91 Civic DX

[mikeiorga]

Hey everybody,

Got a 91 civic with a 1.5l stock motor, and throttle body injection. I've been shocked to get only about 30mpg. And that's babying it. I did notice that the tailpipe gets a black soot on the outside edge which wasn't as heavy on cars I've had that got better mileage. (My old 89 civic SI used to get 37mpg in the same driving conditions and I would have expected it to do worse with the shorter gears)

I'm thinking that the settings on the computer are not for economy. Any ideas on how to change that? I've heard you can use a spacer on the O2 sensor and trick the computer a bit. How far would one space it? What about computer settings? I'm pretty mechanical, and can fabricate most anything in metal, but the electronics take some doing for me.

Any help would be appreciated

Mike

[hondaguy72]

Were you running on flat tires?


J/K

Get her tuned up first. There's no reason that car can't get high 30's all the time if it's all stock. Correct plugs, new cap, new Honda distributor rotor (aftermarket are junk), good wires, etc.

Don't mess with the O2, other than to make sure it's working. It could possibly need replacement due to excessive oil burning. Good thing it's a cheap one, unlike the vx 5 wire model.

[mikeiorga]

That flat tires comment made me laugh out loud. Good one!

New plugs, cap, rotor. I think the wires are an original Honda set, but doubt if they're the ones from 1991. Distributor was replaced with one from Oriley's before I got the car and had only about 10k miles on it. Hadn't heard about having to get one from Honda before. I'll have to check the O2 sensor and see if it's working. Since I live in California, I'll have to smog is soon, so that's a good thing to make sure of anyway.

Also been considering putting headers and a large exhaust as well as a bigger intake since all that stuff is pretty cheap for it. Any suggestions for or against?

Thanks

[Ryland]

Originally Posted by mikeiorga

Also been considering putting headers and a large exhaust as well as a bigger intake since all that stuff is pretty cheap for it. Any suggestions for or against?

If you want to see a drop in your mileage and power then a larger exhaust will do that for sure, for that car stock exhaust from the dealership is your best option, it fits right, it gives the best mileage and it has a warranty for as long as you own the car.

Your fuel injectors are 20 years old so altho replacing them might be over kill it might be worth pulling them out and cleaning them up, or running some fuel system cleaner.
If everything else is working ok tho, my first thought with soot in the exhaust is a bad o2 sensor, even tho you have a check engine light it will take forever for it to trip from the o2 sensor.
Last thing is your transmission fluid, when the car was new it called for straight 10w30 motor oil and is supposed to be changed every two years, I tend to use 0w30 in mine because it gets cold here, but you can get a synthetic MTF from the dealership and it should last longer then the 2 years that the motor oil would.

[mikeiorga]

The tranny fluid is fresh. Looks like I dropped the ball with the O2 sensor though. Will check that and go from there.

TY.

[Gasoline Fumes]

I can't offer much advice, but you should be getting much better MPG. My 188k mile Civic Wagon has the same drivetrain and got 38 MPG before I started ecomodding and hypermiling. And I do much better now, with no engine mods. Brakes dragging? Alignment? Timing? How fast do you drive?

I do have a weird timing issue that may help or harm MPG....

[War_Wagon]

I wouldn't mess with anything mechanical other than checking your timing, plugs/wires, air filter etc. My DX wagon averaged in the mid 30s with not too spectacular driving on my part, and it was pretty much stock. Check your front tires for uneven wear as an indication of poor alignment, and make sure none of your brakes are dragging as was stated before. If your car has the original cat converter on it, it could be plugged up, and the excess backpressure could be hurting you. Also, listen for excessive valvetrain noise, if the valves have never been adjusted you will hear a lot clacking under the valve cover and could be due for an adjustment.

As for driving, I found that my 1.5 had enough torque I could experiment with skipping gears while upshifting and keeping the revs low. See what works for your car and driving conditions.

[Gasoline Fumes]

Originally Posted by War_Wagon

Also, listen for excessive valvetrain noise, if the valves have never been adjusted you will hear a lot clacking under the valve cover and could be due for an adjustment.

Sometimes I feel guilty for not adjusting the valves in the Wagon. It's been in the family for 113k miles and I know they haven't been touched that whole time.

[mikeiorga]

Since the front end is basically new, and I just adjusted the valves about 5k miles ago, looks like when I get home from working out of town, it'll be a matter of checking/replacing the O2 sensor and trying mileage then.
I tend to skip gears if I don't need the power to go up a hill as a habit, but living in the foothills can ding you on mileage, and I'm prepared for that. I just wasn't prepared for 30mpg...
Many thanks for all the suggestions.

[War_Wagon]

Sometimes I feel guilty for not adjusting the valves in the Wagon. It's been in the family for 113k miles and I know they haven't been touched that whole time.

I think these cars are generally made to not be maintained, of all the ones I have owned I have only ever adjusted the valves on one that needed it. The rest never showed symptoms so they got left alone.

As for OP's issues, most of the other causes I can think of should set off the check engine light. A faulty O2 sensor should, though it could have an internal short in it like the one in my '98 had, caused all sorts of weird problems but never set the light off. The '91 would also have a mechanical VSS, with a module in the back of the speedo that sends the electrical signal to the computer. The one in my wagon was failing, and it would cause the car to feel like it was running way too rich, but again this set off the CEL. Sounds like a basic question, but the Check Engine Light does come on prior to starting the car, right?

The only other thing I can think of is that while we have asked about tire pressure, we didn't ask about tire/wheel size. Does the car have stock sized tires and wheels, or has someone put a larger aftermarket setup on it? A set of big, wide, low profile tires and heavy wheels is going to cost you some of the missing MPGs.