MPG highway.

another thing that came to mind is does your axle still have the tag on it? your gearing might be really high. the tag should (hopefully) still be on one of the bolts that holds your diff cover on.
 

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Discussion Starter · #8 ·
That's more like it!

I put about 130 miles on the car and topped it off again, this time with some 93 octane Citgo gas. Thinking back, I remembered that, unlike today's cars, the manufacturers used to recommend a minimum of 90-91 octane and I figured it may bump my mileage up a tad bit. It turns out that I'd been averaging about 17.5 mpg. All things considered, that isn't too bad. The gas in the tank is still a bit tainted from the remaining percent of current volume that sat for five months (I've only put about 250 miles on the car and added a total of 15 gallons after it sat for so long).
I'm planning to take the car in to a shop that knows carburetors and get them to adjust the carburetor and timing. I also plan to take the car to a transmission shop for a transmission service (you know... the usual... change the fluid and tighten the bands as necessary). I'd do the fluid myself, but I'm clueless when it comes to transmissions and figure that I may as well have them do that part while they're taking care of the bands.
Things are slowly looking up!
 

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i think some of it may have to do with the difference between summer gas and winter gas.
 

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Discussion Starter · #11 ·
Dominick-
Yeah, I thought .050 seemed a little wide myself, but the emissions sticker does specify .048 to .052. Come to think of it, though, if the coil is pumping out a weak spark just do to age, that could be a contributing factor.
You mentioned an EGR plate. Do you mean the EGR spacer that's sandwiched between the carburetor and manifold? If that's what you mean, then the EGR plate is fine. Over the course of getting a new EGR tube fabricated and pulling the head because of the plastic washer I'd dropped inside the intake manifold, I took the spacer off, cleaned it, made a new gasket, and put it back on. I suppose I could have missed something subtle, but it looked pretty good to me.
Thanks for giving me some advice on things to look out for. I think I'll look into that coil...
 

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did you get a chance to look at the axle for a tag?
 

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Discussion Starter · #13 ·
Jonzo77-
Didn't check to axle for any tags yet, but will by default either today or tomorrow. I haven't changed the gear oil in the rear axle yet and I suspect that it's overdue for a change of oil.
I replaced the EGR valve even though it seemed to be working right. It was pretty old-looking anyway and I thought that it might be causing problems in that it might have been leaking by. I try not to shotgun repairs like this but figured that it couldn't hurt.
I'm taking it in to Autozone today to get the ignition coil bench-tested.
I'll keep you all posted of any further developments.
 

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If you know the axle to be original to the car and the gear ratio hasn't been changed then you can go off the sticker on the pillar with all the vehicle information on it. Or you can spin the tire 1 revolution and count the number of times the driveshaft turns. It would probably be 2.26, 2.47. 3.08 or 3.45 times.

I saved the stuff for 1980 because that is the year of my car. 1980 Fairmont/Zephyr, Thunderbird/Cougar XR7
 

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jon says 2.73, 3.08, and 3.45 were the 3 that came factory, and the 3.45 might be a factor in the mileage depending on size of axle too... either 6.75 or 7.5. the 6 came with either, just like it came with either the c3, or c4, or even the c5...
 

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Discussion Starter · #16 ·
Gonna REALLY try to get to the rear axle tomorrow. Attended a local cruise night today and was trying to replace my gauge cluster illumination bulbs (that's another thread). I did top off the tank today and found that I had gotten 18.1 mpg. Not particularly stellar, but it's still a lot better than 13 mpg. I discovered shortly before heading out to the gas station that the return spring for the kickdown lever had popped free when I took the head off and, consequently, the kickdown lever was not being pressed against the carburetor linkage. Might this have been a negative factor in the mileage? Perhaps I'll see next time I top 'er off. Oh, BTW, she took about 9 gallons of Citgo 93 octane premium this time. I'd say that the percent concentration of the five month old gas is now small enough as to not really be a factor. The saga continues...
 

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Discussion Starter · #17 · (Edited)
A culprit has been identified

I took the Fairmont into Performance Plus More in Danielson today. The guy who owns the shop is a great guy and is very experienced with carburetors, something you just don't find at shops like Midas and Firestone (among others). Before making any adjustments, the shop owner said that he thought he heard a hint of pinging as I pulled the car up to his shop. He found that the timing was set at 24 degrees advance. Yikes! Not only that, it seems that the timing had been out of whack for some time... he had a bit of trouble getting the distributor to turn and had to tap it lightly with a soft-faced mallet.He set it back to what it should be (6-8 degrees advance). Sadly, it seems that Pony Carburetors left the fast idle screw on the bench when they put my carburetor back together. I'll have to give them a call and get them to send one out. The good news is that the car idles only 50 rpm low (650 vs. 700) when the transmission is in drive. I can deal with it for now until I get the screw. Initial impression is that it seems to run quite a bit better.
Now I just have to put some miles on the car to see what effect today's shop time has on fuel economy. I still haven't checked the rear axle for that tag. But I can say that the tire pressures are all set right. Only time will tell...
 
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