93 Honda Civic Mileage gone bad

I recently started getting very low milage in my 1993 Civic EX (1.6 L
SOHC VTEC).

Previously I've seen between 32 and 38 (34 AVG.)

Now I'm getting about 25 MPG (it has steadily declined the last three
tanks).

Things I have checked/replaced:
* Replaced Plugs and gapped them properly (The old ones looked fine)
* Checked throttle body (no build-up)
* Checked PCV valve: When I squeezed the tube going to the PCV, the
idle went up, I guess that means it's open
* Checked EAVC valve: I removed it and examined it, cleaned it and
tested it. It checks out, the intake port was a little clogged and it
was a little oily inside, but it tests fine (opens with a voltage).
* Timing: It's pretty rock solid, though it's hard to get an actual
reading cause the "pointer" or "indicator" is so far away from the
pulley with the marks.
* Haven't done a Compression Test yet
* Fuel injectors: I was grabbing them with the engine running and
could feel the clicking of the valve in only three of them. I removed
the odd one and tested it and it seemed to work fine and fast when
supplied with a few volts.
*The air filter is super clean
*Oil + Filter was replaced recently (1500 miles)
*Performance seems top-notch, as fast as ever.
*Valves: I just adjusted them and they weren't hardly off at all, only
the valves on one cylinder were .001" too tight.

Note: About a thousand miles before the low milage, I jump-started the
car and had noticeable engine shake after that. Recently, I replaced
one of the motor mounts cause it was busted all the way through.
Wierd, but this didn't really help much. It could be just that the new
mount isn't as stiff as the old one was, or there are more busted
mounts.

So, any ideas about the milage?

Thanks in Advance

 

How's your catalytic converter/exhaust system? Maybe you're getting some
restriction.Also,cold weather will cause a decrease in mileage.

Check your tire pressures,too.

 

I could have sworn I answered this for somebody else today.

Anyway, on to the questions.

Is this 25mpg city, highway, or mixed?

Are the new plugs OEM? What about the cap, rotor, wires? How are they?

What are your tire pressures? Are they the right size?

Are you a lead foot driver?

If you are in a part of the world where winter is settling in, that will
decrease your mileage.

 

I could have sworn I answered this for somebody else today.

Anyway, on to the questions.

Is this 25mpg city, highway, or mixed?

Are the new plugs OEM? What about the cap, rotor, wires? How are they?

What are your tire pressures? Are they the right size?

Are you a lead foot driver?

If you are in a part of the world where winter is settling in, that will
decrease your mileage.

 

ok, please clarify that point: are the timing marks correctly aligned or
not? the pointer & the pulley marks need to line up under the timing
light. if the timing belt's skipped, the "so far away" will be your
problem - and indication of belt skippage. you don't state mileage.
presumably the belt /has/ been replaced at some point. my experience is
that some shops are not practiced at getting belt tension just right.

 

ok, please clarify that point: are the timing marks correctly aligned or
not? the pointer & the pulley marks need to line up under the timing
light. if the timing belt's skipped, the "so far away" will be your
problem - and indication of belt skippage. you don't state mileage.
presumably the belt /has/ been replaced at some point. my experience is
that some shops are not practiced at getting belt tension just right.

 

I have not checked the catalytic converter, I'm not sure how to go
about checking it except looking for holes. The sound of the car
hasn't really changed.

It's not cause of winter, I've driven in the cold before and haven't
got milage like this.

Tire pressures are correct. 30 PSI.

 

Mixed

I didn't get the OEM plugs, but I just replaced the plugs today, Old:
NGK's New: Bosch

Cap, rotor, wires, and coil were all replaced 30 or 40K miles ago.

Tires: 30 psi. Yes, they are the right size

No, I'm not a lead foot, The milage changed, my driving habits
haven't.

Winter, yes but it doesn't get that cold here (Central California) and
I've driven in colder weather and still had great milage.

 

Mixed

I didn't get the OEM plugs, but I just replaced the plugs today, Old:
NGK's New: Bosch

Cap, rotor, wires, and coil were all replaced 30 or 40K miles ago.

Tires: 30 psi. Yes, they are the right size

No, I'm not a lead foot, The milage changed, my driving habits
haven't.

Winter, yes but it doesn't get that cold here (Central California) and
I've driven in colder weather and still had great milage.

 

I don't know why it is this way... but. The little plastic indicator
on the timing belt cover is 3 or 4 inches away from the edge pulley
having the marks. Because of this, it makes it hard to see if the
marks on the pulley are lined up with the indicator correctly. What I
should do is scratch a line from the indicator toward the center of the
pulley. I haven't yet because it's super tight in there.

 

I don't know why it is this way... but. The little plastic indicator
on the timing belt cover is 3 or 4 inches away from the edge pulley
having the marks. Because of this, it makes it hard to see if the
marks on the pulley are lined up with the indicator correctly. What I
should do is scratch a line from the indicator toward the center of the
pulley. I haven't yet because it's super tight in there.

 

That's quite a drop. How many miles are on your car?

IMO, you need a complete shotgun approach. You're halfway
there. In order, this is what I'd do:

Check the coolant levels. Purge the cooling system of air.

I'd replace the PCV valve, assuming it's the original one.

Is the fuel filter due for replacement? If so, replace it.

I would be tempted to pull off the ignition wire of the
suspect cylinder (with the suspect fuel injector) and see if
engine power sounds like it goes down, as it should. OTOH,
if you say performance is otherwise fine, then all four
cylinders must be firing.

Consider a new oxygen sensor.
https://www.automedicsupply.com/ has great prices for OEM
oxygen sensors.

Stick with NGK plugs in the future. OEM plugs seem to be the
consensus here.

Are cap, rotor, and wires OEM?

My site reinforces a few of these points:
http://home.earthlink.net/~honda.lioness/id11.html

Elle
Original owner, 1991 Civic (1.5 liter, manual transmission),
172k miles, 40+ mpg most of the year; about 39 mpg this time
of year.

 

That's quite a drop. How many miles are on your car?

IMO, you need a complete shotgun approach. You're halfway
there. In order, this is what I'd do:

Check the coolant levels. Purge the cooling system of air.

I'd replace the PCV valve, assuming it's the original one.

Is the fuel filter due for replacement? If so, replace it.

I would be tempted to pull off the ignition wire of the
suspect cylinder (with the suspect fuel injector) and see if
engine power sounds like it goes down, as it should. OTOH,
if you say performance is otherwise fine, then all four
cylinders must be firing.

Consider a new oxygen sensor.
https://www.automedicsupply.com/ has great prices for OEM
oxygen sensors.

Stick with NGK plugs in the future. OEM plugs seem to be the
consensus here.

Are cap, rotor, and wires OEM?

My site reinforces a few of these points:
http://home.earthlink.net/~honda.lioness/id11.html

Elle
Original owner, 1991 Civic (1.5 liter, manual transmission),
172k miles, 40+ mpg most of the year; about 39 mpg this time
of year.

 

Low mileage => more gas or more work by engine for a given baseline hmm ...

I presume you have ruled out obvious driving changes such as
altitude/distance/less hwy/more city/lead footing/avg speed etc...

is that automatic tranny ?

- have you noticed an increase in RPM per given cruising speed ? that is at
60 mph you used to be at say 2500 rpm now you are at 3000 rpm ?

- any chance your transmission is the problem maybe you lost 4th/5th gear ??

- if manual are you forgetting to shift into 5th

- since a vtec any chance your engine stuck in vtec high rpm performance
mode ?
or something wrong with vtec system ?

- has the CHECK engine light come on or blinked recently ?
considering VTEC function is controled electronically (ECM function)
could be VTEC malfunction

i have '93 Civic Si same engine and have never really seen (> 31mpg)

maybe the VTEC was broken before and now it works ?

I usually get about 28-30 , maybe your foot is heavier now

using different grade oil (factory recommended) or any new additives ?

HTH

most of this was out of Factory Service Manual, a very good book if you
like to work on the honda i do not know how chilton/hayes etc compare but
the Honda Service manual seems very good

 

Low mileage => more gas or more work by engine for a given baseline hmm ...

I presume you have ruled out obvious driving changes such as
altitude/distance/less hwy/more city/lead footing/avg speed etc...

is that automatic tranny ?

- have you noticed an increase in RPM per given cruising speed ? that is at
60 mph you used to be at say 2500 rpm now you are at 3000 rpm ?

- any chance your transmission is the problem maybe you lost 4th/5th gear ??

- if manual are you forgetting to shift into 5th

- since a vtec any chance your engine stuck in vtec high rpm performance
mode ?
or something wrong with vtec system ?

- has the CHECK engine light come on or blinked recently ?
considering VTEC function is controled electronically (ECM function)
could be VTEC malfunction

i have '93 Civic Si same engine and have never really seen (> 31mpg)

maybe the VTEC was broken before and now it works ?

I usually get about 28-30 , maybe your foot is heavier now

using different grade oil (factory recommended) or any new additives ?

HTH

most of this was out of Factory Service Manual, a very good book if you
like to work on the honda i do not know how chilton/hayes etc compare but
the Honda Service manual seems very good

 

Doesn't matter how you drive it. You should still be getting better mpg's.

I second elle's opinion about the oxy sensor(s). The engine computer no
longer gets realistic data from the oxy sensor and is running in default
mode.

dan

 

Doesn't matter how you drive it. You should still be getting better mpg's.

I second elle's opinion about the oxy sensor(s). The engine computer no
longer gets realistic data from the oxy sensor and is running in default
mode.

dan

 

Restricted cats show up mostly as the last bit of throttle not doing
anything, or even making the engine stumble and surge at full throttle. The
Haynes manual for the '90 to '93 Accord has a slick diagnostic for
restricted exhaust systems using a manifold vacuum guage. It has you connect
the guage and note the idle manifold reading. Then open the throttle so the
engine is revving about 2000 rpm for a few seconds, and watch the guage as
you release the throttle. If the reading returns to the original idle
pressure within 2 seconds, the exhaust system is unrestricted. If the
reading hovers near the 2000 rpm reading before dropping or if it slowly
returns to normal the exhaust is restricted... could be the muffler or cat.

Mike

 

I'd add an OEM thermostat to the list....

Mike

 

I'd add an OEM thermostat to the list....

Mike