Fuel Economy dropped after spark plugs replaced

how are you observing mpg's? do you keep a record of fill ups and dates? weather plays a major factor.

 

Pull out your spark plugs and check gap .....

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Whatever people are saying that if you go with factory spark plugs you have already preset gap ....that is never right...i have found a lot of factory spark plugs out of specifications

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Yeah, if you drop the plug on its head and the gap goes out of whack. I've never had to gap a plug that was purchased specifically for a car.

 

Does it say the specific plug? One of these (from Owner's Manual):



As far as gap, some of the high end plugs caution against adjusting gap. If the gap is close to spot-on, I would leave it alone; they are set at factory, and barring being dropped on concrete or something, should be ok.

Do you set the pressures to spec, or higher? If the latter: dealerships are known to drop pressures back down to spec.

What does the invoice say for the fluid? Should be:

 

i beg to differ. a lot of things changed. it could be thicker oil/ATF they used. it could be massive "idling" they did that affected your MPG average.

 

Given all maintenance was dealer performed and you did not mention any prior issues with them, I would suspect that they made a tire pressure change that increased rolling resistance.

Could plugs, engine oil and the incorrect "ATF" be involved, sure. But since they are familiar with your car and most are good about getting the correct PARTS on customer's cars, TP seems the most likely.

Also, no OEM suggests altering the gap on "Double Platinum" or Iridium plugs, because the risk of damage to the tips in doing so is VERY high.

 

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.

 

How far away are you from @ericbecky? Maybe he can help you at his shop? Did you change out your PCV valve? Could the throttle body be dirty? MAF sensor dirty?

 

Assuming it's related to the spark plug change, and truely grasping at straws:

1. Did you put dielectric grease on the coil connectors plugs, or between the coils and plugs? Maybe messing up the connections??
2. Did you use a toque wrench? Dry thread torque is 15 ft/lb, and in the absence of any guidance I went with 13 ft/lb when the threads had a super light application of anti-seize. Basically what I did was put on a very thin coat, then wipe it off with a cloth, relying on a slight amount lingering in the threads.

I did say: grasping at straws.

Can't be the new plugs, since problem persists when swapping back to the old plugs.

 

Hi Virginia!
First of all, 52 MPG is outstanding is 47 is very good.
On the Fuelly site, the average for 2005 Prius is 43.6 MPG.
Your 52 and 47 are via display, not via real fuel consumption so 43.6 may be closer for you (display reads high).
Use of the display MPG is part of the problem...as that data is not as good....

I changed my own plugs, did not see a MPG change. I don't think I checked the gaps other than quick look.

 

I did not touch my PCV valve. I cleaned the throttle body and MAF sensor with MAF sensor cleaner. This is my second 2nd gen Prius. My other one had 360,000 and everything was original and I averaged 52-58 mpg. My sister now owns that one with everything still original at 400,000. I did all the same things to that one and didn't lose any mpg. May have gained a mpg or two.

 

Maybe something happened coincidentally, leading you to suspect the plugs? Weird as heck though.

 

I did not put dielectric grease on. I did not use a torque wrench but I know I would be in the ballpark for its torque spec. I can check just to make sure.

 

Yeah I thought maybe I messed up a connection to the coil? But I have no check engine light.

 

I am about 4 hours from Madison WI.

 

when you clean the MAF it typically takes a tank for the ecu to relearn the trims

 

That might be true but it doesnt explain why Mpgs lowered in the first place. It has been about 400 miles since that was cleaned. It didn't do anything. It's just all very frustrating. My 03 Civic Hybrid was far better at easily achieving over 60mpg average. I had a tank with 850 miles on it at just over 68mpg average for the tank. Without my auto engine stop feature ever working and my IMA light on due to a hybrid battery that was under 70%. If you are a hypermiler and don't do all city driving I suggest the Civic or Insight hybrids with manual transmissions. I never had to try in the summer and achieved over 60 mpg.