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Old 04-18-2021, 03:02 PM #1
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Worse Gas Mileage After Spark Plug Change

I just had the spark plugs on my 14 Runner changed at the dealer after some rough idling at 120,000 miles. Now the rough idle is gone, but I've noticed slightly worse gas mileage, when I was expecting it to be the same or a little better. Lately, I had been averaging 18.5 mpg on the computer, 75% highway, 20% small town, 5% off-road, but hasn't recently been confirmed by calculations. I reset the computer at the time of install and I've had about the same mix of driving, but now only averaging 17.0, which I've confirmed with calculations. Nothing else was done at the dealer at the time. Is this at all expected or is that a big enough drop to think there could have been an issue with the plugs or install?
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Old 04-18-2021, 04:42 PM #2
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No, itís not normal to expect a drop in MPGs. Itís possible it has to do with the install even if unlikely. IMO, if you take it back with the complaint that it runs better but mpgs are worse, youíre not going to get a very sympathetic ear. If you take it somewhere else you would get a better inspection/diagnosis but itíll cost you. Are you mechanically inclined enough to do a visual of the vacuum lines? The newer 5th gens with the air pump have plumbing that must be removed and replaced, preferably with new gaskets. Did they charge you for these gaskets? Did you verify the plugs installed, or on your invoice are correct for your year?
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Old 04-18-2021, 11:11 PM #3
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Changed mine cpl thou ago & if anything better ! So they left one of vacuum lines off or worse ? Any different sounds on start up , do visual to see if any of them are hanging off !
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Old 04-19-2021, 12:13 AM #4
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Any chance your computer had learned to deal with bad plugs and now learning to work with new plugs?
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Old 04-19-2021, 03:28 AM #5
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Did they install the same part number OEM Denso plug that they removed? There are a few different manufacturers for our 4.0L. If not, this may be a problem based on the heat range of the plug. Although, most of these plugs come pre gapped to the correct specs, I always double check the gap. If it is running smooth, I doubt a vacuum line or a plug connection is loose. Did they inspect your plug wires and coil packs? I doubt it. Being 7 years old with 120k I might have considered a new set of plug wires.
There are so many variables to onboard or self calculations, I.e.;air temp, humidity, wind, tire pressure, etc.
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Old 04-19-2021, 08:38 AM #6
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How many miles since the plug change? May take a while for the MPG's to creep back up. I have noticed it is quick to drop MPG's and slow to raise but over time, especially while computer re-learns your habits. Mine have always come back up slowly, but not instantly.

May also want to clean your MAF sensor and throttle body. Cans of cleaner have instructions on them and there are some write ups on here.
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Old 04-19-2021, 10:16 AM #7
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There is a possibility of counterfeit spark plugs:

PLUG STUDIO / NGK

https://www.denso-am.eu/media/137417...paper-_pk3.pdf

just google it.. its out there
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Old 04-19-2021, 01:20 PM #8
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it runs smoother and now you are jamming on the throttle more.
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Old 04-19-2021, 01:43 PM #9
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Did you change anything else like new tires or change psi? I'd clean MAF (although this 5th gen airbox is always amazingly clean when I check) and remove battery negative overnight then drive like a grandma for a few days. What was said about counterfeit parts is why I do this stuff myself and only buy parts from the dealership or big place like Napa or Summit for Denso/NGK. If its still low I'd probably replace the air filter (non-amazon OE), o2 sensors (non-amazon Denso) and pcv valve (OE) because its cheap and easy. And check all vac hoses were put back properly.
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Old 04-19-2021, 02:20 PM #10
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Quote:
Originally Posted by wcjeep View Post
Any chance your computer had learned to deal with bad plugs and now learning to work with new plugs?
Yeah I think if there was old or bad plugs it could have adjusted to run lean due fuel not burning properly so mpg would be "better". And now its still trying to figure out the correct mixture with new plugs which takes some driving to learn.
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Old 04-19-2021, 03:26 PM #11
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18.5 to 17 is probably within the variance margin, where wind direction, slight traffic pattern difference and even temperature changes and gas source may have an impact.

I'd say give it more time/miles and see how consistent it is.
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