Ok, I searched for this. I did not do the newbie thing.

After I did the 30k service ( replaced sparks, changed the oil, changed differential oil, check air filter) everything is fine, the truck runs smooth, but my MPG have gone down dramatically. People tell me that the truck's ecm needs to relearn and that would take time, but how long? Its been4 weeks, WHAT THE DEUCE? I gapped accordingly the sparks. I dont get it.

 

Funny you mention that......

I did my 30K service at 39K. I bought the truck at 36K and figured (correctly) that the previous owner did squat with it.

I noticed no change in anything.(other then mentally and piece of mind) Technically, you shouldn't. The computer adapts to changes in the truck's tune condition, which is exactly why all vehicles went to a closed loop ECU system, in order to comply with EPA emission standards and to run more efficiently, longer, despite how worn plugs are, or how clogged air filters are, etc.

If it (tune) gets that bad, and the parameters of change for the ECU are exceeded, the vehicle throws a code.... simplification, but the general idea.

I *thought* I noticed a decline in MPG when I added LT rated tires. (heavier, and more rotating mass) I almost went back and got lighter, cheesier tires cause I dreaded the thought of getting 11 mpg around town. I apparently never noticed that before, rather saw about a 13.2 mpg rating most of the time.

Do this.......: IGNORE the overhead computer that tells you your AVERAGE mileage, and not your true mileage per gallon.

Write down your mileage and fill the tank. Run the truck to half, 3/4 empty or empty, and refill. Take the mileage driven and divide by the number gallons you filled it with. You'll note that it's better then overhead monitor and gives you a true, overall MPG reading, not an AVERAGE reading. City driving ruins the average, even when you're getting decent mileage on the highway, or outside the city.

I got 17.8 mpg with half city/highway on my last tank (18 gallons used) of fuel. My overhead had me at 14.5 mpg, but that's simply explained by a few hard accelerations and a stint of 5 mpg slow traffic rides. It's not my true mpg rating, when compared to the fuel that I've burned, vs the miles I've driven.

Try it and see what you get.......... Hope this helps.

 

Thanks OldMopar. I wish it would be the overhead, but my ST does not have that. Everytime I got my mpg was with a pen and paper. I used to get 15.5mpg with 20's, I used to get 17.5 with 17's, now I am closer to 13mpg.
Before I did the tune up with my new intake, I was getting close to 17 mpg. So this gets me thinking some'ing aint right.

(I aint talking to you OldMopar in the following)
PLEASE, I dont need an asswipe telling me "WOW, MY HEMI GETS ME 18MPG". We know you read your overhead well and that I have a V6 on quad, which sucks, but I dont need to be reminded every freaking post. SO PLEASE, keep your hemi mpg on other posts. . .

 

My overhead average actually reads higher than what I am getting. I am wondering if my ecm know I have 33" tall tires. I will program it when I put the shorter winter tires on and see what it gets.

 

Man.....I would be under the impression that a V6 with stick would do pretty good...

My best advice then, is go back over basics.

Make sure you used the correct heatrange sparkplugs, and installed everything correctly. Check stupid stuff, like maybe an E-brake shoe dragging. It's all rediculous stuff, but in order to have such a drastic change, and no code, would cause me to work backwards at any changes that I may have done to effect it.

Resetting the ECU may not do a thing, but it probably wouldn't hurt, again.

Keep us posted.

 

Well, Eltupac, I'll throw a few suggestions out there, do with them what you will. Are you sure your air filter is clean? you said you checked it, did you replace it? Is it in correctly? Is the housing sealed properly? What is your tire pressure? this can effect gas mileage quite a bit. Did you reset the ecm after all your maintenance? If not, try pulling the negitive battery cable, turn the key to the start position a couple of times, reconnect cable after 5 minutes or so. This will let the computor start its calibration from scratch, instead of trying to relearn the paramiters. Are the plugs the correct heat range? double check, dont trust a guy at a parts counter, find 2 independant sources. Double check plug wire / coil pack connections. Just because it "seems" to be running ok, doesnt mean you didnt miss a connection by a fraction of an inch. A weak spark will have a large adverse effect on A 6 banger, much more so than an 8 cylinder.

oh yeah, my HEMI gets 18 mpg, sorry, couldnt resist!

 

Yep, I agree, just double-check. Is a wire loose, outta spec, etc?

 

You ***. But thanks for the suggestions. I will try them. the spark plug was the champion double platinum, I dont know how to get the heat thing.

 

I might be wrong, but i thought we couldn't use platinum spark plugs in our trucks?

 

I don't know for sure, but I was under the impression as well, that the 4.7 and Hemi ran poorly on the Platinum plugs..... The V6 might fall into that catagory as well, obviously.

Iridium tipped is OK, but not the Platinums. That may be an issue...

The heat range of a plug is determined by its number. Different companies/makes of plugs determine their numerical system differently. Any plug site can explain it in better, more accurate detail then me.