Bad economy after car sat for 70 days. Changing plugs. Ruthenium? Other Suggestions?

[Spadesheart]

My car has steadily lost about 0.25 liters/ 100 km of fuel economy steadily every year since I bought it. I just figured this was the car aging normally, as I don't exactly drive it a tiny amount. When I parked it in December before I went to travel, it was getting 6-6.5 L/100 km, which isn't great, but it's colder and I do put on winter tires. It usually goes back into the 4s in the summer. However when I got back and drove it after my trip, the economy has just been unacceptable. It's getting like 8+ L/100 km.

I've done some research. There have been signs that the spark plugs may have needed a changing for maybe 70000 km. The car sometimes does rev on idle which apparently indicates misfire. Apparently that itself can cause a 30% decrease in economy. I'm deciding on the plugs. I can buy the OEM (8 CAD), cheaper iridium ones from bosch (6 CAD), or upgrade (?) to ruthenium (11 CAD). I'm deciding if it's worth it or if there's a big difference. The prices are so low that if there is even a marginal benefit, I'll do the ruthenium, but there's not a lot of consensus online.

I will also change the air filter, it's probably just time anyways.

It's so bizarre that the mileage just tanked after sitting there. I've gone through like 4 tanks of gas since I've been back and it hasn't really corrected.

Does anyone have any other strong suggestions of things I should do?

Thanks!

[mohammad]

Changing an air filter normally won't make a difference, i basically never get fuel economy in the 4's. Is it possible your winter tires are a different size than your summer ones?

the other obvious guess is a change in your driving habits, shorter trips, higher speeds, stop and go are all bad for fuel economy.

If you think over 1L/100km of gas is being wasted due to bad spark plugs maybe check your oil dipstick? the gas must be going somewhere and gravity probably guides it down to the oil pan, this is a big problem with Honda, https://www.motorverso.com/honda-oil-dilution/

[klroger]

Just for a goof, jack up each of the wheels & give them a spin to make sure you don't have a brake kinda stuck on. You don't say where in Canada you are, but where I live, they use more salt then necessary on the roads, & if my car sat for a couple months, I'm sure stuff may start to stick on... Just thoughts...

[Fummins]

I’d point out that winter fuel is less efficient and cold weather causes far worse fuel economy but I assume after multiple years you’d have figured that out by now. The other obvious things to check are tire pressure. For spark plugs I’d just replace them with what came factory, ngk iridium. I haven’t tried more expensive ones but don’t see the point.

[Spadesheart]

Changing an air filter normally won't make a difference, i basically never get fuel economy in the 4's. Is it possible your winter tires are a different size than your summer ones?

the other obvious guess is a change in your driving habits, shorter trips, higher speeds, stop and go are all bad for fuel economy.

If you think over 1L/100km of gas is being wasted due to bad spark plugs maybe check your oil dipstick? the gas must be going somewhere and gravity probably guides it down to the oil pan, this is a big problem with Honda, https://www.motorverso.com/honda-oil-dilution/

Oh I'm pretty hypermill-y. Don't even put the heat on unless its -20. No hard breaking. I will check the oil too, though I don't think that's one of our problems.

[Spadesheart]

Just for a goof, jack up each of the wheels & give them a spin to make sure you don't have a brake kinda stuck on. You don't say where in Canada you are, but where I live, they use more salt then necessary on the roads, & if my car sat for a couple months, I'm sure stuff may start to stick on... Just thoughts...

Wouldn't driving, probably 800 or so km now have done this for me at this point?

[Spadesheart]

I’d point out that winter fuel is less efficient and cold weather causes far worse fuel economy but I assume after multiple years you’d have figured that out by now. The other obvious things to check are tire pressure. For spark plugs I’d just replace them with what came factory, ngk iridium. I haven’t tried more expensive ones but don’t see the point.

Yup! It's in the post there. I on average lose about 0.25 l of fuel economy every year since I've had the thing. The first year summer economy was around 4.4 L/100 km, with pretty hard driving, as I bought it on the other side of the country and road tripped it back, and winter was about an even 5.0L/100 km. This summer I was around 4.9-5.5 on average, and this winter before parked it was at about 6-6.5 on average. We're in the 8's right now though, which is trash.

I've never changed the spark plugs, but I do change the oil at intervals, and have changed the transmission oil twice. The transmission oil doesn't make a huge effect I hope? Haven't changed that for a while since the warranty went out. Mitsubishi oil is so dang expensive.

[dspace9]

The car sometimes does rev on idle which apparently indicates misfire.

That doesn't sound good. Hope you get it all back to normal Spadesheart.

[klroger]

Wouldn't driving, probably 800 or so km now have done this for me at this point?

If something is sticking (eg, wheel cylinder, brake cable) you'll have to take it apart to lube, clean, adjust or replace it. If something is sticking in the brakes, driving it won't make it better... at least it never did for me

[twister77]

We're in the 8's right now though, which is trash.

My 2019 G4 with total of 7000km on it does about 9 L/100 km in Canadian winter, so you are all good.