cyotani

I recently picked up 94 civic hatchback DX with the d15b7 100% stock.
I have been averaging 25 mpg over the past 4 tanks which is well bellow what it should. I drive fairly conservative so this isn't due to lead foot syndrome.

Also the car failed CA smog 3 times for high hydrocarbon readings and it took a brand new cat to barely scrap by in HC but was extremely low in NOX and CO. So it's clear due to the high HC and low mpg that I have un-burnt fuel due to something not functioning correctly. (usually bad spark)

here is what I have done/changed so far:
-cap and rotor
-spark plugs
-fuel filter
-intake filter
-oil change and filter
-fuel injector cleaner
-sea foamed oil tank and vacuum system (before oil change and spark plugs)
-new Denso oxygen sensor
-checked timing (its at the stock setting)
-checked compression (180psi across the board)

Even with this maintenance I'm still getting bad gas mileage.
Any ideas of what It could be or what to try next??

Former User

CEL codes?

Replace plug wires?

Engine running cold?

cyotani

Thanks for the quick response RonJ

The plugs look to have been replaced recently so I have not replaced them yet. I doubt those would cause a 10mpg drop but I will get some new ngk ones if other thing thats plausible.

No check engine lights at all. The car seems to run great. Although it struggles going uphill and this is the first civc ive owned so I dont have anything to compare it to.

Engine temps are perfect. Warms up to the correct temperature and never goes any higher or lower.

I should note that the cold start up is not smooth at all. It turns over a few times more than my old 94 accord ever did and has a rough low idle for the first few seconds until it smooths out.

Former User

How old are the plug wires? That's the only basic ignition part you haven't yet replaced, right?

Does the CEL work?

Power loss can be caused by a clogged cat. Is that a possibility?

cyotani

Not sure on the age of the plugs, they just looked to be replaced recently from what I can tell but I'll look into getting a new set to make sure its not that.

Yes, the CEL is functioning correctly.

I doubt it's the CAT. With the brand new CAT it still barely passed smog for HC so it's got to be unburnt fuel.

How much could a out of spec valve adjustment affect fuel economy and smog? I feel like it could contribute but wouldn't cause the 10mpg drop I am seeing.

would any sensors like map or throttle body position cause these issues without throwing a CEL?

Former User

Do you mean plug wires or spark plugs? If you don't know the age of the spark plug wires, just replace them.

Did you use a timing gun to set the ignition timing? Was the service connector jumped and the engine idling to spec?

cyotani

sorry, I meant the wires. Okay ordering a new NGK set now.

timing was set with a timing gun with the 2 pin connector jumped and CEL flashing correctly. However I was unable to verify the idle rpm because the car doesn't have a tachometer.

Former User

The flashing CEL indicated that a CEL code is thrown. What is the code(s).

Hook up an external tach to the tach plug in the engine bay.

cyotani

my bad, it stayed solid. I just double checked it right now so no CELs.

Ill get a tach to verify idle speed but that still cant cause that big of a change.

Former User

Bad plug wires and/or way out of spec ignition timing can cause the mpg to take a big hit.

It's critical to rule out the common everyday issues before considering the rare ones.

cyotani

okay, I'll replace the wires and double check timing and idle speed and see if theres any change. thanks for the input RonJ!