94 Civic VX - Not great MPG

[paulhale]

Hi guys. Great site. I have gotten some great info from here in the past year, but today I decided to actually register and post.

I got my VX a little over a year ago with about 137k on it. It had been lowered dramatically (read "dropped on its nuts"), had super wide 17" rims, had a "customized" air breather, and had a non-standard exhaust (read "super loud muffler" and "no cat"). I had it raised up as close to spec as I could get it, replaced the wheels with the lightest aluminum 13s I could find, and fixed all of the exhaust leaks. Also replaced plugs, wires, dist cap & rotor, brakes, air filter, valve cover gasket, fuel filter, fuel pump, steering column, cleaned the O2 sensor, and had the stock 5spd torn apart and rebuilt with all new seals and bearings.

After some serious commuting, it now has 167k on it. For most of that time, I was getting 43 MPG pretty consistently (80% highway driving @ 65MPH). In the past month or so, I have noticed a drop to about 36-38 MPG. Where do you guys thing I should start looking for issues? I have no CELs and the car doesn't run any different than it has been. The car runs Mobile1 10w30 High Mileage.

[graydonengineering]

Summer AC use? Using AC could certainly drop your MPG that much. Also, do you have your tires at max? Gradual presure drop can sneek up and steal your gas too. You should also run a good injector cleaner such as Seafoam if you have not already.

[Ryland]

Over sized exhaust will cause problems that could build up over time like slowly fowling your o2 sensor, also check your spark plugs (every 15,000 miles) and change them as needed with stock copper core V-power plugs.
What fluid did they put in your rebuilt transmission? it should either be motor oil (5w 30 I think is stock, 0W 30 works well) or Honda MTF, gear oil has sulpher in it that will destroy the bearings and bushings.

[paulhale]

Originally Posted by Ryland

Over sized exhaust will cause problems that could build up over time like slowly fowling your o2 sensor, also check your spark plugs (every 15,000 miles) and change them as needed with stock copper core V-power plugs.
What fluid did they put in your rebuilt transmission? it should either be motor oil (5w 30 I think is stock, 0W 30 works well) or Honda MTF, gear oil has sulpher in it that will destroy the bearings and bushings.

The spark plugs were changed 30,000 miles ago. I will check them.

I bought a case of Honda MTF and had them use that on the refill.

[paulhale]

Originally Posted by graydonengineering

Summer AC use? Using AC could certainly drop your MPG that much. Also, do you have your tires at max? Gradual presure drop can sneek up and steal your gas too. You should also run a good injector cleaner such as Seafoam if you have not already.

I use my AC for most of the year here in Florida.

I have run Seafoam through the tank, and I usually do it every couple of oil changes.

The tire pressure thing was mentioned to me on another site. They said I should be at "55psi rear (20psi above sidewall) and 65psi front (13psi above sidewall)". I know I'm definitely not inflated that much.

EDIT: I just checked my tire. The MAX PRESSURE is listed as 51 psi.

[Ryland]

Originally Posted by paulhale

The spark plugs were changed 30,000 miles ago. I will check them.

Owners manual says to change plugs every 15,000 so changing them should help, you might do a basic tune up too, check your PCV hoses and valve if there is one, check the inside of your cap and the rotor tip as they sometimes need to be cleaned up a little as time goes on, check your timing, check your valve lash, I know that with stock exhaust the valve lash hardly ever needs adjustment but with over sized exhaust it's hard to say what kind of wear you could see.

[paulhale]

Originally Posted by Ryland

Owners manual says to change plugs every 15,000 so changing them should help, you might do a basic tune up too, check your PCV hoses and valve if there is one, check the inside of your cap and the rotor tip as they sometimes need to be cleaned up a little as time goes on, check your timing, check your valve lash, I know that with stock exhaust the valve lash hardly ever needs adjustment but with over sized exhaust it's hard to say what kind of wear you could see.

I need to do the valve cover gasket, so I'll do the plugs at the same time and I'll check the timing belt. Where is the PCV valve? I have the Haynes manual for this car. Will the PCV valve instructions be in there?

[Ryland]

Originally Posted by paulhale

I have the Haynes manual for this car. Will the PCV valve instructions be in there?

Yep, altho some VX's didn't have a valve, just the PCV hose with empty joints, others seem to have a valve that is held in place by the web between the two intake runners on the drivers side of the engine, others seem to have the valve on the oil separator box, fallow that hose that runs between those two intake runners down under the intake manifold (have someone with small hands help) and you will find another joint/pcv valve, if your idle bounces between 700 and 1,200 rpm at idle then that grommet where the PCV hose enters the oil separator box is often at fault and causing a vacuum leak.

[paulhale]

Originally Posted by Ryland

Yep, altho some VX's didn't have a valve, just the PCV hose with empty joints, others seem to have a valve that is held in place by the web between the two intake runners on the drivers side of the engine, others seem to have the valve on the oil separator box, fallow that hose that runs between those two intake runners down under the intake manifold (have someone with small hands help) and you will find another joint/pcv valve, if your idle bounces between 700 and 1,200 rpm at idle then that grommet where the PCV hose enters the oil separator box is often at fault and causing a vacuum leak.

Ah. My car does do that sometimes.