Improved gas mileage with new spark plugs & wires???

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Over the last two tanks of gas in my wife's '97 Expedition, the gas mileage has been right at 15 mpg.
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The Expedition has 114k miles. We've owned it since 65k. AFAIK, it is still on the original set of spark plugs & plug wires. What kind of improvement in gas mileage, if any, can I expect once I install a new set of spark plugs & wires?
 
there are too many variables to say that plugs and wires make "X%" improvement. But at 114K on originals it has to make some improvement as they s/b pretty far gone by now.

Just don't expect a big change unless 15mpg was markedly down from previous normal mpg.
 
You must be running the 5.4 engine, because that's about as good as it gets with my 01 F150 Supercrew, which is the same frame and setup as your wifes Exp. Bottom line....15 is about average for that vehicle.
 
I don't remember a huge jump in gas mileage when we changed the plugs on the wife's 98 Expedition 5.4L, but it ran a whole lot better. The biggest jump in gas mileage came when:
- I started making sure that the air pressure in the tires was correct on her new set of tires
- We ran 5W20 vs. the 5W30. We would get another 30-40 miles on a tank of gas with the 5W20.

114K is a long way to run that factory set of plugs. You can bet that they are pretty well shot by now. Good luck.
 
You are probably misfiring a plug here and there 1-2% of the time, under load. Cruising has a much less likelyhood of misfiring [less cyl pressures to overcome].
The O2 sensor compensates by leaning everything out slightly, when it sees this richness.
Mileage will probably improve, but throttle response and power should be better, for sure.
 
Thanks everybody!
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I will be changing the plugs & wires soon. FWIW, the engine is a 4.6L. Hopefully the new plugs/wires will make a noticeable difference in power & some improvement in gas mileage.
 
inflate the tires to about 2psi over, change the plugs but I think if you havent done it, a new O2 sensor will probably get you better mileage.
 
Wayne - Changing plugs on a 4.6L is an order of magnitude easier than changing them on a 5.4L Ford engine. Report back and let us know the impact of the plug change.
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2003TRD has good advice. I over inflated our tires by 1-2 lbs and check them frequently. It made a noticeable differene in MPG on such a big vechicle.
 
quote:

Originally posted by NewGuy:
Wayne - Changing plugs on a 4.6L is an order of magnitude easier than changing them on a 5.4L Ford engine. Report back and let us know the impact of the plug change.
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2003TRD has good advice. I over inflated our tires by 1-2 lbs and check them frequently. It made a noticeable differene in MPG on such a big vechicle.


I've tentatively scheduled this project for next Saturday (8-27), when it will be a little cooler here. I will report back when it's done.
 
quote:

Originally posted by 2003TRD:
inflate the tires to about 2psi over, change the plugs but I think if you havent done it, a new O2 sensor will probably get you better mileage.

The Expy has four O2 sensors. Should I change the 2 pre-cat sensors first?
 
Wayne, HERE is a 'how to' for changing plugs on a 4.6/5.4 Triton engine.
It has some invaluable tips.
Ignore the references to the COP's (coil on plugs).
Make sure you have that can of high quality patience on the top on your toolbox.
 
Fuel filter
Spark plugs
Cap & Rotor
Air Filter
PCV Valve
Transission flush and refill and a new filter
Differential fluid and transfer case fluid change
 
Well, I'm a little behind schedule, but still as determined as ever to get this job done. All materials are assembled.

The "Great White Gas Guzzler", aka my wife's 97 Expedition, will have new plugs & wires installed by me this Saturday.

Wish me success, BITOGers; I've never replaced plugs on a big V8 engine before. Here's a picture of the plugs/wireset:

 -
 
quote:

Originally posted by wavinwayne:
The Expy has four O2 sensors. Should I change the 2 pre-cat sensors first?

Yes, the post-cat sensors have nothing to do with fuel economy. They are only there to verify that the catalyst is working properly. I wouldn't bother to replace the post-cat sensors unless a code says they're bad.
 
quote:

Originally posted by wavinwayne:
Well, I'm a little behind schedule, but still as determined as ever to get this job done. All materials are assembled.

The "Great White Gas Guzzler", aka my wife's 97 Expedition, will have new plugs & wires installed by me this Saturday.

Wish me success, BITOGers; I've never replaced plugs on a big V8 engine before. Here's a picture of the plugs/wireset:

 -


Why are you not replacing the cap and rotor?

PCV valve?
 
I did the dirty deed today on the Expy.. It was a very, very difficult & time-consuming process for me. I should have hired it out.

BlazerLT,

The Expy 4.6L doesn't have a traditional distributor, so I don't believe there is a cap & rotor to be replaced on this engine. I'm too tired to try & explain in better detail.
 
quote:

Originally posted by BlazerLT:
Sweet!
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Ok, If you want to get the most mileage out of that big truck, you might want to change your transfer case and differentials to synthetic. [/qb]

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I'm one step ahead of you my friend.
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The Expy is a 2WD (no t'fer case) & the rear diff. fluid was just replaced one month ago with Motorcraft 75w140 synthetic (same as factory fill).
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