Tune-Up Made Mileage Worse?

My 97 Sport 4.0 with manual shift is new to me, and I wanted to establish a baseline. So after driving for a week, I've determined I'm getting 19.6mpg with a 35/65 mixture of highway/city driving.

Then after an intensive maintenance weekend, I did the following: changed differential fluids (Amsoil), tranny fluid (GL-3), Transfer Case (Mobil 1). As the fluids drained, they looked pretty good, but I changed them anyway. Put in a new Fram air filter (the old one was minorly dirty) until I can decide about a cold air system. Followed up with new plugs, wires, rotor and distributor cap. The old Champion plugs looked like they were original with 118,000 miles on them. Very worn. The resistance of the wires showed they also needed changing. The rotor edge was pitted and the inside of the dist cap looked as it it had salt deposits on the nodes.

After the refresh, my Jeep felt peppier. It didn't "bog" down on acceleration. However, my mileage did not improve. In fact, it actually worsened. This fill-up I got 19.4mpg, and that's with a lot of highway driving, almost 70% highway this time as opposed to last week's 35%. While 19+ mpg is awewome, I expected some gain...

Should I try disconnecting the battery to reset the computer?

Analyzing your fuel economy requires multiple tanks of fuel, not just one. There are too many variables influencing fuel economy to state that mileage is such-and-such based on only one tank.

Run a few tanks through the beast, and calculate the MPG for each tank. Then average the readings for multiple tanks.

I typically disconnect the ground from the battery whenever I do the plugs/tune up thing. Over time, the electrodes on the spark plug wear down, and the onboard computer compensates for this. New plugs will obviously be different, so by disconnecting the ground, I then reprogram the computer at start up with all new tune up parts.

By the time I'm done replacing everything, the computer would have time to "lose" it's old memory, and then reset itself with the new components....

Ivan

Typically, how long should I leave the battery disconnected?

thanks!

Most readings I've found say to disconnect the computer for about 10 minutes or so. I usually leave it disconnected for at least an hour...

Ivan

I'll give that a shot and see if it makes a difference. After another tank, I'll take a look at the timing also.

thanks!

Black & Tan said:

After another tank, I'll take a look at the timing also.

thanks!

Click to expand...

Its not adjustable

Black & Tan said:

I'll give that a shot and see if it makes a difference. After another tank, I'll take a look at the timing also.

thanks!

Click to expand...

There is no way to adjust the timing.All spark advance and retard is handled by the ECM(computer).
Wayne

Wow! That's a surprise. Must be one of the benefits of a newer vehicle. Up until now, my newest car was an '89...

thanks!

As others have said one tank is not enough to make a comparison. ALso look into the situation that depending where you tank which distributor some fill up more some fill up less and so on. The .2 mpg difference should be rounded off to 0 and you should look at it running about the same. On top of everything if you filled up in Shell vs Texaco vs Mobil1 vs 76 with all the different additives that they add you might end up with slightly different results.