G35x Spark Plug changing question

[i30daretodif]

quick question. Changing my spark plugs on an '04 g35x. I don't see any pooling of oil (as I've read about), but the old plugs look pretty dark and smell a bit like oil. #2 is the worst, #4 is medium, and #6 looks ok. I've attached a picture of #2 compared to one of the new plugs (NGK Laser Platinum). I'm just wondering if someone can tell me if I have any oil leak issues based on the picture. If so, what is the impact of that? I've been getting about 17.1 mpg recently - would sure like to improve that.



Thanks

[joe603]

If you don't see pooling, then you're probably okay. How is the oil level? You have a non-REVUP engine, so oil consumption is not as common, but still a possibility.

How many miles are on the plugs? If over 80k, I wouldn't worry too much. One thing I would recommend is getting a plenum spacer. The upper intake has a design issue with a few cylinders not getting the proper amount of fuel...the spacer helps get back some HP/TQ lost.

Recommend these guys for the spacer: http://www.motordyneengineering.com/

[i30daretodif]

Thanks - appreciate it. These are being changed right on schedule at 105k. The passenger side is kind of a PITA, but got it done. Car seems to run well on my test drive last night.

[joe603]

ya, 100k+ is time for the plugs! Still recommend the plenum spacer (do a 5/16" for the best gains). Very easy job...15 min tops.

[i30daretodif]

Thanks Joe - I'll consider the spacer (I looked through the MD site and watched the install video). I'm thinking of getting a new car this summer, so I don't want to dump too much into this one. But it is very, very tempting.

[joe603]

I had an 05 coupe a few cars back...had the spacer, MREV2 plenum and a cold air intake...sounded mean!

[SteveTheTech]

That plug doesn't look bad, a little fouled but not enough to cause a miss. That gap is a little wider than it should be.

I'm going to take a shot in the dark and guess that you long term fuel trim setting was about 120, meaning your engine requires 20% more fuel than it should to achieve a stoichiometric burn. Since plug wear is so gradual most never notice the decreased fuel economy and overall performance that happens gradually as any car ages. Using the computer I can watch the numbers and tell you without opening the hood what those plugs would look like (assuming there are no fault codes). If you have a scanner handy (and do not have an emissions inspection coming up) you can try clearing the cars memory, this should help the cars return to a zero set position quicker. Better fuel will also make a difference in your economy, but beware about fuel injection cleaners and additives for the gas tank. What you should do is (safely) drive the car for a few miles in manual mode, pushing the rpms and engine load high but not to an abusive point. You want to increase the engine temperature to burn off the crud that accumulates as a byproduct of an engine with a pcv system and intake valve overlap.



If you (anyone reading this) are one of the drivers that starts to suspect the car is not running well after about 80K miles it probably is not longer in top shape. I have been inspecting almost everything with a first gen VQ35 at about 80 that run a little sluggish. I suggest pulling the plenums apart (not a job for most) and cleaning them (out of the car) and replacing the plugsand intake gaskets a little early on some. Typically those who use low octane fuel and drive mostly short trips around town develop gunk in the intakes and prematurely need plugs. I do not recommend these early to make money, it really does help. As long as you use the right plugs. I've driven so many of these over the years I can tell within a few feet of moving out of a parking space, what the engine is running like (if it is warmed up).

[i30daretodif]

Steve - thanks so much for the awesome response. In your opinion/experience - what kind of mpg should I be getting if it's in good shape? Full disclosure: I have a JWT Pop Charger and Z-Tube.

I do have a scanner, but was uncertain how to change the fuel trim setting. I live in a place where I don't need to get emissions tested, so I used the scanner's erase code to erase whatever was in there. There were no codes at the time. Most of my driving is highway (55+ mph), I'm averaging about 60+ miles / business day. I did notice some darkness around the throttle body, if that makes any difference.

Plenum cleaning sounds fun - more fun if I was putting a spacer in at the same time.

thanks again!

[SteveTheTech]

There are really too many factors for me to give anything but a shot in the dark.

Honestly they range between 16 and 19ish. You should be on the higher end if you do mostly highway driving. But I have no idea where you are, what your base numbers look like or what the car runs like. If it has no codes I would assume it is doing fairly well (within 20% of federal test points) but 3 mpgs can be a big shift based on small details. Sometimes in highway mileage cars like yours actually start to wear like cars who are primarily used for shorter trips. If the car just sits in overdrive coasting lightly it can buildup carbon and decrease efficiency.

Some down stream O2 sensors may help if you continue to notice decreased efficiency. It may be cheaper to shotgun those than paying someone to test it properly. They get old and control more of the air fuel management than anything else.

Clearing the self diagnosis memory is about all you can really do. Most generic scanners cannot access these levels, because you can do some damage in there if you are poking around blindly.