New to me 04 GT with bad gas mileage - Ford Mustang Forum
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post #1 of 6 (permalink) Old 12-11-2016 Thread Starter
kimrpaige1953's Avatar
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New to me 04 GT with bad gas mileage

I recently purchased a 2004 Competition Orange GT convertible. It has a BBK cold air system and the exhaust is not stock, not sure what has been done, but it is a bit louder than stock. Anyway it appears to be otherwise stock. It has just over 107K miles. My issue or question is the fuel mileage. I am getting 14 mpg just running around. I have no check engine light on, It seems to run great, although I do feel a skip or miss every now and then when in OD and doing about 45 or so. The plugs and wires were replaced about 10K ago. Is there something simple to look at that normally goes bad to create this problem? I am a old school guy knowing carbs and stuff, so the high tech EFI and computers are somewhat new to me. Any help would be appreciated. Thanks in advance, Kim

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post #2 of 6 (permalink) Old 12-11-2016
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You might have a coil pack breaking down. Did you do the plugs yourself? Only reason I ask is because you said wires were done but these cars are coil on plug so there aren't any wires. Also, very common, check your vacuum lines. You could very well have a vacuum leak.

1998 GT
Rebuild in the works. New crank, 2013 Boss rods, srp .020 over flat top pistons with valve reliefs, 12:1 compression, ported pi heads, comp xe278ah cams.
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post #3 of 6 (permalink) Old 12-11-2016
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Usual tune up items.... Plugs (I know you said they were already done....), fuel filter, clean MAF (this becomes a regular maintenance item IMO anytime you are using an oiled cotton gauze air filter). On one of my vehicles I have seen a solid 1.5 - 2 mpg increase just from running a can of Seafoam through the intake..

I assume if the check engine light were on that you would have mentioned it in your post...

As for the misfire, without a check engine light (and more specifically the actual code being generated) it can be difficult to diagnose a bad coil, and thus I personally would be hesitant to recommend you go buy a new set just to try it out and see what happens. Very likely could be a coil or coils going bad, but at this time sufficient reason and information to spend money there is lacking. I would pull each one and inspect the rubber boot that extends down into the plug well. Look for any cracks, deterioration or signs of current arcing through (often a brown-ish dust like substance). The boots can be replaced for about $10 each and are available at your local parts store. While you're doing all of this, look down in each plug well and verify that it's dry - no oil or coolant pooling up in there.

2001 GT Convertible
/1967 Coupe
All the usual bolt-on's /200 I6, C4

And a few Explorers too... 2008 4.0 XLT 4x2, 1996 XLT 5.0 4x2, 2003 Sport Trac XLT 4.0 4x4
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post #4 of 6 (permalink) Old 12-12-2016 Thread Starter
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SoCal2V, Thanks for the quick reply, Sorry I have not paid a lot of attention to the engine as I just bought it recently. I have been a mechanic all my life but really not worked on too many newer cars. As far as the plugs and wires, I was reading over a invoice from the previous original owner. So maybe they charged her for wires and she did not know any better. I will pull the coils and check for oil/coolant, very doubtful as the engine is very clean and looks to be very maintained. I will also clean the maf sensor, any special cleaner? Seafoam in the intake? I have used Seafoam many times and swear by it but only in a can added to the fuel tank and mostly in motorcycles, so could you explain using in intake? And yes there is not a Check Engine Lite on. I was thinking of an O2 sensor but again it would normally throw a code. I am loving the car, it runs and sounds great, just sucks gas. I also own a 98 Mustang LX convertible I use up in Maine in the summer, it is a 3.8 V6 and it gets 32mpg on the hyway. Just thinking the 4.6 should be getting in the high teens or better around town. I am also a older guy and have not even opened it up yet. I will check all hoses and coils/plugs, then get back to you. Thanks again, Kim
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post #5 of 6 (permalink) Old 12-12-2016
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Maf sensor should be unplugged when cleaning and you can either buy the special $8 can of isopropyl alcohol or just get the $1 bottle of it. I would assume the boots are causing the misfire or poor conditioned coils as mentioned, if you go to a junk yard with a big bag of tools you could "borrow" some to test on your vehicle. I would also say bad gas mileage can be contributed to a faulty IAT or CHT, reading too cold would make the pcm use more fuel to compensate. The upstream 02s also can be reading off. If they dont cycle from rich/lean to lean/rich fast enough it will throw off the fuel/air ratio similar to a maf. If none of the sensors has failed completely(which is what im assuming) no CEL is commanded by pcm. Good luck.

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65 GT A-Code Mustang 289
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post #6 of 6 (permalink) Old 12-12-2016
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What spark plugs are you running anyway?

Matthew 26:52-54 / Go Pack Go!

1987 Mustang GT Hatchback - 2V Mod Swap In The Works

1997 Lincoln Town Car, PI Intake/Cams, Sofa-On-Wheels, "Fat Tony" we call it

2001 GT Convertible - D1SC / Geared / Cammed / 440wh 400wt - TOTALED 2/2019
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