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?
Worse Gas Mileage After Spark Plug Change
- Thread starter jkdrew
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1000MPH
Senior Member
No, it’s not normal to expect a drop in MPGs. It’s 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, you’re not going to get a very sympathetic ear. If you take it somewhere else you would get a better inspection/diagnosis but it’ll 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?
oljedi11950
New member
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 !
TRD PANTHER
New member
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.
There are so many variables to onboard or self calculations, I.e.;air temp, humidity, wind, tire pressure, etc.
Jivewalker
Member
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.
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.
There is a possibility of counterfeit spark plugs:
PLUG STUDIO / NGK
https://www.denso-am.eu/media/1374174/identify-counterfeit-denso-spark-plugs-white-paper-_pk3.pdf
just google it.. its out there
PLUG STUDIO / NGK
https://www.denso-am.eu/media/1374174/identify-counterfeit-denso-spark-plugs-white-paper-_pk3.pdf
just google it.. its out there
RichinRidgewood
New member
it runs smoother and now you are jamming on the throttle more.
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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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.
dawudih
New member
I replaced my oem plugs at 124,000 miles & now have 131,500 miles. I also changed my front brake pads for the first time, rear diff fluid, engine oil and the new designed air filter at the same time. All Toyota oem parts. The only variable would be the oil was 5w-20 instead of the usual 0w-20.
My commute to work is 78 miles round trip; most of it is cruise control in the HOV lane at 70mph, and I work for 7 days straight from 0700 to 1900. I also mostly use the same fuel station pump. Very consistent and controlled test.
I was getting 20.8 mpg for the last 2 years & after I serviced the car, it dropped to just under 19.9 mpg (almost a 5% decrease). I blamed the spark plugs, thicker brake pads or 5w oil. I just changed the oil back to a 0w-20 and fuel economy is up slightly to 20.1 mpg, but still less than before the service. Anybody have any real life experience with this reduction?
(The only reason I check my fuel economy is to find potential issues)
My commute to work is 78 miles round trip; most of it is cruise control in the HOV lane at 70mph, and I work for 7 days straight from 0700 to 1900. I also mostly use the same fuel station pump. Very consistent and controlled test.
I was getting 20.8 mpg for the last 2 years & after I serviced the car, it dropped to just under 19.9 mpg (almost a 5% decrease). I blamed the spark plugs, thicker brake pads or 5w oil. I just changed the oil back to a 0w-20 and fuel economy is up slightly to 20.1 mpg, but still less than before the service. Anybody have any real life experience with this reduction?
(The only reason I check my fuel economy is to find potential issues)
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