ideal spark plug gap?

[gothstone]

Has anyone played with the spark plug gap to see it affect on gas mileage? Manual states 0.039-0.043 , most sites say .044 GM sent that letter knocking the 09's down to 0.031-0.035. While I realize it will probably work just about anywhere in that range, surely there is an ideal value. Thoughts?

[kcROB]

Great question and, in my case, timely since I need to upgrade my 2011 factory plugs. Most of my Aveo driving will be city. I too, would appreciate some savvy on the gap question. Would highway mileage be optimized by increasing or decreasing gap? Thanks!

[Thymeclock]

Great question and, in my case, timely since I need to upgrade my 2011 factory plugs. Most of my Aveo driving will be city. I too, would appreciate some savvy on the gap question. Would highway mileage be optimized by increasing or decreasing gap? Thanks!

No, your MPG won't be improved by playing with the plug gap. Usually setting it in the recommended range will work fine. For example, if the recommendation is for .039 to .043, gap the plugs at .040 to account for the increase as the plug wears and the gap widens.

If you narrow the gap more than the recommendation you might experience dieseling, a condition where the motor continues to run after the ignition is shut off. If you examine the plugs and they look black like carbon rather than clean grey ash, you might consider switching to the next level of a hotter plug, but beware: on Aveos there is a recall due to potential failure of the coil pack. Chevy's remedy for the coil failure was to substitute a spark plug that is the next level colder in heat range. So be aware that too hot a plug or too narrow a gap could cause problems.

[kcROB]

Thymeclock, I sincerely appreciate your plug gap advice. My 2011 has had the recall satisified by the previous owner at 9.5k miles.I will examine them carefully before any changes are made. Be well and stay warm!

[thankful_ragamuffin]

I think that Thymeclock’s post is pretty much the definitive answer on the subject, but I’ll just throw out this bit of info that NGK presented at a seminar they held for all the local shops here. I don’t know what the U.S. laws are on fuel labeling, but here the warning “CAUTION. MAY CONTAIN ETHANOL” is all that is required on blended fuels, so the percentage of ethanol going into your tank is unknown. According to the NGK rep, the percentage here could be as high as 20%. They suggested that with that much ethanol a colder heat range plug is required, especially in heavy vehicles that work hard. They also said that the key to success with colder plugs is to tighten the gap 5 – 10 thou. Our shop has made the change to colder plugs for lots of customers with feedback from only one fellow – he said his big block equipped Chevy truck pulls his trailer up the grade somewhat better but he has lost 2mpg (Imperial gallon). So any further thoughts on heat ranges and plug gaps for our little engines?

[kcROB]

Something to consider, reduce gap in presence of ethanol using a colder plug. When the midwest warms up, the plugs will come out w/ approx. 11k miles, Aveo has 20k now, should show combustion clues. Thanks!