Spark Plugs and Fuel Mileage

B

blackfx

Member
Location
Florida
Hello, I have a 2004 Infiniti fx35 rwd. I just recently put in new NGK Iridium spark plugs. Before the install, I was getting 17-18 mpg but after the new plugs I am getting 13-14 mpg. I have driven the car about 30-40 miles with the new plugs under the same driving conditions and do not know why there is a drop in fuel economy. I am not sure if I should wait it out and see if the mpg's go up or take it back apart and check everything out. I followed the instructions to install them perfectly and don't know if this could be a gap issue or loose plug issue. Any help is appreciated!:smile (2):
 
Viperi said:
did you reset your MPG tracker after the spark plug install on the screen? cause it takes the average.
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No, however it was averaging 17.5 before the change and is now averaging 13.2. It would fluctuate between 13.1 and 13.2 so I am pretty sure that is where the mpg is resting as of right now. If anything I thought that plugs should help fuel economy not hurt it.

---------- Post Merged at 03:11 PM ---------- Previous Post was at 02:24 PM ----------

I took the engine cover and air tube off and checked the connections. They all seem to be fine. I'm guessing this is a gap or loose plug issue.

---------- Post Merged at 05:03 PM ---------- Previous Post was at 02:24 PM ----------

I guess I might check the gap. What gap do I need for the VQ35?
 
Platinum vs iridium vs copper, doesn't matter too much. The different materials dictate how long they last. Platinum is supposed to go 105k miles, iridium is 60k, and copper is 30k.

Did you get the correct range plugs? Some people go 1 step colder that could result in the loss of mileage.

Stock range Ngk plug is PLFR5A-11. One step colder is the 6A-11.

Tapatalkin'
 
Hmm. I've installed iridium plugs in my Car when it called for platinum and there was a huge difference . Iridium was nonstop shaking but once I switched them out to platinUm it was fine.
 
catchx34 said:
Hmm. I've installed iridium plugs in my Car when it called for platinum and there was a huge difference . Iridium was nonstop shaking but once I switched them out to platinUm it was fine.
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I don't have any shaking problems. It runs fine so hopefully that is not the case.

---------- Post Merged at 04:35 AM ---------- Previous Post was at 04:30 AM ----------

The plug says LFR5AIX-11.
 
Strange, I'm running iridium plugs right now and it runs great. I know Flxr8 was running copper plugs in his Fx and had no issues. He just liked to replace his plugs regularly.


5A-11 is the stock range.
Tapatalkin'
 
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Ok, thanks guys. I actually ended up breaking a screw while I went in there to re-tighten them. That is why I am taking so long to see if the tightness was the problem. They did seem pretty loose so I tightened them to hand tight plus a quarter turn. Maybe this will solve it since the plugs are the right kind. Thanks for all the help though!

---------- Post Merged at 03:15 PM ---------- Previous Post was at 10:16 AM ----------

Seems I am now getting slightly better mileage. 14.2 city driving. It is also on low grade 87. Just wondering if this is on par? Gonna run 89 next fill and see if this helps. I also ended up tapping a new hole into the old broken screw that holds in the coil that attaches to the spark plug. It is a stainless steel screw just wondering if I should have any worries about it holding it in there. Thanks for the help!
 
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