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Offline 2.3stangii

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2.3 lousy gas mileage
« on: July 24, 2011, 11:03:49 AM »
Ok I've been scratching my head for a while now trying to figure out why my 71 (which has a low mileage 2.3 from a 78 in it) uses twice as much gas as my heavier, high mileage 78 wagon.. I don't know the exact MPG but at our prices of around 3.69 a gallon, it costs me $10 in my 78 to drive about 38 miles to work and back. While the 71, despite being much lighter costs $20 to drive the same distance!

Both are 4 speeds and have and same size carbs. The only differences are the rear ends, 3.55 in the 71 and 3.00 in the wagon but the wagon has 13'' wheels while the other has 15'' which should cancel out the low gears. And the 71 has brand new points dissy and the 78 has duraspark.

I'm thinking its somewhere in the carb cause the 71 won't idle very good, kind of lopes a little. One weird thing I noticed is that when I hit the brakes at say an unexpected light change, the engine dies but the car doesn't have power brakes so I don't see how braking could effect the engine.  ??? Any ideas?
78 Pinto wagon
74 Mustang II
78 Cobra II

Offline blupinto

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Re: 2.3 lousy gas mileage
« Reply #1 on: July 24, 2011, 11:41:54 AM »
This might be dumb, but possibly failing fuel pump?

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Offline sedandelivery

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Re: 2.3 lousy gas mileage
« Reply #2 on: July 24, 2011, 03:45:21 PM »
You might want to check the carburetor float  if it is not the fuel pump.

Offline FCANON

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Re: 2.3 lousy gas mileage
« Reply #3 on: July 24, 2011, 05:00:01 PM »
Smell your oil.. I've had 4 pumps fail on me Due to gas eating the Diaphragm... might be sqriting in your block or out the weep hole if this is your issue.

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Offline 2.3stangii

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Re: 2.3 lousy gas mileage
« Reply #4 on: July 24, 2011, 07:35:42 PM »
Thanks I'll check that, could be the problem since literally everything else on this engine from the oil filter to the plug wires was original, I'm sure the fuel pump is 30 years old too.
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74 Mustang II
78 Cobra II

Offline ToniJ1960

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Re: 2.3 lousy gas mileage
« Reply #5 on: July 30, 2011, 09:22:01 PM »
 Just an idea,every time I had a power valve start leaking in my carberator it ran a little off on idle and of course used more gas.The first time I found it,was when I was trying to adjust the mixture. No matter how much I turned in the mixture screw,it wouldnt affect it.I asked someone and he told me it was too much gas geting through the top half.So I had it rebuilt. Now after all these years,I do it myself.In fact I just redid the top half about a month ago.It idles a lot better and gets abou twice the mileage nearly. With these gas prices it was time to do it :)

Offline JohnW

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Re: 2.3 lousy gas mileage
« Reply #6 on: April 12, 2012, 01:07:42 PM »
I figured I'd bump this back up instead of starting a new thread.

Last fall, I was getting 24-25mpg on a tank average and getting up to 29-30 on the highway. Now my gas mileage is down to a hair over 20mpg. I rebuilt my carb last fall but couldn't get a power valve at the time. I was still getting mid-high 20s after rebuilding it. Now it's dropped down, but nothing else seems to have changed. The performance and drivability haven't changed.

I just put a new fuel pump in last week with no noticeable effect on performance or mileage. I change fuel filters regularly, and it just got a fresh one. It also got a new PCV valve in the fall. I put in an entire brand new ignition system (distributor, plugs, wires, ignition module, internally regulated alternator) last summer and timed it, as well as rechecked timing recently. I'm also using ethanol treatment with every fill-up.

The charcoal canister was disconnected and the port on the carb plugged before I got the car, could that be causing problems by not allowing the carb to vent properly? I'm also pretty sure my cat is plugged and I'm going to rip it out soon. I've got some valve noise but it sounds exactly the same as when I was getting better mileage. Are there any other things I should look at?
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Offline OhSix9

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Re: 2.3 lousy gas mileage
« Reply #7 on: April 12, 2012, 08:51:26 PM »
a plugged cat will kill fuel mileage .  get rid of that or replace it with a high flow first and foremost. 2nd after you free up the exhaust make sure the thing isn't running rich as excess fuel is what destroys cats.  the more unburned fuel they have to deal with the hotter they run and the sooner they die. 3 tune for an octane level with you timing and don't "treat the old girl to a tank of the good stuff" without turning up the timing.  Higher octane resists detonation but also burns slower so if you don't light it sooner it pumps out unburned into the cats and back to the above...

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Offline dick1172762

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Re: 2.3 lousy gas mileage
« Reply #8 on: April 13, 2012, 12:01:21 AM »
Power valve is got a hole in it. Easy to change and cheap too.
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Offline kail

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Re: 2.3 lousy gas mileage
« Reply #9 on: April 16, 2012, 04:33:07 PM »
Ok I've been scratching my head for a while now trying to figure out why my 71 (which has a low mileage 2.3 from a 78 in it) uses twice as much gas as my heavier, high mileage 78 wagon.. I don't know the exact MPG but at our prices of around 3.69 a gallon, it costs me $10 in my 78 to drive about 38 miles to work and back. While the 71, despite being much lighter costs $20 to drive the same distance!

Both are 4 speeds and have and same size carbs. The only differences are the rear ends, 3.55 in the 71 and 3.00 in the wagon but the wagon has 13'' wheels while the other has 15'' which should cancel out the low gears. And the 71 has brand new points dissy and the 78 has duraspark.

I'm thinking its somewhere in the carb cause the 71 won't idle very good, kind of lopes a little. One weird thing I noticed is that when I hit the brakes at say an unexpected light change, the engine dies but the car doesn't have power brakes so I don't see how braking could effect the engine.  ??? Any ideas?

I was having a hard time about this and also got this problem. thanks for asking it saves me a lot of time :)

This might be dumb, but possibly failing fuel pump?

its not dumb. this really fix my problem but i have to test it for a while. thanks for the simple solution

Offline JohnW

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Re: 2.3 lousy gas mileage
« Reply #10 on: April 16, 2012, 04:36:57 PM »
I just ordered the power valve from Napa and I'm pulling the cat out this weekend. After I do my water pump that seems to be going...

Then I'm going to pull the plugs to check how the fuel mixture is and see if that needs any adjustment.
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Offline 2.3stangii

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Re: 2.3 lousy gas mileage
« Reply #11 on: May 02, 2012, 07:55:26 PM »
Wow. Been a while since my last login and I find this year old thread has been bumped lol. Thanks for bumping, cause I still have a miss and lousy idle problem.

Since this thread was first posted I had to let the car sit cause I didn't have time for it, but now that I've moved about 30 miles closer to work and my daily driver wagon has decided to tear itself apart at the floor. I've no choice but to rectify the problem  on the 71. I've learned that it is not the carb, plugs/wires or fuel pump, I've replaced all those. No cat either, just a strait pipe. Not yet determined gas mileage since I'm only a few miles from work but I'm pretty sure it still sucks.
New carb made it idle a little better, it still misses up to about 2000 rpm, but as long as I'm accelerating, it does fine. Plenty of power.
I'm thinking its possibly a weak coil or the timing is still off, hopefully not a head gasket.. Can't think of anything else.

78 Pinto wagon
74 Mustang II
78 Cobra II