Mileage Drop After Tuneup.

MikeInCtown · February 27, 2009

I'm not really sure what may have happened but my mileage has actually gone down after my 100k tune up.

An oil change was done about 500 miles before the tuneup. I also installed a new air filter about 300 miles before the tuneup. With the oil change and filter I was averaging better than 22mpg with my 5.3 extended cab on the highway with the cruise on. I replaced the plugs with OE A/C and even replaced the wires with A/C from the a dealer.

Since the plug and wire change I have noticed that my mileage on the highway using cruise has gone down roughly 2mpg or more. I'm at a loss as to what it could be. When I put the new plugs in I used a slight amount of anti sieze. I also put some of the wire boot lube/protector/sealer on the wire ends when I did the replacement so the boots wouldn't stick. Since I noticed the mpg go down I have removed every wire and cleaned out the boot protector. I have also removed every single plug and cleaned off every bit of anti sieze from the plug. I am sure some still remains on the threads in the block though.

The old plugs all looked exactly the same and had no carbon buildup. The new plugs already had some carbon on the flat part of the plug at the ends of the threaded area. (nothing on the porcelain or electrode) Seeing this carbon buildup makes me think it is running rich and that I am getting incomplete combustion from one or more cylinders. -this is after about 500 miles on the new plugs.

I am at a loss as to what to do next. I just picked up a new set of OE A/C wires (red, not black) and am going to install those. I also plan on taking my meter and checking the resistance of all the wires thinking that I may have gotten a bad wire or two. It certainly won't be the first time.

is there anything else I should be looking at, or am I basically limited to having one or more plugs or wires bad as that is when the decreased mileage happened?

-Mike

ColoradoSkiCountry · March 1, 2009

Mike,

check to make sure that the plug numbers match, soemtimes a wrong plug can get put into the box on accident. 2nd, go ahead and disconnect your battery for about an hour or so to reset the onboard computer.

I've always noticed a slight decrease in MPG after doing a tune up, but after 1000 miles or so it usually returns.

green 788 · March 1, 2009

The first thing I would suspect is that it could be guilt by association... in other words, there could be other factors that came into play long about the time of the tune up--and these factors may be the real culprit.

A large change in the outside temperature and/or humidity will affect gas mileage. So consider that...

Further, if you bought gasoline from a different station which is using 10% ethanol you will almost certainly see a drop in mpg. If you're buying from the same station, it's possible that they have begun using 10% ethanol additized gas recently.

I don't think it's a plug wire issue. It could be that your plugs, though, are gapped differently now, causing the engine to be a little less efficient.

Good luck on solving the problem.

Dan

MikeInCtown · March 1, 2009

Ryan, thanks for the plug suggestion. I did check a bunch but not every number. There were two numbers. The A/C oem number and then the aftermarket number. On a few of the plugs the OEM long number was different but the short 5 digit number for aftermarket was the same. I'll have to do the disconnect thing again. I tried disconnecting for 5 inutes a week ago and holding the cables together. What was wierd was that the radio didn't even lose it's presets. Maybe waiting longer will help.

Dan, to be honest nothing changed at all beween plug changes. I purchased the truck exactly 1 month ago and it had a fresh oil change and 100,500 miles. The week I got it I did a Summit high flow filter change. Within the first two week the temps were all 20s/low 30s. I went for a highway drive of 150 miles+ to get a bedliner and averaged 23mpg strictly highway. I would also notice that when going down the road where I work in the rural areas that I could set the cruise on 50 and get 25mpg instant readings.

After the change I can get 20-22 readings and on the highway the best I can average is 20. (using the business trip computer vs the personal is really cool) What's really odd is that the plugs had that soot on the bottom of the threaded area and the old ones had nothing. It's almost like I'm getting a weak spark and the system is reading more O2 in the system and adding more fuel. This past week I did add a K&N intake that I found for $100 so I have an extra Summit high flow filter now. I'm hoping that with the new intake I can get my mileage at least back to where it was when I bought the truck.

I got some new OEM wires off eBay and used my ohmeter to check new vs old. I'm getting ~834 ohms on nearly every wire. If I would have known a 9" wire would read 800+ ohms I would have just bought a set of MSDs or something as they worked great on my old S-10. I'm going to have to check all the new wires next weekend after I fix my busted Bravada.

It's just so mysterious and if I didn't notice the soot at the bottom of the plugs I really wouldn't be so worried about the mileage drop.

redvett · March 1, 2009

Doing a relearn will not help since the PCM is constantly in a learn mode. Starting from scratch in the fuel tables will take at least 100-150 miles of driving.