So I've had this car for almost 4 years now, up until the past 6-8 months or so it had been getting pretty regularly 360-400 miles per tank (36-41mpg). Now I am barely getting 300 (usually around 270-290 per tank).

It is a completely bone stock, unmolested 1994 Civic Si with D16Z6. Only mod is a K&N air filter in the factory airbox. I change the oil every 3000 miles with Castrol Syntec and Fram oil filter.

My driving habits have not changed at all, and the car still runs great. I have heard that it smells rich when driving behind me.

In the past 6000 miles, to remedy my problems, I have replaced:
K&N Air Filter
NGK Spark Plugs
o2 sensor
Fuel filter
Tires (narrower actually, with proper pressure)
Oil+Filter

This week I am going to run some BG 44K fuel system cleaner, but I am running out of ideas of what could fix my problem. I bought this car to be a MPG King and to take wear and tear off the toy car, so I really need this to be back to fighting shape.

Any help? Thanks in Advance!

 

Clogged cat maybe? Might also need some new plug wires and/or dizzy cap and rotor.

 

I dont think plug wires could cause that dramatic of a drop in gas mileage. And I guess I can check my cat, but its not down on power at all, then engine feels great.

 

Full tune up, cap rotor, wires

These could all affect mileage.

 

Its about time... I would check ignition timing as well

 

i would recommend dizzy cap rotor... alignment if its really bad can affect gas mileage, gas cap, pcv valve... thats all i can think of aside from what you have already....also changing way you drive can affect, are you coasting in gear? and not dropping into neutral when slowing down......i recently started coasting in gear and i gained 5mpg


i would also like to see how everyone on here try to save gas..

 

Clogged IACV clean it out, adjust FITV, check timing, get new spark wires, Dizzy cap, Rotors, clean TB, Clogged CAT, get injectors cleaned, Stop activating Variable Valve timing, I could keep going on.

 

I also would check pcv valve and replace dizz cap and rotor at a minimum

 

you know whats funny ive read several posts like this and when my hatch was stock i never got better than 325 miles per tank. highway id get like 350 maybe 360 but 300 everytime was the norm. i went nuts for a while changing everything even took out the cat and ran a test pipe and a chrome p28 tuned and still no changed. other than the fact that i would get on it more lol.

when i swapped in my gsr motor i started getting like 340 to a tank and that was getting on it all the time. go figure.

 

well I did that 44k stuff, and it didnt seem to make a difference. Yesterday I seafoamed the engine through the brake booster line, and changed the oil.

I just ordered some Denso Plug wires, and new cap and new rotor from our local parts store. Going to try to install that tonight. I hope this makes a difference! with gas prices going up, it should would be nice to be getting 40mpg again!

 

How many miles do you have on the motor? Thats a big factor.
My D15b7 has 230k right now, I've put 20K on it and from day one I have been doing every bit of maintenance. the Exhaust was new from cat to tail, K&N in stock box, NGK plugs, Honda wires/dizzy cap/rotor, Fuel filter, PCV, Adjusted valves, I do a full seafoam treatment every other oil change (3-4k), Cleaned TB & ran countless injector cleaners through the system, New tires, alignment, new suspension-bushings/arms, etc.

After all that, I STILL average 300mpg +/-20mi as a daily. I do 5mi city and 10mi hwy.

 

Well at 65 miles on the new tank, I changed spark plug wires, distro cap, and rotor, and changed the oil, and seafoamed the intake system and cleaned the fuel system with BG 44k.

This tank I got 364 miles!!!! So without even a full tank of the car being fixed, I have already seen a benefit of 6mpg!

Im expecting to get 370-390 miles to this next tank, and should be right back on par with what I was hoping for!

HOORAY!

 

Yeah my car is running very bad right now, it bogs/studders easily when I give gas and even in the summer it did too but wasn't as bad and it still got 45mpg mixed driving but like 35-40 street always and like 45mpg highway only. I need to do a tune up very bad and I should be seeing regular 40-45. Nowadays Im getting 270-330 to a tank

I plan on seafoming it as well! How much did the tune up cost you?

 

I'm in a similar situation. Going to try the above methods.

 

Just an update on this, I seafoamed my engine and changed the plugs+oil and the car drives so much better. Its just smoother and no bogging anymore, I think I gained a bit of power but yeah its a lot better now. MPG wise Im not sure but I would say Im hitting 35-40 more constantly.
Careful when seafoaming your engine, theres a proper method to use and this is the method I went by and can vouch for this guy as well:

Quote, originally posted by gourley »
"Ive been doign seafoam for many years now and I can 100% I will reccommend it anyday. It just has to be used right.
My method (5 motors later no damage)

1/3 in the oil
1/3 in the gas tank
1/3 sucked through the brake booster line while car is running (just letting the car sip slowly at the seafoam, then when theres a bit left in the can, shut the car off and equaly pour some into the spark plugs to let sit and dissolve the carbon on the piston tops)
Wait 15 minutes about
Turn the car on and rev motor to 3k RPM for a couple of minutes
Then drive around town spirted (not too fast to draw attention with the Police)
When you stop seeing smoke return home
Change the oil and filter (seafoam will thin it out alot)
Change your spark plugs (Dont forget to gap then to .044)
And your set!"

This is how it works though,

It comes in a bottle and you put it in the gas tank, vacuum hoses in the intake manifold, in your oil, and maybe even inside where the spark plugs would go. You can do different mixes. Some people will put 1/3 can in the gas tank with full fuel, 1/3 in the crankcase (oil) and 1/3 through the brake booster hose. Other just put 1/3 or 1/2 in the tank. Some people don't trust it in the oil.
Basically it liquifies carbon build up so it will clean ur injectors, carbon around the valves, moisture from the crankcase, carbon from the piston rings and things like that. Basically cleans up your motor and makes it run smoother and gets power back and runs more efficient.

Then you rev the car a little bit until there's no more smoke going out of the exhaust and then change the oil and it's all set. It makes the oil thin out so you gotta change it. It also fouls spark plugs so they gotta be replaced

 

For me, the biggest difference was spark plug wires and cap/rotor.

Im back to 34ish mpg now I think.