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Tune up leading to worse gas mileage?

I’ve recently tuned up my 1999.5 pathfinder only to have the gas mileage decrease from around 17MPG to 14-15MPG. While I know this is not normal, I can’t figure out what went wrong. I was expecting equal or improved gas mileage and that has definitely not happened.

I changed out the plugs, wireset, distributor cap and rotor. I replaced the originals with NGK Iridium plugs, NGK wireset, Beck & Arnley dist cap and rotor.

I’ve done practically everything to troubleshoot this issue, but to no avail. Fuel injection cleaners, disconnecting the battery, removing certain components and replacing them with the old parts, taking out the plugs and reinstalling them, etc.

Overall the ride is fine and smooth…but the mileage really tailed off. I will try to bring it in for an oil change to see if that helps. Any tips would be much appreciated.

If the mileage dropped, changing out old parts with new and then changing out the new parts with the old makes 0 sense in my book.

BTW, what makes you think an oil change will solve the loss of mpg?

I suspect the click you hear MAY be from a sensor.

Possibly from the engine temp sensor telling the computer that it is getting a temp reading lower then the normal operating temp causing it to run cooler, thus feeding more fuel than normal to keep the engine running in a cold mode.

Did you have the battery disconnected while tuning the engine? Were any adjustments made to the timing? I have occasionally seen similar problems immediately after minor and major repairs and if all the adjustments are back to spec the system usually puts itself back to its prior, normal running mode in a few trips driven from cold start to fully warmed and cruising. That’s just my SWAG on the situation, though.

I’m curious though, how do you drive the pathfinder? I would have expected better mileage than the 17MPG you began with.

The engine may not like those iridium plugs. Try putting the old platinums back in and see if it solves the problem. Some engines just don’t like certain spark plugs.

So the rational behind swapping the old parts back in was to process of elimination. Prior to the tune up, the old parts were still getting me roughly 17-19MPG. After the tune-up it dropped down to roughly 15MPG. The drive of the car felt more smooth, but the gas was being used up at a far faster pace. I wanted to see if any of the new parts were somehow defective, but that was not the case.

I did not mention any clicking sound or anything of that nature. Do you think this is a sensor issue?

The battery was disconnected only after the tune-up was completed. I read in one of the forums that sometimes disconnecting the battery completely can help reset things. I believe it is similar to what you had stated about the system being back to spec and taking a few trips to adjust itself. My 1999.5 Pathfinder has a MPG of 15/19 for city/highway driving. I usually get anywhere from 17-19MPG. Once hit 20MPG, but that was very rare. After the tune-up it dropped down to 14-15MPG. I don’t speed and I don’t tow anything around, this model just has 170 horsepower for a vehicle that weights over 4000lbs. Thanks for the feedback

Thanks BustedKunckles. I have been wondering all along whether going with the iridium plugs was a good move. All of the reviews that I’ve read have stated that the iridium plugs are far superior to the platinum ones and also last longer. My OEM plugs were NGK Platinum. I might try your suggestion and swap them out. Although it’s going to be a pain in the butt to get the last plug in the back of the engine swapped out. Thanks for the feedback!

That’s simply not a significant change in mileage. Assuming you’ve been tracking mileage for the last year, drive it for another year, THEN compare mileage.

By the way, disconnecting the battery is not going to help the mileage.

Are there any mechanics where you live? How many gallons of gasoline could you buy with the parts you bought?

It’s still not clear to me. What they’re telling you is that disconnecting the battery erases the long term trim values and sets them back to default. Over running time the computer optimizes those settings to match engine performance and usage patterns. If you disconnected the battery before the lower mileage results were measured it might explain the discrepancy. It will take some run time to regain optimum efficiency. If it’s running ok you may want to take a wait and see approach rather than continuing to mess around with it.

[i] The engine may not like those iridium plugs.[/i]  

Yea, it has been my experience that OEM plugs and wires and both cheaper and more reliable than the fancy expensive stuff.

How do you measure your mileage?  Are you using a the old fashion method of miles between fill-ups and gallons purchased?  Have you had only one fill-up or several? 

Did you change the plugs before or after you noticed the change in mileage? How has the weather been in your area? Any change?

I have filled up my tank roughly 8-10 times since the tune up. I am using the old fashion method of calculating MPG with an app on my phone that keeps track of the stats for me. The drop in mileage was almost instantaneous after I switched out the plugs…and it seems to only be getting worse. Before the tune-up I was averaging 17-19MPG. Directly after the tune up it dropped down to 15MPG…and now I even see times where it dips into the 14MPG range. I even tested this on strictly highway driving…and it was still only 15MPG.

The weather has been decent. Temperatures have been in the high 60’s to high 70’s. I’m handling the car the same way I always have. I’m not a speed demon by any stretch. No change in driving habits at all.

I’m ordering a set of platinum plugs and seeing if that resolves the issue.

Hi TwinTurbo,

I’ve been waiting quite some time already for the engine to return to optimum efficiency. The car has been running like this for the past 4 months or so and I have already gone through about 8-10 fill ups. The MPG is still averaging 14-15. Going to try and swap out the Iridium plugs for Platinum ones.

At first that’s what I thought as well. Going from 17-19MPG to 14-15MPG is not a huge drop off. However, everyone I have spoken to has said that there should not be a drop off in MPG. It should be at least as good if not better than before the tune up. I calculate the MPG of my car all the time. I was consistently getting 17-19MPG for the last 10+ years prior to the tune up. The main reason I tuned-up the car was because occasionally when I started the engine it began to rumble.

So this drop off of about 3-5 MPG is very apparent to me since I have never dipped this low EVER with this car. It is very apparent that the tune-up caused the MPG levels to dip to all time lows. I will replace the iridium plugs with platinum and see if that works.

I’m a big believer of OEM parts and fluids.
If an original part gave good service for a long time why introduce uncertainty by using something different?

I have filled up my tank roughly 8-10 times since the tune up. I am using the old fashion method of calculating MPG with an app on my phone that keeps track of the stats for me.

Good job, that should result in an accurate figure. Unless you have had a change in the weather, or driving habits, I believe you are on the right track.

* * * * * Is it possible that you ended up mixing up a pair of plug wires?

“I have filled up my tank roughly 8-10 times since the tune up.”

So what you’re saying is you have NO idea how much gas you’ve put in the car since the tune-up and therefore NO idea what kind of mileage you’re getting. End of story.

ZombieWolf, funny how you jump to conclusions and assume I have no idea how much gas I put into my car. I didn’t have the numbers handy so I was just using the 8-10 estimate to show that this was not just happening after 1 or 2 fill ups. As I’ve stated, I consistently calculate my gas mileage and over the past 10+ years, I have seen my MPG range anywhere from 17-19. If you don’t want to be helpful, then please don’t waste anyone’s time posting anything condescending. You have not been very helpful and please keep your comments to yourself if you’re not willing to help.

Appreciate your feedback circuitsmith. I had read that Iridium plugs were far superior for durability and potentially improved gas mileage, so I decided to try them out. I figured that my OEM plugs were NGJ and if NGK produced superior plugs, why not give it a try. This is the first time I’ve heard that some engines don’t take well to Iridium plugs. I guess you live and learn! I’m going to revert back to the NGK Platinum plugs.

That is the part that I made completely sure I paid special attention to. I swapped out a plug at a time marking each wire with masking tape and a sharpie to ensure it was put back together in the same manner. Thanks for asking though.