[Weber_58]

So what would cause my MPG to go down after installing a new distributor cap, rotor, 8.2mm wires, and autolite 3923's? I though those upgrades were supposed to help the mpg a little?

 

[Alltornup]

It's probably going to take a tank of fuel or more for your computer to adjust to the changes. You probably should have reset the computer after the tune up.

 

[Catlin987987]

whats the best way to reest the computer in a 99? Pull the PCM fuse or maybe the battery for an hour?

 

[SD_R/T]

Same thing happened to me. Changed the autolites out for NGK's and everything went back to where I expected.

Worth trying the computer reset, though.

 

[Weber_58]

I just drove it over 500 miles so everything should be reset. I kind of figured it was the plugs. Now I have to wait until its not freezing outside to switch them back out...

 

[realdeal7369]

whats the best way to reest the computer in a 99? Pull the PCM fuse or maybe the battery for an hour?

pull the batter pos. get in and turn the key to start the truck and hold it for 15-20 seconds. then release the key. take it out, re-attach battery, start truck.

OP:
may also be a fluke thing. stranger things have happened. check for a vacuum leak, clean the tb out, check the cat (bang on it, see if it sounds loose inside...(may be starting too, depending on how old it is, and milage on the truck too). worth a shot to just check things over.

 

[Weber_58]

The cat is less than a year old. I'll try to reset the computer tonight when I get to work (its going to be really, really slow being thanksgiving and all). How do I check for a vacuum leak?

 

[realdeal7369]

how is it idleing? is it somewhat rough? you can just check alot of the thin hoses runing to various things ..check the condition of them. also is a small one goin to the map sensor i think (front of the tb), check that too.

 

[R/Tdurango02]

to hell with autolite plugs, i dont care what anybody says theyre junk

 

[Wacker33]

do you have the mpg digital console, if so, press and hold both buttons if not reset computer

 

[SD_R/T]

do you have the mpg digital console, if so, press and hold both buttons if not reset computer

I think they were referring to the pcm, not the overhead console :huh:

The overhead console is just a close approximation to actual mileage. You'll have to actually do the old-school math method if you want an accurate reading.

 

[Wacker33]

thats true, but its pretty close and will go down to 0 when stopped at light

 

[SD_R/T]

Yes, it's a decent approximation. But my overhead console routinely gives me tank averages around 1.5 less than my actual mileage.

I think what I was originally after though was that the overhead console has no relationship to the spark plugs and their effect on mileage. The theory being discussed was resetting the truck's pcm so that it could 'learn' the new sparkplug parameters.

 

[Wacker33]

I agree the tank average is a bit wacky it really fluctuates with your driving habits/conditions

 

[SD_R/T]

"You know something that would really make me applaud? A guy gets stuck in quicksand, then sinks, then suddenly comes shooting out, riding on water skis! How do they do that?!" -jack handey

sorry OP.