My 99 civic ex was getting about 30-32mpg and now its getting about 25mpg. The only modifications that were made before the drop in fuel efficiency were the wheels, tires and spark plugs.

The old spark plugs looked bad and were the stock NGKs. All four were replaced with the NGK platinums.

The wheels and tires were the stock 14"s I believe and I changed to 15" rims and new tires. Since I used the Tirerack.com wheel and tire selection package, the total diameter of the wheel+tire shouldn't have changed. I've been running with the tires around 35psi, so the tires aren't low.

My plan is to check the spark plug wires for resistance >25k ohms. Then maybe take a look at the primary O2 sensor. I would like to try to clean the IACV, but I'm not exactly sure how to do that.

I did install an AEM short ram intake almost two months ago. The air filter does look kind of black, but it was brand new. I'm not sure if the air filter needs to be replaced. It would be pretty costly to have to replace the filter every month or two, so I'm in denial about the air filter being the cause of the drop from 30mpg to 25mpg.

Any ideas or suggestions why the fuel efficiency would just drop off a cliff from the low 30s to the mid 20s?

 

well i just measured the resistance on the spark plug wires and from left to right 7.72k, 8.3k, NC, 70k ohms. i can't imagine how bad a no connect is, but i'm going to buy a new wire set today. unfortunately i really only need one or two replacements, but i guess they're only sold in sets of four.

 

you should be able to wash the air filter and reuse it, it's pretty easy especially if it's a dryflow kind you don't need to put oil on it, even if you got to oil it up it's still not that hard. Check the document that came with the aem sri.

I'm haveing similar problems with my car and gas mileage, but I think it's from the seafoam that I used and possibly an o2 sensor got dirty. Also got to give it an oil change but don't think that has much to do with gas mileage....

 

well, i bought some new spark plug wires at the store. i couldn't find anyone with NGK and the dealership was charging $110, so i bought some bosch for $51. each of the four wires were under 1500 ohms so they were an improvement over my old worn out wires. i guess i should have changed the wires when i changed the plugs.

the car "felt" like it was driving better with more power and the fuel gauge seems to be higher than usual after 100 miles, but i won't know for sure until i refill the tank.

btw, i did do the seafoam treatment, but i did use a little at a time for the intake manifold, so it looks like my o2 sensors might still be okay. they're probably due for replacement although i'm not sure how to do it yet or what parts to buy. i also should look into the iacv cleaning, but i haven't figured out how to go about it yet.

 

i forgot to ask. what happens when you've been driving with a broken spark plug wire for a month or two? i guess there is no spark and its like only 3 cylinders are working?

 

wow, i dunno....surprised no one else is helping you on this post. I would listen to how the engine is running, at idle and driving(pings and miss fireings). Prolly depends on how long you were driving with just 3 cylinders. Could put extra strain on the other pistons.

Did your car smoke at all while you were feeding it into the vac line?Or just afteryou restarted it?

I think I fed it into my intake manifold too quickly the first time I did it and it smoked a bunch. The engine light came on and after that is when I think I've been getting worse gas mileage.....I don't want to go through the trouble of getting another o2 sensor just yet. Or another cat convert, especially if it's not conclusive that their the reasons for it.

 

Yup, you were simply driving on 3 cylinders...been there, done that.. I would change out your plugs now that you know you're getting spark to all four. Plugs are cheap, and easy to replace. Changes are the one that wasn't firing is probably fouled out pretty bad at this point. Change all 4!

 

as far as the seafoam goes. i only got smoke after i re-started the car. nothing happened while i sipped the seafoam in through the vacuum hose.

ugh. i just put new spark plugs in before finding the broken ignition wire. i'll have to check out the plugs to see if they're already messed up because of the broken wire. thanks for the advice.

 

could also be the weather.... has it been preaty cold around your area lately?
i know almost all winter ive been runnen ruffly 25-26mpg... and now with the warmer area im back up to 28-29mpg...



tho it might be alittle from your wheels BUT depends on thier weight.... my stock wheels weighed 32pounds and my 17inch rims weighed 37pounds... so 5 pounds more rotating so i can see why alitle milage drop... id say pull one of your wheels off and weigh them to see how much they weigh with the tire on them.........

 

While we are revisiting this topic, the original poster shoudl DITCH THE PLATINUM PLUGS! They're not helping, in fact, in the long run they're worse than standard NGKS