93 Altima stalling at random

On 07/10/2012 03:30 AM, mark wrote:
> Update: Car has not stalled since that oil change a few weeks ago
>
>
> New problem though: Failed emission test on very high Nox reading , 3x the
> maximum allowed. Cat was changed<15k ago so I doubt that's the problem,
> next most probable is the O2 sensor. I ran a quick test at various rpm's
> and it varied between .2 and .6 vdc. Manual says .1-.9 vdc

if the sensor was faulty enough to give you bad smog, you'd very likely
get a check engine light. voltage testing of sensors is a crap shoot
unless you've got them on a scope. and even then, the computer isn't
looking for voltage, it's looking for the stoichiometry point, and thus
the injection percentage that flip-flops sensor readings between high
and low voltage, not voltage per se.

high NOx is usually either lean combustion or bad cat. maybe, if you
have some badly clogged injectors, some injector cleaner will help
reduce lean combustion and thus NOx.

but much more likely, you've got a cheapo cat and it's simply worn out
already. platinum is really expensive. used oem cats have scrap values
of up to $100. if you're buying an aftermarket cat for $130, you can
figure that there's not a lot of platinum or longevity in the thing.


--
fact check required

Update: Car has not stalled since that oil change a few weeks ago


New problem though: Failed emission test on very high Nox reading , 3x the
maximum allowed. Cat was changed <15k ago so I doubt that's the problem,
next most probable is the O2 sensor. I ran a quick test at various rpm's
and it varied between .2 and .6 vdc. Manual says .1-.9 vdc

mark wrote:
> Update: Car has not stalled since that oil change a few weeks ago
>
>
> New problem though: Failed emission test on very high Nox reading ,
> 3x the maximum allowed. Cat was changed <15k ago so I doubt that's
> the problem, next most probable is the O2 sensor. I ran a quick test
> at various rpm's and it varied between .2 and .6 vdc. Manual says
> .1-.9 vdc




Check the EGR valve for proper operation/ plugged passages.

mark wrote:
> Update: Car has not stalled since that oil change a few weeks ago
>
>
> New problem though: Failed emission test on very high Nox reading , 3x the
> maximum allowed. Cat was changed <15k ago so I doubt that's the problem,
> next most probable is the O2 sensor. I ran a quick test at various rpm's
> and it varied between .2 and .6 vdc. Manual says .1-.9 vdc

Was NOX the ONLY high item or just the only one over the limit? Without
seeing the trace it is difficult to pinpoint the problem, but here are a
few items to check before you get panicked.

Check the EGR system ( EGR valve, EGR solenoid, Transducer and
Temperature sensor), and vacuum lines for leaks. If the EGR isn't
operating it can cause high NOX due to the increased combustion temps.

For the EGR valve you apply 5-10 in.hg of vacuum and watch the valve. It
should open and hold open until you release the vacuum.

Testing the solenoid is a bit more involved due to the three lines. You
should have two lines in an L shape on one end and one straight out the
other. You need 12 volt power and either vacuum or air pressure.
With NO power applied the L shaped port should allow air through either
way, with none going out the straight port on the other end.
With power APPLIED the air should NOT flow through the L shaped port
like it did before. Now the air should go through the port on the other
end and the port sticking out the side of the L shaped port.

The transducer gets tested by plugging one of the ports on top. Now
blow into the bottom port gently while applying a vacuum to the
unplugged port on top of the valve. While you blow into the valve the
vacuum should hold steady. When you stop blowing it should open and
allow vacuum to pass through. DO NOT USE AIR PRESSURE TO BLOW INTO THE
UNIT. It operates using exhaust pressure and higher pressures will
damage the diaphragm inside the unit.

Temp sensor is a simple unit. Should read high resistance when cold and
much less when hot. At freezing it should read 7.5 up to 10 meg ohms.
With it at the boiling point of water it should read (POINT) .20 meg or
less.


Check for vacuum leaks below the injectors (this will usually show up
with a slightly high idle speed as well but may not), ignition timing,
excessive carbon in the cylinders, look at the exhaust for leaks before
the cat. Check for anything that could be causing a lean condition.

Even using cheap gas could cause problems. What I do with a marginal
fail is tell the owner to fill up with higher octane fuel, then take the
vehicle out and run it HARD for a few miles, make sure it is up to
temperature and everything is working. Do this a few times then bring it
back for the retest (if it's a close test I'll toss the 5 gas tester on
it prior to the test as a precaution) This works wonders on the grandma
cars that set around and hardly get brought up to temp.

--
Steve W.

There is a leak in the Transducer between the line to the egr valve and
another 1 going to the tee fitting on the Transducer. (EGR valve itself
works fine) Can't find the part online so probably horribly expensive. I
am thinking of bypassing it and running vac direct to the EGR. Will try to
disassemble it and repair 1st.

mark wrote:
> There is a leak in the Transducer between the line to the egr valve and
> another 1 going to the tee fitting on the Transducer. (EGR valve itself
> works fine) Can't find the part online so probably horribly expensive. I
> am thinking of bypassing it and running vac direct to the EGR. Will try to
> disassemble it and repair 1st.
>
>

Bypass it and you will make things worse. That transducer is designed to
regulate the egr flow using the back pressure of the exhaust.

Standard Motor products part # VS137 $62.99

http://www.rockauto.com

--
Steve W.