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'88 Sentra engine oscillation?

 
Sam Lewallen
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      14th September 2003, 01:05 AM
I have a 1988 Sentra with the 1.6L fuel-injected engine, manual
transmission. Here's what's happening (it's kinda long-winded so please
bear with me): the car starts and runs fine up to about 2000 or 2500 RPM
(guessing based on engine noise). At that point, it starts oscillating
between that RPM and what sounds like idle about twice a second:
"vroom...vroom...vroom...vroom", like the throttle is being pumped, even
though the accelerator pedal is held at a steady pressure. If I press
the gas pedal more, it just keeps doing it (the revving/idle
oscillation), not sounding any different, until I let it go back to
lower RPMs, where it runs fine again.

Another strange thing is that it seems to be RPM-related, since if I'm
in, say, 4th gear, I can floor the pedal and it accelerates fine up to
about 40 or 45 MPH, which is the speed it goes when the engine is at
about 2000-2500 RPM, at which point the oscillation starts again (makes
for a mighty jerky ride, too). It doesn't happen all the time though;
sometimes the car runs just fine and I can rev the engine all the way up
to around its redline (no tach so can't say for sure) without the
oscillation starting again. I've not been able to pin down the specific
conditions that cause it to happen. Sometimes it works fine, sometimes
it has this problem. I thought it might be temperature related, but
it's happened on cool and hot days, with the engine both cool and hot as
well.

I've replaced the air and fuel filters and changed the oil in the last
couple of days trying to pin this problem down, but it hasn't helped.
I'm assuming it's not a timing issue (slipped or loose belt or chain)
since it doesn't happen constantly.

In an attempt to track down the source of the problem, I've taken off
the cover for the air filter and looked down into the intake as I pull
on the accelerator cable manually. When I run the engine up, I can see
the butterfly valve move with the throttle, and that valve does not
oscillate when the problem starts happening. Instead, I can see what
must be the fuel/air mixture coming from the injector housing (I think
that's what it is) in the middle of the intake throat, and it looks
almost like that is what is fluctuating. I.e. the throttle valve stays
constant but the injector seems to be oscillating between putting enough
fuel/air in and not enough; I can see the gas running across the surface
of the butterfly valve, and the amount seems to oscillate.

If anyone has some ideas on what to go after next, I'd sure appreciate
it. I'm hoping to avoid having to take it into a mechanic. Might it be
a problem with the injector (dirty injector maybe)? Clogged fuel lines?
Some kind of air intake problem? I have a replacement PCV valve, but
haven't installed it yet (can't seem to find where it goes on the
engine... :-/). Anyway, any advice would be most welcome.

Thanks,
~Sam

 
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Peter Hill
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      14th September 2003, 08:01 AM
On Sat, 13 Sep 2003 18:05:09 -0700, Sam Lewallen
<(E-Mail Removed)> wrote:

>In an attempt to track down the source of the problem, I've taken off
>the cover for the air filter and looked down into the intake as I pull
>on the accelerator cable manually. When I run the engine up, I can see
>the butterfly valve move with the throttle, and that valve does not
>oscillate when the problem starts happening. Instead, I can see what
>must be the fuel/air mixture coming from the injector housing (I think
>that's what it is) in the middle of the intake throat, and it looks
>almost like that is what is fluctuating. I.e. the throttle valve stays
>constant but the injector seems to be oscillating between putting enough
>fuel/air in and not enough; I can see the gas running across the surface
>of the butterfly valve, and the amount seems to oscillate.


Get a fuel pressure test done when it's doing it. If the pressure
fluctuates it's either pump, pressure regulator or pump power signal
from the ECU.

--
Peter Hill
Spamtrap reply domain as per NNTP-Posting-Host in header
Can of worms - what every fisherman wants.
Can of worms - what every PC owner gets!
 
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Sam Lewallen
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      15th September 2003, 07:16 AM
Peter Hill wrote:
> On Sat, 13 Sep 2003 18:05:09 -0700, Sam Lewallen
> <(E-Mail Removed)> wrote:
>
>
>>In an attempt to track down the source of the problem, I've taken off
>>the cover for the air filter and looked down into the intake as I pull
>>on the accelerator cable manually. When I run the engine up, I can see
>>the butterfly valve move with the throttle, and that valve does not
>>oscillate when the problem starts happening. Instead, I can see what
>>must be the fuel/air mixture coming from the injector housing (I think
>>that's what it is) in the middle of the intake throat, and it looks
>>almost like that is what is fluctuating. I.e. the throttle valve stays
>>constant but the injector seems to be oscillating between putting enough
>>fuel/air in and not enough; I can see the gas running across the surface
>>of the butterfly valve, and the amount seems to oscillate.

>
>
> Get a fuel pressure test done when it's doing it. If the pressure
> fluctuates it's either pump, pressure regulator or pump power signal
> from the ECU.
>
> --
> Peter Hill
> Spamtrap reply domain as per NNTP-Posting-Host in header
> Can of worms - what every fisherman wants.
> Can of worms - what every PC owner gets!


Thanks for the advice. I think I may have solved the problem; I found
the following on a Google search:

Question: 1988 Nissan Sentra 4 cyl mileage: 120000. Car sputters and
jerks/lurches in a rhythmic fashion (like I'm revving the engine even
though I am applying constant pressure to the accelerator). Typically
does it when I get to above 40 mph, but has done it accelerating from a
dead stop and other speeds as well. Also, doesn't do it all the time.
Have changed fuel filter & replaced air flow sensor. Nothing. Mechanic
has said it is neither "gunk"/moisture in the gas tank, nor fuel
injector problems. We are at the point of fishing...HELP!

Answer: The three things we would check would be the ignition module,
the pick-up coil and the O2 sensor.

The oxygen sensor appealed to me as a good place to start, since it's
easy to get to. I pulled the old one this evening, and it definately
needed replacing. It's blackened, and the slits that let the exhaust
gas go through the sensor are nearly swollen shut. I put in the new
one, started it up, and immediately noticed that it's running smoother,
with quicker response to the accelerator pedal. I took it for a long
drive around the block, taking it up to about 50 MPH and revving the
engine frequently with the clutch in, and the oscillation did not
reappear. Doesn't necessarily mean it's fixed, of course, but I have a
good feeling about this. I'll keep watching it for the next couple of
days and hopefully the problem won't come back. Thanks again for your
response.

~Sam

--
If a man will begin with certainties, he shall end in doubts; but if he
will be content to begin with doubts, he shall end in certainties.
-Francis Bacon-

 
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Sam Lewallen
Guest
Posts: n/a
 
      16th September 2003, 07:02 AM
Sam Lewallen wrote:

> Peter Hill wrote:
>
>> On Sat, 13 Sep 2003 18:05:09 -0700, Sam Lewallen
>> <(E-Mail Removed)> wrote:
>>
>>
>>> In an attempt to track down the source of the problem, I've taken off
>>> the cover for the air filter and looked down into the intake as I
>>> pull on the accelerator cable manually. When I run the engine up, I
>>> can see the butterfly valve move with the throttle, and that valve
>>> does not oscillate when the problem starts happening. Instead, I can
>>> see what must be the fuel/air mixture coming from the injector
>>> housing (I think that's what it is) in the middle of the intake
>>> throat, and it looks almost like that is what is fluctuating. I.e.
>>> the throttle valve stays constant but the injector seems to be
>>> oscillating between putting enough fuel/air in and not enough; I can
>>> see the gas running across the surface of the butterfly valve, and
>>> the amount seems to oscillate.

>>
>>
>>
>> Get a fuel pressure test done when it's doing it. If the pressure
>> fluctuates it's either pump, pressure regulator or pump power signal
>> from the ECU.
>>
>> --
>> Peter Hill
>> Spamtrap reply domain as per NNTP-Posting-Host in header
>> Can of worms - what every fisherman wants.
>> Can of worms - what every PC owner gets!

>
>
> Thanks for the advice. I think I may have solved the problem; I found
> the following on a Google search:
>
> Question: 1988 Nissan Sentra 4 cyl mileage: 120000. Car sputters and
> jerks/lurches in a rhythmic fashion (like I'm revving the engine even
> though I am applying constant pressure to the accelerator). Typically
> does it when I get to above 40 mph, but has done it accelerating from a
> dead stop and other speeds as well. Also, doesn't do it all the time.
> Have changed fuel filter & replaced air flow sensor. Nothing. Mechanic
> has said it is neither "gunk"/moisture in the gas tank, nor fuel
> injector problems. We are at the point of fishing...HELP!
>
> Answer: The three things we would check would be the ignition module,
> the pick-up coil and the O2 sensor.
>
> The oxygen sensor appealed to me as a good place to start, since it's
> easy to get to. I pulled the old one this evening, and it definately
> needed replacing. It's blackened, and the slits that let the exhaust
> gas go through the sensor are nearly swollen shut. I put in the new
> one, started it up, and immediately noticed that it's running smoother,
> with quicker response to the accelerator pedal. I took it for a long
> drive around the block, taking it up to about 50 MPH and revving the
> engine frequently with the clutch in, and the oscillation did not
> reappear. Doesn't necessarily mean it's fixed, of course, but I have a
> good feeling about this. I'll keep watching it for the next couple of
> days and hopefully the problem won't come back. Thanks again for your
> response.
>
> ~Sam
>


Well, that wasn't it; back to the same problem today. Next on the
chopping block is the fuel pump and distributor... People at work have
suggested that the injector might be clogged, but it looks like a little
more of a chore to pull that out and inspect it (requires new gaskets,
etc.) and I'm limited by the fact that I have to keep driving my car to
make it to work and school, not to mention it's my only real
transportation to the auto parts store! Anyway, I'll keep plugging at it.

~Sam

--
If a man will begin with certainties, he shall end in doubts; but if he
will be content to begin with doubts, he shall end in certainties.
-Francis Bacon-

 
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