Kawowski

I'm trying to determine if using 93 octane while driving conservatively (mostly highway) caused this carbon buildup. My thinking is that the Maxima calls for 91 octane... my area only carries 93... and that the extra octane isn't allowing the fuel to completely burn off leading to this ^. I am very knowledgeable about engines in general, but do not have experience breaking down and evaluating many engines.

I only use Shell 93. 40k miles on the engine. No mods, fuel treatments, etc.

My thought is now that I have a new engine, I should consider switching to 89 octane for more complete combustion and less carbon buildup. Is this amount of carbon acceptable? Is is mild buildup? Moderate? Severe? Does anyone think the buildup is something else besides carbon buildup?

My goal is not to start a "which grad fuel do you use" debate. I am only considering switching to a lower grade (89 is the lowest I would go) to prevent this ^. Any comments?



Below is what I was expecting to see... this is a 2010 Murano... same engine as our Maximas but with a lower compression ratio (87 octane requirement).

Richard66

Just my opinion and I ask this as a question but wouldn't the carbon buildup be equal across all the pistons if was the gas or the octane level?

LtLeary

I know this may be a silly question, but why the engine change? But more on point, this doesn't look THAT bad but pictures of the plugs and reason for the swap would help.

Kawowski

Kawowski

The engine was replaced due to a main bearing knock. The heads were removed for a head gasket replacement and the #1 and #4 bearings were damaged. For more details, see:

http://forums.maxima.org/7th-generat...ts-w-pics.html

Sorry... don't have a pic of the spark plugs and didn't see them.

What really looks bad to me is the first picture, far right piston and second picture, middle piston... there appears to be raised portions of the carbon that actual breakup the "flatness" of the piston. That doesn't look good to me... but again, I'm no expert. Thanks for the comments.

13Maximasv

There is a bearing knock issue per a TSB that occurs in these motors. You can read more here. I also had my short block replaced on my 2013 at 3000 miles because of the bearing

http://forums.maxima.org/7th-generat...-problems.html

LtLeary

Bummer...did the plugs look pristine? I have replaced mine twice..once at 100,000 and at 220,000 and the plugs looked pristine but then again, I did NOT pull the heads off to look at the cylinder crowns. I do admit that the relatively low mileage, and most of it highway driving you said you do, I would not have thought they would have as much carbon on them as much as they do, but perhaps having the bad bearings wasn't the only issue you had (or may have contributed!)

I don't know that changing to mid grade would help but then again, I believe that only the 09 had the "premium required" moniker.

Kawowski

Wow... 220,000 miles... how many miles do you have now? Any problems along the way with your Maxima?

13Maximasv

My motor is not the one in the picture and mine was just replaced.

Kawowski had his motor torn apart and then replaced, As far as I know his bearing knock returned on the new short block within 6 months

LtLeary

Just went over 226,000. Very few problems compared to many of my previous vehicles. Radiator fan was the most serious failure and a few problems with my climate controlled seat. Other than that, battery, oil changes, cvt drain and fill, and tires...normal expendable items. Brakes are still original and other than a suicidal raccoon and a few trucks throwing rocks and breaking windscreen, those are the only "non routine" items. Having said that, the original paint shrugged off the stones thrown by trucks, but could not survive the racoon! Definitely the best car I have owned in 40 years.

esco115

iTrader: (1)

Kawowski - you have the octane theory confused, 93 burns quicker then 92, 91, or 89. And burns at a cooler temp. as well. Your build up looks to be more oil based not full carbon, nonetheless, its pretty bad and its a call out to Nissan since VQ35s have been problem makers since their first introduction.

Kawowski

Wow... that mileage is great to hear. Don't know if you've followed any of my threads or posts... I had major CVT work done and a short block replacement. I'm hoping to get half as many miles as you without anymore problems. I'd really like to get out of this particular Maxima (it was obviously a lemon), but just can't afford to. How often did you flush the CVT? Sorry about the raccoon... sometimes they just want to end it all...

Kawowski

Thanks for the comment on the possibility of oil... I didn't think of that. Now I'm worried my heads are faulty leaking oil into the cylinders. They replaced my shortblock, but of course by definition, reused my heads. I haven't noticed obvious oil loss, but I'll keep an eye on it.

I thought the definition of higher octane was it's resistance to detonation... therefore ignition. It requires more energy to ignite (spark plug firing vs. only hot pistons). This prevents the fuel from igniting in high compression (and hotter running... think temperature vs. pressure) engines sooner than desired. I could be wrong, but that is my understanding.

Max2013

This is also my understanding of octane. Lower octane may cause "pinging" because it combusts too easily and pre-ignites before the spark plug fires. The higher the octane the more resistant to detonation, combusts when it should, and thus is less likely to "ping". Right?

MaximaDrvr

iTrader: (1)

You are both correct essentially. Diesels ignite from compression, and heat, not just heat. You aren't going to ignite fuel from just piston retained heat unless you are running a drag car. Higher octane resists ignition, but burns faster than lower octane.

That picture looks like oil leaking, maybe a bad valve seal. If it was gas related it would be the same across all cylinders. That looks like a head problem to me.

Kawowski

Thanks for the input... problem is they replaced the short block, so the same heads went back on the new block.

Around and around we go...

RCM78

Actually, he was right. The lower the octane the faster the fuel will burn...