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[2011] Spark Plug replacement causing bad fuel economy.

3K views 9 replies 3 participants last post by  amc49 
#1 ·
after driving for both the old plugs and new plugs, i found out the gap was wrong, the initial spark plugs that were in there were gapped at .038. After putting in the correctly spec'd new plugs, my fuel economy dropped from around 24ishmpg to 14-17mpg. I'm going the initial gap in the new plugs and see what happens then...but anyone have any idea for how this could happen. I know it could be a weak ignition system, so probably old or non-oem ignition coils, or it could be with the timing. What do you all think?
 
#2 ·
There might be underlying issue not necessarily the spark plug replacement. This new spark plus are pregapped already when you buy them, is not necessary to gap them again. Check other issues with it, perhaps you have introduce a vacuum leak as a result of installing the new spark plugs.

Sent from my VS995 using Tapatalk
 
#3 ·
Okay so, yes, spark plugs are supposed to be pre-gapped. However, I bought really cheap $3 spark plugs off rockauto, SKP brand to be exact. There was no vacuum leak created when replacing the plugs. I believe, for a brief moment, I changed the gap from .044 back to .038 since thats what the old ones were gapped at (which by the way is what I'm trying to emphasize, if the problem was a vacuum leak i may have introduced or anything in regards to changing the plugs, that doesnt answer the question as to why the original plugs were gapped wrong). This lasted for less than a day because, while working with my mechanic friend, we were waiting around for something, so we decided to do some preventative maintenance and clean out my throttle body and replace my PCV valve (although my old PCV was fine and I wish he had left it with me so I had a backup just in case, but whatever its an inexpensive part). After all that, when driving my car, around 20mph the car would hesitate/stumble, like it was losing power, and then it would kick back and kind of jump. It wasn't the transmission because while it happened, instead of the RPMs going up, they were going down. My friend told me the issue was my plugs which I had just changed the gap on, so I decided to say screw it and just put the original plugs back (being a new "mechanic" or car enthusiast I didn't know the importance of using OEM plugs at the time I ordered them) and it didn't solve the issue. I did the idle relearn process a few times as well and it still didn't solve the issue. I live in Las Vegas and it was like 112 degrees that day. It turned out the issue was the heat and that I had less than a quarter tank of gas, I believe the car would lose power because it stopped getting fuel for a second or two. I read online that nissans had this problem, but I'm not sure if it's a nissan-specific problem or if it's a problem with the new fuel pump I just installed which isn't exactly a real quality part or well known brand, but whatever, I'll just fill up the tank every time it gets halfway now. And, re-installing the original Denso plugs that were inside my car when I bought it, seemed to bring my gas mileage which before was as low as like 12mpg, to currently an average of about 22mpg. However, like I said, the gap on these plugs is at .038 not the factory .044. I made sure to check the gap on a new gapping tool at autozone to verify my tool was giving the correct reading, both gapping tools gave the same measurement. And, I had went to the junkyard and pulled the plugs on an altima there around the same year as mine, and the plugs measured about .044, so basically factory spec. After cleaning the throttle body and doing the idle relearn process, my car no longer starts at 1500rpm and drops to 750 once its warmed up, it seems to start at about 1000rpm. So I guess at this point...here's my question..

Should I take the original Denso plugs out and gap them to .044 then re-install them and see what happens? Or, since my gas mileage is kind of back from the dead, and that's the way the plugs were gapped when I bought the car, should I leave it like that?

Any advice would be greatly appreciated. I'm kind of already assuming you guys are going to want me to re-gap them to factory spec and see what happens so we can try and get an idea as to why they were gapped wrong in the first place and also make sure im not doing any damage to the engine or ignition system from leaving the plugs the way they are...

Thanks everybody!
~Vince
 
#4 ·
Okay so, yes, spark plugs are supposed to be pre-gapped. However, I bought really cheap $3 spark plugs off rockauto, SKP brand to be exact. There was no vacuum leak created when replacing the plugs. I believe, for a brief moment, I changed the gap from .044 back to .038 since thats what the old ones were gapped at (which by the way is what I'm trying to emphasize, if the problem was a vacuum leak i may have introduced or anything in regards to changing the plugs, that doesnt answer the question as to why the original plugs were gapped wrong). This lasted for less than a day because, while working with my mechanic friend, we were waiting around for something, so we decided to do some preventative maintenance and clean out my throttle body and replace my PCV valve (although my old PCV was fine and I wish he had left it with me so I had a backup just in case, but whatever its an inexpensive part). After all that, when driving my car, around 20mph the car would hesitate/stumble, like it was losing power, and then it would kick back and kind of jump. It wasn't the transmission because while it happened, instead of the RPMs going up, they were going down. My friend told me the issue was my plugs which I had just changed the gap on, so I decided to say screw it and just put the original plugs back (being a new "mechanic" or car enthusiast I didn't know the importance of using OEM plugs at the time I ordered them) and it didn't solve the issue. I did the idle relearn process a few times as well and it still didn't solve the issue. I live in Las Vegas and it was like 112 degrees that day. It turned out the issue was the heat and that I had less than a quarter tank of gas, I believe the car would lose power because it stopped getting fuel for a second or two. I read online that nissans had this problem, but I'm not sure if it's a nissan-specific problem or if it's a problem with the new fuel pump I just installed which isn't exactly a real quality part or well known brand, but whatever, I'll just fill up the tank every time it gets halfway now. And, re-installing the original Denso plugs that were inside my car when I bought it, seemed to bring my gas mileage which before was as low as like 12mpg, to currently an average of about 22mpg. However, like I said, the gap on these plugs is at .038 not the factory .044. I made sure to check the gap on a new gapping tool at autozone to verify my tool was giving the correct reading, both gapping tools gave the same measurement. And, I had went to the junkyard and pulled the plugs on an altima there around the same year as mine, and the plugs measured about .044, so basically factory spec. After cleaning the throttle body and doing the idle relearn process, my car no longer starts at 1500rpm and drops to 750 once its warmed up, it seems to start at about 1000rpm. So I guess at this point...here's my question..

Should I take the original Denso plugs out and gap them to .044 then re-install them and see what happens? Or, since my gas mileage is kind of back from the dead, and that's the way the plugs were gapped when I bought the car, should I leave it like that?

Any advice would be greatly appreciated. I'm kind of already assuming you guys are going to want me to re-gap them to factory spec and see what happens so we can try and get an idea as to why they were gapped wrong in the first place and also make sure im not doing any damage to the engine or ignition system from leaving the plugs the way they are...

Thanks everybody!
~Vince
A problem arising of a quarter of gas that makes the car run haywire is false to the best of my knowledge. I own Nissan Sentra 2008 being driving it most times with low gas and have never encountered that problem, but aside this is a bad habit to always be running your gas low.

Lastly let the car run a while and check feedbacks, sometimes letting the car relearn for a while helps. Am sure after relearning your gas mileage will be better .

Sent from my SM-G925T using Tapatalk
 
#6 ·
From higher up post #2...........

'....is not necessary to gap them again.'

Referring to new plugs out of the box.

NO. NEVER assume the gap is correct, I used to sell them and if you had any idea how many get dropped......I had to scold stupid kids who got into plug wars throwing the boxes at each other in the store.

BESIDES THAT, a certain plug part number may find itself in 50 different makes of engine, the gap chosen for that plug out of the box will be the most common one used by most of those engines. if yours is not in that group the gap WILL BE WRONG and up to you to catch that.

ALWAYS check the gap on ALL new plugs, it often is how you also catch the fact that if iridium or platinum spots are welded to them they are all there; many are crap welded and come off in the boxes and you then just paid $10 for a $1 plug.

Sloppy work always has a cost. It fits right into a Trump world though.
 
#8 ·
You actually should go to the factory stated plug gap, it is matched to the coil strength to give maximum spark with max coil life. You can go .005" closer to extend the tuneup time.

Brown residue showing up quick may be too dark once the entire porcelain has colored up solid. I look for very light tan myself. As you widen the gap the plug color should lighten up unless you go too far to then begin to not jump spark because gap is too wide, then plug will go dark. Wider gap = more spark voltage, at least until the coil can't do it.
 
#9 ·
You actually should go to the factory stated plug gap, it is matched to the coil strength to give maximum spark with max coil life. You can go .005" closer to extend the tuneup time.

Brown residue showing up quick may be too dark once the entire porcelain has colored up solid. I look for very light tan myself. As you widen the gap the plug color should lighten up unless you go too far to then begin to not jump spark because gap is too wide, then plug will go dark. Wider gap = more spark voltage, at least until the coil can't do it.
Alright cool, finally some advice! Thank you sir. I feel like at this point, I should take a picture of the plugs and post it (which I will do within 18 hours, in the morning or afternoon probably). Here's the thing, I threw out my new plugs cause they weren't OEM and gave me bad gas mileage and were just bad lol. So I returned to using the old plugs, however, they're not OEM either AND they're iridium. OEM calls for platinum, which is fine and all, but I don't know if the timing on the engine is perfect or if they're using OEM or aftermarket coils. That being said, I don't know if I should touch the gap since there is all these variables. The ones in there now, the iridium ones, have been in there since I bought the car. is there any way to check the coils and see if they're OEM or aftermarket, or possibly iis there a ratio of voltage to spark plug gap? I guess what I'm trying to ask is, aside from changing the gap and seeing how the car runs afterwards, is there anything I can do beforehand that will tell me what the gap SHOULD with the information coming from the car rather than just the service manual??
 
#10 ·
Even if aftermarket coil they have to match the OEM specs making them, so you use OEM gap spec. Use the type and BRAND of plug specced for the car, not doing so you get into variances never planned for when the gap spec was picked. Platinum and iridium are simply longer lasting plug tips, platinum about 60K mile and iridium the first true 100K mile plug. There is no ratio of voltage to gap size other than the coils you have will fire more voltage the wider the gap gets, It's a rule of spark. The coil never makes any more voltage than it takes to jump the gap and smaller gap is easier to jump. But they don't burn as hot nor expose as much fuel to the spark with smaller.
 
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