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Discussion Starter #1
REF: 3.0L Vulcan Flex Fuel Engine
My friend has a 2005 Ford Taurus and she just got her spark plugs changed about 3 weeks ago. Before she got her spark plugs changed, she says she was getting 28-29 miles per gallon. Car has over 105,000 miles on it; she is now getting ~23 miles to the gallon (five tankfuls). Any idea what is going on? She had the oil changed the week before with 5w20 as required. No warning lights and no driveability issues. Her mechanic put in Motorcraft SP -500 spark plugs as specified. I don't know what to tell her...
 

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i'm lost too... at 105,000 it should be getting thicker oil in it though...like 10W30
where are you from?
 

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Does she have any receipts or a log book to confirm her past/present MPG?
 

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I checked her math on the MPG data and it is correct. The car trip computer is never correct but within 3 mpg most of the time. She lives in SW Ohio where the weather is moderate. As far as the oil goes, 5W-20 is all she ever used (spec). Why should she change to a thicker 10W30 at this time?
 

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I checked her math on the MPG data and it is correct. The car trip computer is never correct but within 3 mpg most of the time. She lives in SW Ohio where the weather is moderate. As far as the oil goes, 5W-20 is all she ever used (spec). Why should she change to a thicker 10W30 at this time?
:confused5:

So there is no way to show what this car's MPG was BEFORE the s/plug change...?
 

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Discussion Starter #6
She has sufficient before and after data to show the decrease. One reason i convinced her to do the spark plug replacement was the decline in MPG (over the past few months) even though the car ran great with original plugs and 105k. Still runs great but she expected an improvement. Ran a can of SEAFOAM last time she filled up so we hope that may help. car has been re-checked by her mechanic twice and they find nothing wrong with it. Just looking for something we may have missed...
 

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once you get past a certain point in your mileage, you need thicker oil to keep the compression up. higher compression, better MPG's... Get it?
 

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Jonzo, I don't really agree with you on the change in oil. Ford says 5w20 in almost all its engines since the late 90's. This has to do with bearing clearence and oil passages. I drive two fords, my daily driver is a 98 Explorer Limited with a 5.0L and has 252,000 miles on it. Still gets around 20MPG on the road. I have seen these newer engines fail when the heavier oil is run in them. The main problem is cold start, takes too long to get to the bearings and causes premature failure.
If I read the original poster correctly, she was having problems with fuel economy falling off before the tune. Best to start checking the computer system to see if something is not correct
 

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Check the fuel filter, air filter and tire pressure. Sounds fundamental, but I've seen "mechanics" ignore the basics (one shop put a new battery in my mom's car without cleaning the terminals, and couldn't figure out why it wouldn't charge!) Its up to you....

I agree with staying with the 5-20 oil. If you need heavier oil, you need an overhaul. I understand one of the reasons Ford went to the lighter oil was to reduce parasitic power loss from pushing the heavier oil around inside the engine.
 

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Verify the plugs are the right ones. If they don't extend into the cylinder far enough, it will run like the timing is retarded. Happened on one of the trucks where I work. (A Chevy, done at a GM/Goodwrench dealership shop.) It ran poorly and got horrible mileage.
 

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There's probably 50 different reasons whay the fuel mileage went down.Anywhere from hit a pothole and knocked the front end out of alignment to a dirty airfilter.Have her mechanic monitor the O2 sensors and fuel trims to see if he/she see's anything obvious.Then start looking for the not so obvious reasons like tire scuffing and such.

And as far as oil,stay with the 5-20 as recommended by Ford.Thicker oil is tough on a motor when cold. I've got 335000 mile on a 90 Ranger using 5-20 synthetic oil every 5000 miles and engine is clean as a whistle inside.
 

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Discussion Starter #12
We've done all the easy stuff. Air filter, fuel filter, tire psi, no engine codes (checked at advance auto parts). Is there an easy way to reset the computer? Last resort is going to the FORD dealer - that's why we are on here.
 

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We've done all the easy stuff. Air filter, fuel filter, tire psi, no engine codes (checked at advance auto parts). Is there an easy way to reset the computer? Last resort is going to the FORD dealer - that's why we are on here.
yes. to reset the computer all you have to do is disconnect the negative battery cable for about 15 minutes. this will reset the fuel trims and might work for a few miles, but its likely that whatever is causing the drop in mpg will cause the computer to revert back to what it was doing before disconnecting the cable.
 

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How about check the spark plug gap ? Do a real fuel injector cleaning where you have an auxiliary fuel/cleaner hooked up to the vehicle. Fords stuff is pm-5 .
 

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REF: 3.0L Vulcan Flex Fuel Engine
My friend has a 2005 Ford Taurus and she just got her spark plugs changed about 3 weeks ago. Before she got her spark plugs changed, she says she was getting 28-29 miles per gallon. Car has over 105,000 miles on it; she is now getting ~23 miles to the gallon (five tankfuls). Any idea what is going on? She had the oil changed the week before with 5w20 as required. No warning lights and no driveability issues. Her mechanic put in Motorcraft SP -500 spark plugs as specified. I don't know what to tell her...
 

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REF: 3.0L Vulcan Flex Fuel Engine
My friend has a 2005 Ford Taurus and she just got her spark plugs changed about 3 weeks ago. Before she got her spark plugs changed, she says she was getting 28-29 miles per gallon. Car has over 105,000 miles on it; she is now getting ~23 miles to the gallon (five tankfuls). Any idea what is going on? She had the oil changed the week before with 5w20 as required. No warning lights and no driveability issues. Her mechanic put in Motorcraft SP -500 spark plugs as specified. I don't know what to tell her...
crazy thing happened...only changed plugs...went 28 to 23??????
 

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I would double check for correct plug application and make sure they set gap to proper specification. If you dont know the gap it should be on your catalyst label under hood. Also make sure nothing obvious like air filter is plugged or something was forgot to be put back together.
 
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