Mpg drops on 87 Fifth Avenue

[johnnypopwell]

Hello just bought a new 1987 fifth avenue . Has a new computer carburetor and spark plugs. The mpg drops very quickly when the tank is below half. The previous owner cut off the muffler. The mpg just drops really fast. Is there a way to fix this or is it common?

Thanks

 

[Big_John]

The mpg drops very quickly when the tank is below half

The mpg just drops really fast.

By this, do you mean the fuel gauge drops faster?

If so, that's just a gauge or sender issue. Probably the geometry of the float arm changing as the level drops.

 

[bronze turbine]

I think most cars the fuel gauge drops faster the lower it gets. I've never heard of the fuel mileage changing between a full and half-empty tank, but most people probably just take an average for the whole tank of gas. If this car is averaging about 17 or 18 mpg for the whole tank I'd say that's in the ballpark.
My dad had an '87 Fifth Avenue.

 

[CBODY67]

MPG does not change with the fuel gauge needle position. Never has. BUT once the needle passes a certain point, it can move faster in particular parts of its movement than in others. Something which is common with most every vehicle on the road . . . then, now, and in the future.

Most common is when the tank if Full, the needle can stay there for about 100 miles or so, but after it starts to move, another 50 miles and it can be at 1/4 down, by observation. Yet if the tank is filled at that time, it'll take more than 1/4 of the tank's rated volume to get the pump to click off.

I believe that car has a Trip Odometer? Does it work? If so, "zero" it when you fill the tank completely. That can give you a better idea of how many miles have been traveled per each mark on the fuel gauge.

Just some observations,
CBODY67

 

[bronze turbine]

Here's an interesting note about fuel gauges: My Nissan 300ZXs had 2 fuel gauges. On the car with analog instruments there was a smaller one inside the bigger one that registered full until the big one dropped below 1/4, then it started going down to give you a more accurate reading. The one with the digital instruments had a fuel gauge of fluorescent bars, but when it dropped to 1/4, a little digit would light up "5" and count down to "0" at which point I guess you would be out of gas. I've never seen another car that had that.

 

[volaredon]

Every Chrysler product I've ever owned (too many to recount) the gas gauge has fallen faster once it reaches the "1/2" mark
I thought it was just a "Mopar thing"
From late 70s B bodies to full size vans to jeeps to diplomats to D/W series trucks they've all acted like that for me.

 

[Toolmanmike]

Hello @johnnypopwell are you there. Don't leave us hanging. Is it your fuel level that goes down or your engine magically get's bad mileage the last half of the tank?

 

[johnnypopwell]

Hello @johnnypopwell are you there. Don't leave us hanging. Is it your fuel level that goes down or your engine magically get's bad mileage the last half of the tank?

The gas tank needle drops very quickly once it gets to half a tank…. I cant figure out why. If i drive 20 miles on half a tank its on E by the time i get there.

 

[CBODY67]

Perhaps what you might do is put 10 gallons in the tank when it hits the 1/4 full mark. See where the needle goes after that addition. Then set the trip odometer to zero and re-fill it when it gets to 1/4 full again. Of, if possible, when the needle gets to 1/8 tank, fill the tank completely and see how much it holds. That can give you a good idea of how much fuel is left at that reading.

Is this "driving" really highway driving (continuous over 45mph), stop and go traffic, and idling of the motor? Just curious,

I once rented a new Buick LeSabre. I normally checked the fuel mileage back then. I was getting a little worried when I'd driven over 150 highway miles and the fuel gauge needle had not budged. When it did move, the closer it got to "E", the faster it moved.

Just some observations,
CBODY67

 

[Toolmanmike]

The gas tank needle drops very quickly once it gets to half a tank…. I cant figure out why. If i drive 20 miles on half a tank its on E by the time i get there.

I could almost 100% guarantee it's the tank unit that is causing the issue. Replacements now days are rarely accurate. In fact yours may be a replacement. As long as it's consistent, you can head to the gas pump when it get's so far down before it runs out. Otherwise it might be time to get yours rebuilt.

 

[Big_John]

The gas tank needle drops very quickly once it gets to half a tank…. I cant figure out why. If i drive 20 miles on half a tank its on E by the time i get there.

Easy fix... Buy a Meter Match and install it in the sender wire. TechnoVersions - MeterMatch for Analog Gauge Correction

The nice thing about it is you can "recurve" the gauge so it's accurate through the range. It takes a little messing around, but once you get it, it's good.

Here's one good thread on it. Fuel Sender calibration using Meter Match

And my adventures with it. Gas Gauge Fix

 

[johnnypopwell]

Thanks for all the advice. The gas tank looks pretty old so im guessing nothing got changed out.. luckily its not that rusted. The sensor for the tank is way under the tank so the whole thing would have to be removed and changed out.

 

[furious70]

Fwiw I replaced the sender in my charger and it acts just like this. Annoying but I fought the lock ring for an hour to get it all to seal up so I'm not touching it! I just keep track of the miles

 

[300rag]

My experience has been that it takes as long to get from full to 3/4 as it does from 3/4 to empty on most of the Chrysler Corp vehicles I have owned/driven.