1992 cher ltd i6 4.0 - HUGE MPG Swing

[cmk2112]

Short and sweet.

Had it for 5 years. Always got between 15 and 16 mpg. Took it to Jiffy Lube (no garage at old place) for a quick oil change before my 1500 mile move from Norhtern VA to Houston. My MPG suddenly INCREASED by over 5mpg to 20+. After about 3 tanks it's back to what it was before. This isn't due to highway driving as the change was evident regardless of type of driving. I am under the hood and checking things with my manual, but I don't see anything obvious.

Has anyone ever heard of this? Any suggestions on what to look for/check? Gotta get back that milage.

I'm hangin with the astronauts and feeling like a fool...

[fredjacksonsan]

Hard to say, but the highway thing makes a big difference. I drive mostly highway to and from work, and average about 17-18. But if I am on a long trip and only stopping for gas (AND on level ground, like in east Texas) I have gotten up to 24mpg.

[longfellow]

Can you think of anything different you may have done as far as driving posture on your trip. Were you most always in cruise. The trip may have cleaned your spark plugs off real good and now 3 fill ups later your plugs
have loaded up again. Just a thought.
I,d pull a couple of plugs and see what they look like at electrode tip.

[cmk2112]

Thanks for the help to all...

As far as things that I may have done differently...

I made the same trip in mid May without my truck loaded down with the items that I brought with me this last time, and the MPG was the 15 to 17 'norm' with the lighter load. I also replaced the plugs before I drove down in May, and the same plugs are still in use. Also, on this last trip the gas mileage remained at those wonderful levels until about Texarkana, and then they went back down to the 15 to 17 'norm.'

So it appears that things are upside down; same trip, same plugs, a much HEAVIER load, yet phenomenal gas mileage. I was thinking that maybe it was a vaccum hose connection or something that they 'messed with' under there that the beast liked, but they couldn't tell me anything at Jiffy Lube because they obviously didn't remeber me, and the notes on the receipt gave nothing away.

A couple other things to note: For the first time they used 15w-40 oil to replace the old. I noticed immediately that my oil pressure was and remains right around 40 when it used to drop very low after it warmed up and was at idle, and only got to around 35 when driving. Also, I dumped some fuel injector gas treatment into the tank when I fueled up just after the oil change. It was after these 2 things that I noticed the drastic change during the 2 days prior to my departure.

When I gas up I always put in exactly 10 gallons so that I always know what the MPG is, and this is why I am certain that something changed and then went back to its deficient state after about a 1000 miles...

I am going to replace the fuel filter tomorrow...

Puzzling, ay?

[GregA]

Originally Posted by cmk2112

...When I gas up I always put in exactly 10 gallons so that I always know what the MPG is, ...

How does this work? How do you accurately calculate the mileage?

I FILL my tank to the very top - drive for a couple hundred miles, then fill it to the very top again. Then I do the calculations (miles traveled divided by the gallons just added).

If you are only adding 10 gallons each time, how do you know how much gas you used?

Take Care,

[fredjacksonsan]

Originally Posted by GregA

How does this work? How do you accurately calculate the mileage?

I FILL my tank to the very top - drive for a couple hundred miles, then fill it to the very top again. Then I do the calculations (miles traveled divided by the gallons just added).

If you are only adding 10 gallons each time, how do you know how much gas you used?

Take Care,

That's my thought also. If you don't fill up the tank then you can't be sure of how much gas you used.

[longfellow]

This may be a silly question but on your trip was the wind with you or against you.

[fredjacksonsan]

Originally Posted by longfellow

This may be a silly question but on your trip was the wind with you or against you.

Have to be one hell of a wind, that blew all day.

The difference could be in the terrain...rolling hills will sap your mpg, while flat terrain will enhance it.

[lussenden]

yeah you couldve had a full tank, gone 200 miles, and put in your 10 gallons, then thought you got 20mpg.

[cmk2112]

Thank you all for the enthusiasm...

It's simple: 10 gallons purchsed at the pump that produces 150 miles equal 15 mpg... And this is less in the summer when I use the a/c. I guess I should've mentioned that I have a trip milage counter on the odmtr that I reset each time so I don't have to track odometer readings. Then it went form the average 150-155 to 207, 205, 217, 198... ... ...

And I assure you I did not fill it up one time and forget that I did so. Agian, this lasted for 1000 miles or so. I have been putting in 10 gallons at a time for a while because it's just the way that I do it. Various reasons including Cash Flow and a 1 mile commute. Also, on long trips it works because I need to stop often to move around. (Don't ever get accidentally knocked off a building... ) But it's simple and it works for me.

And as far as wind goes... This was not a factor. This included 2 days of city driving before I left on the trip and I accounted for the drivnig conditions. I know this truck well, which is why I am surprised.

But I guess it could be brain damage... It was quite a fall...

Thanks again...

[fredjacksonsan]

Originally Posted by cmk2112

It's simple: 10 gallons purchsed at the pump that produces 150 miles equal 15 mpg... And this is less in the summer when I use the a/c. I guess I should've mentioned that I have a trip milage counter on the odmtr that I reset each time so I don't have to track odometer readings. Then it went form the average 150-155 to 207, 205, 217, 198... ... ...

And I assure you I did not fill it up one time and forget that I did so. Agian, this lasted for 1000 miles or so. I have been putting in 10 gallons at a time for a while because it's just the way that I do it.

Ok if I may, here is the flaw in your calculations:

Say you have 5 gallons in the tank when you fill up. You put in 10 gallons. Now you drive 300 miles and it's on empty. You put in 10 gallons again, and it seems you may have gotten 30 mpg, but in actuality you used all or most of the 5 gallons, and only got 20 mpg. Now that you were almost empty, you only have 10 gallons in the tank. So you drive 200 miles, it's empty again, so you now think you got 20 mpg and are wondering about the change.

The error occurs because you don't actually know how many gallons are left in the tank when you get your 10 gallons.

To correctly check the MPG, start with a full tank. Then you KNOW it is full. Drive.... now stop and fill up again. You now know almost exactly how many gallons you used.

If you think about it some, I think it will make sense.