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Discussion Starter · #1 ·
High all. My son's 1999 DOHC has just about got me whipped. It has been one thing after another and just when I think I have it all straightened out something else pops up. I checked his mileage a while back and it has been averaging 12 to 16 mgp. I tried to blame that on a heavy foot and intown driving. I just filled it up and he drove 28.3 miles, I filled it back up and he used 3.15gal. Thats 8.98 MPG :confused:. I read a post a while back someone said they were getting 8MPG and I laughed to my self saying no way. I asked my son if he was letting his friends siphon the gas out :p. I was not happy with the 16mpg but now I am at my wits end. The car runs good. Has no codes. Fuel trim values and O2 raedings look good. Tires are at rated air pressure. Trans rebuilt, brakes not dragging as far as I can tell. If the brakes were dragging that bad they would be on fire. I do notice a rich smell from the exhaust and inside the pipe is fairly heavy with carbon although you don't see any smoke. The engine has good compression and recent plugs. I am at a loss to figure this out. I am going to start over in the morning and pull the plugs to see how they look. Any ideas would be appreciated.
 

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Driving 28 miles and refilling the tank will not give statistically significant results. Different pumps / auto shutoffs, if the car is uphill / downhill during fill will alter how much gas goes in. Temperature of gas in the tank makes a difference due to gas having a high thermal expension coefficient. Realistically, it takes several tanks / hundreds of miles to get a truely accurate average MPG reading.
 

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Discussion Starter · #3 ·
I agree Jeff, I just don't want to give my son a full tank very often. He will find a way to burn it up regardless how the engine is running LOL. I have been tracking the gas and the best it ever got was 16MPG. Still not acceptable. I would start feeling better if I was getting at least 20mpg. I do believe the engine is running rich by the smell of the exhaust. I just don't understand that I am not getting any other clues to what may be going on. If I had two or three bad injectors would I not get some kind of code?
 

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Were the pre cat O2s ever contaminated with coolant or some other chemicals? Some chemicals will induce what is called a chemical shift in the O2 output, basically moving the A/F versus voltage transfer function either toward the rich or lean side. The "shifted" O2s give the PCM incorrect A/F info (i.e., telling it the A/F is too lean so the PCM ups LTFT to go rich. The PCM then thinks the A/F is correct, but in reality, it is running rich.
 

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You won't see black smoke because the cat's will burn it up, but they will run very hot. The spark plugs will tell the tale if it's really rich.
 

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That is why when the hg went south on the old 3.8 essex eng., techs automatically replaced the o2s. Coolant took them out.
 

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Discussion Starter · #8 ·
A year ago I swapped in a jy motor to replace my dead motor. When I hooked everything back up, I forgot how the O2 sensors were wired. I thought I hooked them up right by the way the wires laid. I was wrong and had them switched about as bad as you could get. Fired motor up and it ran for a while and died. Would not restart. Pulled plugs and they were gas soaked. Computer didn't know what to do. Took a while but figured out what I did and hooked them up right. Readings I get from my scanner lead me to think they are okay. I will get specific readings tomorrow. The gas mileage just seems to keep getting worse/

Ed
 

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Only way your gonna get a good read on MPG is to drive it yourself, and you only, for a tank and see what's going on.
My style of driving was way different than my old man's style, hence "your mileage may vary" . Green light = mat it back then.
 

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Only way your gonna get a good read on MPG is to drive it yourself, and you only, for a tank and see what's going on.
My style of driving was way different than my old man's style, hence "your mileage may vary" . Green light = mat it back then.
I agree, this would be the best "1st" step the process.
 

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I get 15 mpg in the city with my 99 and up to 29-30 pure interstate @ 65 mph.

These V6s are hard on the fuel in town with the compressor on.
 

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My 2004 is bad on gas as well...

I was consistantly getting 24/25mpg on the highway, this is a best scenario. This is my daily driver. I am a creature of habit. I would buy gas from the same place until they give me a reason to go somewhere else. I took it to the Stealership, for the oil change/tire rotation/etc. I forget what Ford called it, but it only cost me $35.99. I also had them check my AC, it wasn't putting out what it should. Now for the last 3 weeks since the Stealership, I'm getting about 25 to 30 miles less for the distance driven. In other words, when I reach the 3/4 mark I usually have at least 100 miles on my trip meter, now its down to about 70 to 75 miles. I get no CEL's and it seems to be running fine and my AC puts out like it should...??

Joe D.
 

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mpg can be affected by alot of things. tire inflation is one of them. if the dealer changed the psi to a lower amount then this can be a part of them problem. also oil can play a part here also. ( hopefully this wont start a war. ) full synthetic (imo) lubes better and reduces friction, dino oil not so well.
are you using pure gas or a station that adds corn juice? mpg increase with pure gas. stations that adds eth ( up to 10% ) fluctuates on the % so one fill up will vary from another fill up.
start with the easy. tire inflation. when at max psi you get better mpg. a little less then max that ride comfort improves.
then look at the air filter. a clean filter allows better fuel air mix.
check the gas station you use and see if it has pure gas. if not i would switch to another that does. it maybe a higher price but the engine runs better. if none of these things helps you mpg there are other things to look at. these are just the simple.
 

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I was consistantly getting 24/25mpg on the highway, this is a best scenario. This is my daily driver. I am a creature of habit. I would buy gas from the same place until they give me a reason to go somewhere else. I took it to the Stealership, for the oil change/tire rotation/etc. I forget what Ford called it, but it only cost me $35.99. I also had them check my AC, it wasn't putting out what it should. Now for the last 3 weeks since the Stealership, I'm getting about 25 to 30 miles less for the distance driven. In other words, when I reach the 3/4 mark I usually have at least 100 miles on my trip meter, now its down to about 70 to 75 miles. I get no CEL's and it seems to be running fine and my AC puts out like it should...??

Joe D.
I would check for dragging brakes. Easy and often overlooked.

Old school, but look for most likely combined with easy to check items.

Easy to check items should be done before most likely. I could relate couple of stories about charging, and one PS related where lots of $$ was spent only to find a slipping belt. Example only.

For brakes, start the car cold and drive around the block a couple of times with minumum braking. Then check for the hot feel of the rotors and drums. CUTION, some items might be HOT!:lol2:

Another: when started cold, put it in gear, hold the brake and let idle settle a bit, then take your foot off and see if it rolls freely with only idle pull on level ground. That one jumped out for me on my '01. It was a surprise when it was slow moving, but once around a turn it was free. It was a stuck guide pin.

Recent case. Relative called and told me he just drove a Buick van one mile from cold and the front brakes seemed too warm for that short run. I said fine, be right over and use his floor jack. Jacked up one front, put it in "N" and I could barely turn the wheel. Pads were stuck in the guides, both fronts. Low miles, no rust, bad pads.

Good Luck.

-chart-
 
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