I recently had my spark plugs changed and have noticed a dramatic decrease in power and performance. Om top of that, my gas mileage has dropped about 10%. Any ideas on what the cause may be? Moreover, any ideas on how to fix this? Thanks in advance.

 

Redo, re-gap, make sure all coil packs are on snug and all plugs are plugged in. What kind of plugs did you put in?

 

A little more info...
I had the work done at a Nissan dealer. I haven't the tools or the knowhow to do it myself. I took it back and they checked it over and found nothing wrong. Figures. From the receipt the new plugs are...
22401-EW61C.

 

So you had them changed twice with the same results? Hmm..

 

No, just once. Sorry for any confusion. When I took the car in it ran fine but when I drove it away with the new plugs I noticed immediately that something was not right. Drove it for 2-3 days, took it back and complained. They made sure everything looked OK and declared that there was nothing they could do. They suggested that I had a bad tank of gas even though it was the same tank of gas that I drove in with. I'm stumped and bummed cuz I've lost my "rocket sled" as I like(d) to call it.

 

They're definitely giving you the run around because they see that you don't know about cars. Truly dispicable. A bad tank of gas? Seriously? Bunch of idiots. I worked at a gas station for years and that is not a thing. As if somehow you got the only 16 gallons that didn't get filtered thru the many filters that are in any pump.

Anyway, enough of my rant.
I would take it to an independent mechanic and don't let on that you don't know about cars. It seems that you aren't getting sufficient spark somehow thereby not burning the gas efficiently enough to make the power you should be getting and spitting a lot of it out of the exhaust.

Fortunately this would only be due to a few things: bad plugs, one or more failing coil packs or a gap that isn't big enough to deliver enough spark.

 

There is a bigger issue here.

On a side note, a bad tank of gas is seriously apparent in rural gas stations!

 

No bigger issue. OP changed the spark plugs and now something's wrong. It would be mighty odd for something else to happen to go wrong at the same time. Its the only variable. I would just start the job over and go over everything. Just too bad OP doesn't know how to work on it themselves, cuz plug jobs on our cars cost a fortune.

 

OEM ngk iridium from the part number OP gave.

 

Truth be told my car (2007 g35x) is pretty fussy when it comes to fuel. kwik-fill gas and gas at local supermarket make it run poorly but Sunoco seems to be the ticket. Been running Sunoco for the last 2 weeks, perhaps some improvement but nothing like before when hitting the gas was actually fun. Oh well, I'll try to find a good mechanic and get ready to break out the credit card again. Thanks again for the comments.

 

no kidding
I would never run that garbage gas. Pops owned two Mobils and would always warn against it. Shell for me only. If nothing then Exxon or Sunoco
That's it.

 

Here's another twist...the car seems to take off from a dead start OK, the problem really presents itself when I'm tooling along 30-45 mph and decide to gun it. VERY slow response, often it seems to downshift very reluctantly, and when it does, that's where I no longer feel the beloved g-forces. Almost feels like I'm dragging an anchor. And again, it didn't do this before I had the plugs replaced. Kinda weird but that's the type of product I usually wind up with. Problems that nobody has ever heard of and have no idea how to address. Very frustrating.

 

Just a thought...could it be a transmission problem based on the info in my last post?

 

I'd pull one plug and confirm it's the correct p/n and heat range. Getting one that the wrong heat range might do this

 

Just did a search, the plugs the Nissan dealer installed (22401-EW61C) are NOT COMPATIBLE with my car. WTF! Boy did they ever lose a customer! $350 down the drain because they're never going to see my car again!