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Old January 11th, 2013, 08:43 PM   #1
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Default 5.4 exhaust leak - power loss

I have an 05 f250 super duty with a 5.4 has engine. 316, 000 miles, New plugs and could around 275k. I'm running 315s with a bully dog programmer (not happy with the programmer).

It's got a nasty exhaust leak at the manifolds and I'm wondering if that is likely to be killing my fuel economy (such as it is). I'm only getting 8-9 mpg on average, 12 on the highway at 70. I believe the concept is that the o2 isn't reading enough expended fuel so it pushes more through.

I'm hoping that someone has experienced this first hand and has seen an improvement with new manifolds.

I was hoping that it was a clogged cat, but I've got a lot of flow if the plume of white smoke on really cold mornings is any indicator.
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Old January 11th, 2013, 08:50 PM   #2
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i have a 5.4 in my 01 f-150 an i had a bad manifold but got the same mpg. but i only get around 16-17 if i'm lucky
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Old January 11th, 2013, 08:54 PM   #3
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Yeah those weren't much for mpg's new, lol, if you do the manifolds have a welder handy, I have pretty good luck welding nuts onto the broken studs in the head and walking them out
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Old January 11th, 2013, 08:55 PM   #4
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It definitely could affect how it runs and fuel economy, it pulls Oxygen in and makes the computer think it is leaner than it is so it will be running rich. I would probably run a backpressure test on each side before you fix the manifolds just cuz of the mileage. It would be easier to replace the cats at the same time you pull the manifolds if needed.
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Old January 12th, 2013, 02:46 AM   #5
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an there a bitch to do?
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Old January 12th, 2013, 05:52 AM   #6
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Yes.
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Old January 12th, 2013, 09:47 AM   #7
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The parts aren't expensive so I may go ahead and change them. Thoughts on headers versus cast manifolds?
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Old January 18th, 2013, 10:26 AM   #8
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We have a 99 f250 reg cab long box with the 5.4. It has a contractor cap on it and a couple thousand lbs of tools in it. It only has 60,000 miles on it and has never got more then 10 mpg. It is a heavy truck and the 5.4 is a under powered pig in my opinion.

The exaust leak you have is common on every ford I work on lately. Doesnt matter if it is a 5.4, v10 or a deisel. Ford cant build a manifold stud to save their life.
The leak could be causing you to loose fuel economy, as well as loose power.

I wouldnt put headers on it unless you spend money on some really nice coated ones. Michigan winters are hard on headers. If you buy cheap ones the salt will eat through them in a few years. It is also hard keeping headers from leaking imo.
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Old January 18th, 2013, 11:58 PM   #9
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I loved the 5.4 in my f150 SCREW. That's why I chose it over the V10, but had I known the mileage was no better I'd have poured for the two extra cylinders.

Thanks for the advice on the headers. I'll take that and save money.
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Old January 19th, 2013, 08:56 AM   #10
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love my v10 power wise.
had the same exhaust leak you got.
got it fixed pretty cheap.

look for a better stud, possibly arp, or dorman waveloc.
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