Reinstall old spark plugs for better MPG?

[JEDI]

2005 Mazda3i AT, 130k miles

bought the mazda3 new and got 34mpg (70% hwy/30% city).
it came with NGK Itr5f13 (Iridium) spark plugs.

at 80k miles i changed to NGK TR6IX (Iridium). Advanced Auto didnt have the Itr5f13. website said the TR6IX was compatible.
i only changed it because you're 'supposed' to change it at 75k miles. Car was still running fine.

unfortunately my mpg dropped to 31mpg with same 70% hwy/30% city.

i bought new Itr5f13 but that made the mpg worst. it dropped even more to like 28mpg.
This was in the summer.

Went back to TR6IX. MPG back to 31mpg.


but now that gas prices are going back up, i'm thinking of switching back to the original Itr5f13.

Yes/no? WHY?
THX!

 

[Jetster...]

Fuel is changing. Higher Ethanol %. Not saying this is the cause but somthing to think about. Use what works best

 

[SpatiallyAware]

As long as you're in the same heat range it shouldn't matter, especially on a non stressed engine such as that. However, I believe the TR6 is going to be cooler than the OE LTR5... Which means you technically have to run your car a bit harder to burn them off but I would guess you're fine just stepping up one range like that.


I would be very surprised if the new plugs caused it, and 10% mpg difference could easily be just a few more uphill battles on that tank of gas... Are you 100% sure it was the same amount of city/hwy and all that? Is this averaged over a few tanks or only one?




Personally I would go with whatever is stock. FWIW NGK markets your stock plug as a 'laser iridium' which is the EIX model, not the IX you purchased which is 'single iridium'. Most people say this only has to do with longevity of the plug, but in my experience it can affect the way the motor runs pretty drastically depending on the motor. My liter twin supersports (TL1000, 1098) were both very picky about plugs.

 

[SpatiallyAware]

Fuel is changing. Higher Ethanol %. Not saying this is the cause but somthing to think about. Use what works best

Fuel changed months ago, in most places in the US

 

[unokitty]

If you have the time, it would be an interesting experiment to clean, regap, and reinstall your old plugs.

Whether or not, it would help your milage isn't clear ... But it would be an interesting experiment.

My experience is that some cars, and motorcycles, like particular types of plugs. (Though, NGKs were always my favourite brand.)

And if it doesn't work out, you can always put your new plugs back in...

Best of luck.
Uno

 

[janas19]

I suppose in theory, if you buy better quality/more powerful plugs, the spark will be hotter and perhaps combust the fuel spray in your cylinders more fully than weaker plugs. However, this doesn't take into account how efficient the engine is at using that fuel once it burns. I suppose it's worth a shot.

*Edit* After re-reading your post, I would go right back to the original model plugs you first had! Sometimes, sometimes you have to forget what the manufacturer recommends (it's more risky, but it can also pay off - I mean that's how overclocking basically started, right?)

 

[Apex]

You should check the gap difference between your used and the new plugs.

 

[Ronstang]

At 130K miles change the spark plug wires and see what happens first.

 

[foghorn67]

At 130K miles change the spark plug wires and see what happens first.

Coil on plug?

 

[JEDI]

Coil on plug?

dont know???

 

[Pacfanweb]

I suppose in theory, if you buy better quality/more powerful plugs, the spark will be hotter and perhaps combust the fuel spray in your cylinders more fully than weaker plugs. However, this doesn't take into account how efficient the engine is at using that fuel once it burns. I suppose it's worth a shot.

There is no such thing as a "more powerful spark plug". All a plug is, is a gap for the electricity to jump. A plug's design in no way can make the spark "more powerful".

The only thing that you can do with a plug to make the spark "bigger", is to increase the gap. When the gap is bigger, the coil has to build up more energy to jump that gap.....and to a point, that will mean you have a bigger, hotter spark.

That is probably why you experienced a slight fuel mileage loss after replacing high-mileage plugs....as the miles rack up, the plug gap gets bigger and bigger, and the spark gets bigger as a result.....and that helps combustion.
It used to make a much bigger difference 20 years ago, before fuel injection was so precise, but it still makes a bit of difference.

I remember at the Ford dealership I used to work at, when Explorers came out, they had 60,000 mile platinum plugs. Before, the copper plugs were 30k plugs. The gap doesn't get that big in 30k, but 60k on regular platinum is a different story.
I remember we had multiple examples of the first couple years of Explorers come in for 60k services, we'd clean injectors usually, too, trans service, the whole nine yards. And they'd come back a few weeks later complaining that their gas mileage dropped after the tune up. We'd of course find nothing wrong....finally we figured out that the spark had gotten so hot by 60k that they'd picked up some mileage, and when we put the freshly-gapped new plugs in, the spark was weaker, and mileage dropped noticeably. Car still ran fine, but just a little less efficient combustion.

Today, fuel injection and ignitions are so precise that this isn't as noticeable from just the plug gap, but it still can make a little difference if you are paying close enough attention.

Heat range is another thing that really doesn't matter as long as it's close. Heat range has nothing to do at all with the spark. The tip of the plug needs to be cool enough at operating temperature to not pre-ignite, but hot enough to avoid carbon fouling. That's all the heat range is for.....to prevent fouling but doesn't let the plug get hot enough to spark the mixture prematurely. It affects the spark itself in no way.

 

[JEDI]

Pacfanweb

so gap new spark plugs wider to improve mpg???

 

[Eli]

You could try it. It would be interesting.

I'm going to go with another theory - or possibly a combination of both.

Mark where the open end of the ground electrode is with a sharpy on the outside of the plug, then torque them home. Note where the electrode sits. Try and note correlations with all three sets?

Possible the engine responds well to spark plug indexing?

 

[Pacfanweb]

You could try it. It would be interesting.

I'm going to go with another theory - or possibly a combination of both.

Mark where the open end of the ground electrode is with a sharpy on the outside of the plug, then torque them home. Note where the electrode sits. Try and note correlations with all three sets?

Possible the engine responds well to spark plug indexing?

That's an old trick, but it does nothing, esp with today's super-hot ignitions. You'll need several sets of plugs to get them all indexed the same way, too.

 

[Pacfanweb]

Pacfanweb

so gap new spark plugs wider to improve mpg???

Possibly, but there are variables:

One, you'll be starting with a larger gap, so they plugs won't "last" as long before the gap wears big enough to misfire. If you're the type who replaces them more frequently than required, probably not a big deal.

Two, as I mentioned before, today's fuel injection and and ignitions are so precise that it MIGHT not make as much difference as in years past....in other words, even if you get a hotter spark, the computer may simply adapt to how well the mixture is burning and you end up with the same performance. Probably will vary from car to car, too.

Three, today's ignitions are so hot that this also might mean it doesn't make as much difference as in years past ON SOME CARS. In other words, the spark is already so hot that getting it hotter doesn't matter.
But it MIGHT. It certainly won't hurt to try.


All ignitions are designed to have some "head room" if you will, to allow for the plug gap getting larger. So by "head room", I mean that basically, the coil has enough ass to push a hot spark across a larger gap....to a point. Once you pass that point, which usually is well after the recommended replacement mileage, the spark will still jump the gap, but you get a weaker spark, as the amperage decreases.

So larger gap=Makes the coil build up more voltage (pressure) to jump the gap. And for awhile, the spark is actually hotter, but then the amperage comes down and it is weaker. But that's a BIG gap in today's ignitions to get to that point. You'd more easily find that amperage drop in an old GM HEI or other factory electronic ignition.

I will give this example: My 2004 Suburban has almost 160k miles, and it still has original plugs. Hasn't been touched, tuneup-wise. Still gets the same gas mileage as it did at 30k miles. Still runs fine. I can only imagine what the plug gap must be.

The wife's 1999 Tahoe ran misfire-free until 150k or so, and the only reason it started missing was the little spring-loaded connector/thingy in the distributor cap that sends spark to the rotor fell out. It was still firing the plugs just fine. Their gap was between .090 and .100". Stock is .060".

So today's ignitions are plenty powerful to jump a larger gap. Only way you can tell if it'll help yours is to try it.