LOSS in fuel economy after brand new spark plugs and wires!
10-23-2012, 09:05 PM
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LOSS in fuel economy after brand new spark plugs and wires! So for my 04 GTO I purchased some NGK Laser Iridium plugs pre gapped at 0.040 (since iridium is what I pulled out), and got MSD 8.5mm wires (since 2 of mine were splitting at the outer layer of rubber). Everything sounds fine, however, I'm trying to improve my recent loss in fuel economy, and this time it actually made it even worse! I usually get anywhere from 17-20 mpg, then I started getting 14-15, now after the plug/wire change, I'm averaging around 13! WTF! I tried the fuel system cleaner, did nothing. Don't think it's the air filter since I just installed a new K&N system 6 months ago. Fuel filter is inside the tank, no knowing what's up there, so my next guess was spark plugs. I was running lean which could explain my issue somewhat, but I still don't know what's causing the car to run lean. And no my foot isn't getting any heavier lol, in fact it's gotten lighter since I've noticed this mpg loss. I only have intake and mufflers so I don't think I need a tune to rework the air/fuel ratios etc, but plz tell me if anyone believes otherwise. I've heard o2 sensor could be the culprit, I've heard mass air flow sensor, vacuum lines, but I can't put my finger on it. Car has 74,500 miles if that helps any...any help would be great as these are rough times for loss in mpg's! $5 a gallon out here in Cali 
10-24-2012, 10:10 AM
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Thanks I'll try that, it has 74,500 miles...I also smell a funny scent I'm not familiar with...maybe its the anti-sieze that touched the block as I was installing them? Idk, I just hope it's not a loose connection within the wires
10-24-2012, 10:16 AM
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You'd have to be driving in some pretty dusty/dirty conditions, and/or putting on A LOT of miles to have a clogged air fiter in just 6 months. But it's possibile that some of the K&N filter oil has gotten on the MAF wires. Usually this is the result of over oiling the filter after cleaning it rather than a new, factory oiled one out of the box though. But it's still worth cleaning the MAF.
How do you know the car was running lean? Did you scan LTFTs? If they are elevated then it may be to cover a vacuum leak so you could start by checking that. O2 sensor might be an issue as well, but without any codes present to indicate a bad sensor, all you can do is change both fronts and see what happens.
How do you know the car was running lean? Did you scan LTFTs? If they are elevated then it may be to cover a vacuum leak so you could start by checking that. O2 sensor might be an issue as well, but without any codes present to indicate a bad sensor, all you can do is change both fronts and see what happens.
10-24-2012, 01:08 PM
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Over-oiling of K & N air filters is very common. When you buy a new K & N it comes out of the box a nice orange color, so people wash the filter clean and spray it with oil until it looks like a new one.
That is way too much oil. The filters are lightly sprayed at the factory and that oil migrates through the filter while it is in the box. And then once you open it up it's all pretty orange. Most people clean and re-install in one day and end up over-oiling. A light spray of oil only on the intake side of the filter is all you need, don't spray the back/top side that faces the MAF.
This may not be your problem, but it is always a good time to clean you MAF sensor. Spray it, don't touch it with Q-tips or anything else.
That is way too much oil. The filters are lightly sprayed at the factory and that oil migrates through the filter while it is in the box. And then once you open it up it's all pretty orange. Most people clean and re-install in one day and end up over-oiling. A light spray of oil only on the intake side of the filter is all you need, don't spray the back/top side that faces the MAF.
This may not be your problem, but it is always a good time to clean you MAF sensor. Spray it, don't touch it with Q-tips or anything else.
10-24-2012, 01:34 PM
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The way the spark plugs looked after I removed them showed I was running lean.
I haven't cleaned the filter since it's been installed so it can't be oil on the maf. I have a bluetooth obd reader that connects to an app on my phone, could I use that to accurately scan for any troubles? If so what would I need to check and what readings should I be getting from those sensors?
I haven't cleaned the filter since it's been installed so it can't be oil on the maf. I have a bluetooth obd reader that connects to an app on my phone, could I use that to accurately scan for any troubles? If so what would I need to check and what readings should I be getting from those sensors?
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10-24-2012, 04:27 PM
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Not sure about the NGK iridiums, but the ones from GM are supposed to remain at an .040" gap even for an LS1 application. Or at least that's what the GM service bulletin says. I saw no loss in MPG going from .060" stock platinum plugs to .040" GM iridium plugs.
10-24-2012, 04:31 PM
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I would try cleaning the MAF either way, since it can't hurt. I have used rubbing alcohol and Q-tips to clean my MAFs for about 13 years now, I have not broken one yet. You have to be careful though.
10-25-2012, 08:08 AM
#11
This can be accounted for just in the switch to winter gasoline. Its that time of year...
OP, take a look at your trims if possible and maybe replace your o2 sensors. Those have more to do with proper gas mileage than spark plugs.
10-25-2012, 10:35 PM
#13
K&N air filters = complete garbage and only kills gas mileage and performance. Nuff said.
The gap you chose is terrible. You should be gapped to .050 at a minimum. Get some NGK TR55's and gap them YOURSELF to .050 with a gap tool. Don;'t trust that they are gapped properly.....check them all.
You may have a bad wire or two. Causing misfiring at medium throttle....thats very hard to hear/feel and you won't always get a code. But gas mileage will go out the window.....
Broken ceramic part of any of the spark plugs will kill performance too.
Absolutely the 02 sensor(s) can be on their way out. They will make you run rich and KILL gas mileage. Have it scanned to see if they are switching properly with the engine running........make sure the guy who does knows what he's looking for.....Take it to a race shop if possible, they will read it properly and in most cases NOT charge you a dime.
.
The gap you chose is terrible. You should be gapped to .050 at a minimum. Get some NGK TR55's and gap them YOURSELF to .050 with a gap tool. Don;'t trust that they are gapped properly.....check them all.
You may have a bad wire or two. Causing misfiring at medium throttle....thats very hard to hear/feel and you won't always get a code. But gas mileage will go out the window.....
Broken ceramic part of any of the spark plugs will kill performance too.
Absolutely the 02 sensor(s) can be on their way out. They will make you run rich and KILL gas mileage. Have it scanned to see if they are switching properly with the engine running........make sure the guy who does knows what he's looking for.....Take it to a race shop if possible, they will read it properly and in most cases NOT charge you a dime.
.
10-29-2012, 08:08 PM
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My camaro is averaging 25 mpg!
PSSST if what you were using was good for you why did you change it all?
10-29-2012, 10:30 PM
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.040" is not too tight for GM iridiums.....Check GM Bulletin 03D-J-171 (which applies to RPOs LR4, LM4, LM7, L59, LS1, LS6, LQ9, LQ4):
"A new spark plug has been released for use in the above vehicles. The new spark plug has an Iridium tip instead of the current Platinum tip. Due to the different tip design, the gap of the spark plug has also changed. The new spark plug, ACDelco Part Number 41-985 (12571164), is gapped to 1.01mm (0.040 inches) when the spark plug is made. The spark plug gap is set during manufacturing and should not be changed to the gap required with the Platinum plug, or damage to the spark plug may result"
10-30-2012, 05:48 AM
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I gotta set you straight...listen carefully.....a closed gap is going to cost you fuel mileage! The reason you were getting better mileage with the old stuff was the gap was worn wide!
PSSST gm only speced the 40 thous gap to shut up all the people complaining about poor idle quality. I don't make this stuff up.............
I'm not posting up just to move the keys around on my keyboard, i'm trying to help you in your search for an answer to your problem .
ASK LS6427 how enlightened he was after he discovered the real reason his starter broke off.
PSSST gm only speced the 40 thous gap to shut up all the people complaining about poor idle quality. I don't make this stuff up.............
I'm not posting up just to move the keys around on my keyboard, i'm trying to help you in your search for an answer to your problem .
ASK LS6427 how enlightened he was after he discovered the real reason his starter broke off.
Last edited by Camaroo1; 10-30-2012 at 05:53 AM.
10-30-2012, 08:53 AM
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Based on some of the quotes below, I think you could use some straightening yourself.....
You have this wrong, I never said that mileage was better with "the old stuff". I cleary stated that mileage has gone up a bit with the new plugs. The old plugs were still gapped very close to .060" when I removed them.
It was the OP of this thread that mentioned a loss of MPG, but apparently he was having issues in this regard even before the plug swap....so there is clearly something going on besides just the plug gap in his case.
I'm not searching for an answer, I don't have a problem, and I never asked a question. All I have stated is that the Delco iridium plug with the GM recommended smaller gap works without issue. MPG is actually up a bit, power and driveability are the same.
I think you have the wrong thread here.
It was the OP of this thread that mentioned a loss of MPG, but apparently he was having issues in this regard even before the plug swap....so there is clearly something going on besides just the plug gap in his case.
I'm not searching for an answer, I don't have a problem, and I never asked a question. All I have stated is that the Delco iridium plug with the GM recommended smaller gap works without issue. MPG is actually up a bit, power and driveability are the same. I think you have the wrong thread here. Last edited by RPM WS6; 10-30-2012 at 09:13 AM.
