Lower MPG after replacing spark plugs

Did you use a gauge to ensure the spark plugs were gapped properly before installing them?

SCH-I535

 

Probably the wrong assumption. I went the same route a few years back and found a discrepancy on MPGs also.
Solution was to remove the plugs and re-gap accordingly, there are only 4 in line......
This was in my wife's 2011 about 6 months ago.

 

how many tanks have you used since changing?

 

If one was off, that's 25% in a 4 cylinder, and so on...

 

Its possible you damaged a wire or dont have the wire on correctly(easy to do) or cracked a plug.
Easy check to see if wire/plug is arcing is open hood with engine running in very dark area(garage with door shut) and make sure you dont see any spark jumping around from the wires to the block or any other areas....

 

Then ensure that they are gapped at 0.044

SCH-I535

 

I'm having a very similar issue. No one in any of these threads has answered as to why people who just change spark plugs are getting a dramatic drop in mpg. I changed my spark plugs from Denso Iridium to NGK Laser Iridium and instantly lost about 10mpg. They were gapped properly I did check. I also know my tires are above 45 psi, I have a new yellow top battery, traction battery is a Dorman which was just put in, no check engine lights. So before changing the spark plugs I noticed over 52 mpg at 60 mph. Now it is about 42 mpg. My average has dropped the 10 mpg also. I did that for multiple tanks with no increase in fuel economy. I do feel like it has a miss or something when it is using the ICE but not when electric. So I replaced all 4 coil packs still no change. I drove like that for 3 tanks or so with no change. So I tried to put the old plugs back in. I've had these back in for about 2 tanks now with no change.

 

I would remove the spark plugs and examine them closely for cracks or abnormal deposits for any clues. Believe it or not, I cracked 2 expensive plugs in a row in a boat because of poor access and use of flexible wrench extension.

Btw, Prius mpg is surprising sensitive to minor changes. I went from 60 to 50 mpg in city after new tires. I wouldn't believe, if someone else told me.

And finely, could it be something else that changed? Oil battery, etc.

 

Did you apply dielectric grease? Where did you apply it?

One thing to check: pull the four plugs and see if one hasn't fired at all. It should be obvious. You may have poor contact (hence, the dielectric grease question)

 

FWIW I'd go with the above suggestion to re-check the spark plug gap.
At a second level I check the spark plug wires.....eg intermittent spark.
Third possible damage to the fuel injector wires.

Another best guess is the tire air pressure was lower.

Good hunting.

 

If you use the correct plugs, they are pre-gapped. I think one of them isn't firing or poorly firing from misapplication of anti-seize or dielectric grease. Also possible that a sensor wire wasn't reconnected during reassembly, like the MAF sensor, which I think is disconnected in this procedure.

 

I changed the battery months before optima yellow top, the hybrid battery was replaced with a dorman, oil was changed before plugs along with air filter. None of that changed its mpg. Tires only have a couple thousand miles on them but also before the plugs were changed. 0w30 full synthetic oil and Toyota oil filter. Toyota replacement air filter. None of that changed how the car drove until changing the plugs. Since then I tried cleaning the MAF sensor with MAF sensor cleaner, and cleaned the throttle body. I still feel the hesitation. It was actually getting better gas mileage with the junk hybrid battery than it is now. This is my second 2nd generation prius. My other 05 had 330,000+ when I got it. I did all the same things I have done to this one and I was getting 52-58 average per tank depending on how much highway I drove. It is still on the road with my sister driving it with over 400,000 and never had the engine, transmission or hybrid battery replaced. Not even the 12v battery in the past 4 years at least. The 05 Prius I have now achieved even better mpg at 60+ than that one but after the plug change it dropped significantly. The only thing I can think of is damaging a wire to the coil packs but I have no check engine lights.

 

Uh... asked you some questions, they were sorta ignored... can't help you if you won't answer them.

Did you apply dielectric grease to the plugs? Did you use anti-seize compound on the plug threads? Did you pull the plugs and see if one hasn't fired? Did you check the cable to the MAF sensor and make sure it was reattached? Are you sure all four ignition coils were reattached?

 

I have reconnected the plug wires multiple times with each time I took the plugs out and already checked the gap, cleaned the NGK's once after I didn't like the drop in mpg. Then took them out a few tanks later and went back to the Denso plugs. It still hesitates. So yes possibly a wire issue? I didn't change the tire pressure. Weather has become warmer by a little not colder so not a temperature issue. It doesn't get any better after the vehicle is warmed up either. Is there an easy way to check the plug wires without taking their covering off?

 

All coils are connected and MAF sensor. I did not use dielectric grease nor did I use anti-seize. I have taken the plugs out 4 times total now. They all look the same. All look to be firing. The old Densos were very white the first time they came out, that's when it was achieving over 50 mpg at 60mph and above. Now 60 mph or above is around 40-43 mpg. That's with pulling and changing the plugs multiple times now. And new coil packs.

 

I had the same issue in my 2010 Prius. After reading this thread I did some research. If you're replacing the spark plugs make sure you go to Denso website. They recommend not using anti seize and they have specific torque and a specific way to check Gap. You're not supposed to touch the tip with a feeler gauge and you're supposed to open or close the gap with pliers. With that said I double-checked my spark plugs everything was fine. Went on a drive and I still had low MPG. I unplugged my battery and did a full reset. That's when I noticed that I still had low MPG but after I filled up y tank it said my miles to empty we're almost 200 miles more than normal. I watched a video of a guy doing a battery reset and through his test he said the car learns you're driving and takes about 6,000 miles before it settles in. In the meantime even though it's saying I'm getting low MPG my gas gauge is not dropping very fast at all. So I'm currently calculating the old fashion way my MPG and when I get done I'll post my results.

 

How do you calculate your mileage?