1991 Nissan Sentra- reduced gas mileage- where to check?

robbie

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Nov 12, 2011, 12:19:30 PM11/12/11

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the factory rating for my 1.6L 4 speed manual sentra claims 25 city
33 highway. I have always maintained over 30mpg when I check the
mileage between fillups. I generally do this using a full tank of gas
so as to get a better average. Now my mileage is under 25 and has been
as low as 22mpg. I have done a tuneup, installed a new O2 sensor,
checked the MAF sensor, but haven't found anything out of the
ordinary. I was wodering if I could have leaky injectors, despite the
fact that the car runs and idles fine. I have replaced an injector
about a year ago because it quit working entirely (kind of obvious
when you lose a cylinder's worth of power on a 4 cylinder engine!) but
the other 3 are original and have 160k miles on them. Any advice out
there for me? This is my daily driver for work (30 miles/day) and I
would love to have those mpgs back!

John Henderson

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Nov 12, 2011, 2:32:02 PM11/12/11

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A few other things I can think of:

Air filter blocked by dirt
Tyre (tire) pressure low
Spark plugs or plug leads faulty
Thermostat over-cooling

A leaky injector might be inclined to cause hard starting
(overfueling the engine while stopped as it releases residual
fuel-line pressure).

John

John Henderson

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Nov 12, 2011, 2:45:35 PM11/12/11

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robbie wrote:

> I have replaced an injector about a year ago

You can get an idea whether all cylinders are running at approx
the same mixture ratio by examining the colour of the deposits on
the internal spark plug insulators.

John

al

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Nov 13, 2011, 8:50:28 PM11/13/11

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Defintely check the plugs as their condition can reveal a lean or rich
mixture as well as a malfunctioning cylinder. Assuming that the plugs
look fine and given that the engine seems to be running well, then
excessive drag is likely. Tire pressure is a possibility especially
at this time of year when temperatures fall. Every 10 F change in
temperature causes about a PSI change in tire pressure so it would
take a pretty drastic change in temperature to drop mileage 20%.. A
much more likely possibility is a dragging caliper. If the calipers
are not kept well lubricated and the slides free to move, then
excessive drag can occur. Does one or more tire feel hotter to the
touch than the rest after running a while? Is there any pulling while
braking? Simply jack up the car and rotate each wheel by hand. They
should be fairly free, ideally all should rotate easily. If one or
more wheel will barely budge when you try to rotate it by hand, you
found the problem. Good luck. Al

Plague Boy

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Nov 15, 2011, 9:33:17 PM11/15/11

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On 11/12/11 12:19 PM, robbie wrote:
> the factory rating for my 1.6L 4 speed manual sentra claims 25 city
> 33 highway.

That is about what I get with my '92 with 180K. My milage went up when
I replaced the faulty Coolant Temp Sensor, which was also causing hard
starting in cold weather.

I don't think there's any good way to test it, but it was about
$25 and was simple to change, I didn't have to drain the coolant.

PB

nucleus

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Nov 16, 2011, 7:07:08 PM11/16/11

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have you determined if the ECM is operating in closed-loop-control?

robbie

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Aug 28, 2012, 11:57:55 AM8/28/12

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There is no CEL on, and I have replaced the O2 sensor recently as it was still the factory one. There are no error codes when I manually check them by turning the diagnostic screw on the ECU under the dash- I have always gotten 5-5-5 (all clear) any other diagnosis to be done?

Your Name

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Aug 28, 2012, 5:17:13 PM8/28/12

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In article <bcce3a79-79da-45c4...@googlegroups.com>, robbie

<robbie....@gmail.com> wrote:

> On Wednesday, November 16, 2011 7:07:08 PM UTC-5, nucleus wrote:
> > have you determined if the ECM is operating in closed-loop-control
> >
> >

> > On Nov 12, 11:19=A0am, robbie <robbie.h.wil...@gmail.com> wrote:
> >
> > > the factory rating for my 1.6L =A04 speed manual sentra claims 25 city

> > > 33 highway. I have always maintained over 30mpg when I check the
> > > mileage between fillups. I generally do this using a full tank of gas
> > > so as to get a better average. Now my mileage is under 25 and has been
> > > as low as 22mpg. I have done a tuneup, installed a new O2 sensor,
> > > checked the MAF sensor, but haven't found anything out of the
> > > ordinary. I was wodering if I could have leaky injectors, despite the
> > > fact that the car runs and idles fine. I have replaced an injector
> > > about a year ago because it quit working entirely (kind of obvious
> > > when you lose a cylinder's worth of power on a 4 cylinder engine!) but
> > > the other 3 are original and have 160k miles on them. Any advice out
> > > there for me? This is my daily driver for work (30 miles/day) and I
> > > would love to have those mpgs back!
>

> There is no CEL on, and I have replaced the O2 sensor recently as it was st=
> ill the factory one. There are no error codes when I manually check them by=
> turning the diagnostic screw on the ECU under the dash- I have always gott=

> en 5-5-5 (all clear) any other diagnosis to be done?

There was a brief article in the newspaper here in New Zealand about one
of the American companies having quality issues with their gas / petrol /
fuel. Maybe that's what is causing your mileage problem.

Jim Yanik

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Aug 29, 2012, 11:53:13 AM8/29/12

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Your...@YourISP.com (Your Name) wrote in
news:YourName-290...@203-118-187-95.dsl.dyn.ihug.co.nz:

First,the US is mostly on E10;gas with up to 10% ethanol(usually 5.9%),so
it gets less mileage than pure gasoline.

In what shape are your plug wires? a bad plug wire can cause a weak spark
and poor fuel burning.
could be the catalytic converter clogging up,too.
before 1996,the cat is not checked by the ECU.
They DO wear out eventually.

Have you checked cylinder compression? you might have leaky rings or valve
seals.

--
Jim Yanik
jyanik
at
localnet
dot com