Long vs short term fuel trim & hesitation?

Hi all,

I'm still trying to figure out the cause of the slight hesitation off idle
in my 1999 Altima. I'll skip the history and the details of what I've tried
so far, but suffice it to say that all the regular tuneup items have been
replaced, EGR valve and passages have been cleaned, coolant temp sensor is
new, etc etc. I've posted in detail before if anyone wants to know more
background.

Anyway, I've got an Autotap interface for my laptop now so I'm able to
monitor a few of the OBD paramaters (when the software isn't crashing).
Everything looks as I would expect as far as sensor readings; the TPS, MAP,
MAF, coolant and intake air temp sensors all seem good.

I have a question about the short term fuel trim though. When I first
started the car with the scanner connected (it was warm, and had only been
off for maybe 5 minutes) the short term trim was reading around 0%, and the
hesitation was at its worst (as it always is when the car is just started).
When I blipped the throttle slightly, which is when the hesitation manifests
itself, the short term trim dropped to around -10% after the engine
recovered from the miss, and then after returning to idle, the short term
trim was back to about 0%.

After a few minutes, the short term trim worked its way down to around -8%
at idle, and would still drop a little lower when I blipped the throttle.
The hesitation at this stage was less apparent though, as it normally is
after a few minutes of running.

So, I'm wondering if the ECU is compensating for an over-rich mixture, and
that's over-richness is what is causing the hesitation.

Interestingly, the long term trim is actually close to + 10%. So is the
short term trim just a fine tuning either side of that long term value, in
which case maybe there is no problem here? Or is there something fishy
going on with the two values working in opposite directions?

Thanks in advance for any insight!

does the slight hesitation result in a miss-or more importantly a
'ses' light to illuminate? For what it's worth-suspicious of a fuel
injection issue-does your equipment show any specific cylinder
missing? Wonder if an injector cleaning of some magnitude is in order.
jerry@peru-motors




On Feb 21, 5:20 pm, "JM" <j... (AT) ns (DOT) sympatico.ca.ns> wrote:

Hi all,

I'm still trying to figure out the cause of the slight hesitation off idle
in my 1999 Altima. I'll skip the history and the details of what I've tried
so far, but suffice it to say that all the regular tuneup items have been
replaced, EGR valve and passages have been cleaned, coolant temp sensor is
new, etc etc. I've posted in detail before if anyone wants to know more
background.

Anyway, I've got an Autotap interface for my laptop now so I'm able to
monitor a few of the OBD paramaters (when the software isn't crashing).
Everything looks as I would expect as far as sensor readings; the TPS, MAP,
MAF, coolant and intake air temp sensors all seem good.

I have a question about the short term fuel trim though. When I first
started the car with the scanner connected (it was warm, and had only been
off for maybe 5 minutes) the short term trim was reading around 0%, and the
hesitation was at its worst (as it always is when the car is just started).
When I blipped the throttle slightly, which is when the hesitation manifests
itself, the short term trim dropped to around -10% after the engine
recovered from the miss, and then after returning to idle, the short term
trim was back to about 0%.

After a few minutes, the short term trim worked its way down to around -8%
at idle, and would still drop a little lower when I blipped the throttle.
The hesitation at this stage was less apparent though, as it normally is
after a few minutes of running.

So, I'm wondering if the ECU is compensating for an over-rich mixture, and
that's over-richness is what is causing the hesitation.

Interestingly, the long term trim is actually close to + 10%. So is the
short term trim just a fine tuning either side of that long term value, in
which case maybe there is no problem here? Or is there something fishy
going on with the two values working in opposite directions?

Thanks in advance for any insight!

On Feb 21, 5:20 pm, "JM" <j... (AT) ns (DOT) sympatico.ca.ns> wrote:

Hi all,

I'm still trying to figure out the cause of the slight hesitation off idle
in my 1999 Altima. I'll skip the history and the details of what I've tried
so far, but suffice it to say that all the regular tuneup items have been
replaced, EGR valve and passages have been cleaned, coolant temp sensor is
new, etc etc. I've posted in detail before if anyone wants to know more
background.

Anyway, I've got an Autotap interface for my laptop now so I'm able to
monitor a few of the OBD paramaters (when the software isn't crashing).
Everything looks as I would expect as far as sensor readings; the TPS, MAP,
MAF, coolant and intake air temp sensors all seem good.

I have a question about the short term fuel trim though. When I first
started the car with the scanner connected (it was warm, and had only been
off for maybe 5 minutes) the short term trim was reading around 0%, and the
hesitation was at its worst (as it always is when the car is just started).
When I blipped the throttle slightly, which is when the hesitation manifests
itself, the short term trim dropped to around -10% after the engine
recovered from the miss, and then after returning to idle, the short term
trim was back to about 0%.

After a few minutes, the short term trim worked its way down to around -8%
at idle, and would still drop a little lower when I blipped the throttle.
The hesitation at this stage was less apparent though, as it normally is
after a few minutes of running.

So, I'm wondering if the ECU is compensating for an over-rich mixture, and
that's over-richness is what is causing the hesitation.

Interestingly, the long term trim is actually close to + 10%. So is the
short term trim just a fine tuning either side of that long term value, in
which case maybe there is no problem here? Or is there something fishy
going on with the two values working in opposite directions?

Thanks in advance for any insight!

Indeed, a rich mixture at idle can cause the hesitation. Check the
behavior of the oxygen sensor at idle vice at speed e.g. 2000-2500
RPM. If the O2 sensor is getting tired it may switch rapidly, which
is normal, at speed when it is hot but then switch slowly at idle when
it is cooler. That slow switching rate in turn causes the mixture to
be wrong at idle and results in a hesitation when you initially rev up
the engine from idle. Good luck. Al

"blazerman" <jerry (AT) peru-motors (DOT) com> wrote

does the slight hesitation result in a miss-or more importantly a
'ses' light to illuminate? For what it's worth-suspicious of a fuel
injection issue-does your equipment show any specific cylinder
missing? Wonder if an injector cleaning of some magnitude is in order.
jerry@peru-motors

Unfortunately, no, there's no CEL or any stored codes.. that might make the
diagnosis a little easier.

"al" <abuonag (AT) msn (DOT) com> wrote

On Feb 21, 5:20 pm, "JM" <j... (AT) ns (DOT) sympatico.ca.ns> wrote:
Indeed, a rich mixture at idle can cause the hesitation. Check the
behavior of the oxygen sensor at idle vice at speed e.g. 2000-2500
RPM. If the O2 sensor is getting tired it may switch rapidly, which
is normal, at speed when it is hot but then switch slowly at idle when
it is cooler. That slow switching rate in turn causes the mixture to
be wrong at idle and results in a hesitation when you initially rev up
the engine from idle. Good luck. Al

Well, it is a new sensor, but I did notice that the sensor seems to be
switching slowly at idle vs. at 2000 rpm or so. I seem to remember reading
that the system is only designed to work above that speed anyway though, so
maybe that's normal. Or maybe there's an issue with the O2 sensor heater
causing the sensor to cool at idle speeds?

One thing I forgot to mention that may fit with the rich mixture theory is
the fact that the car seems to be slow to warm up, compared the '94 Altima I
had before this one. In the winter I can be driving for 30 minutes or more
before the needle reaches the normal point on the gauge, or before it even
comes off the cold mark on really cold days (-20 C). The rad hose to the
thermostat on these days is usually barely warm. I replaced the thermostat
last summer, but saw no real change. Maybe this is normal though.

Thanks for the input.

JM wrote:

"al" <abuonag (AT) msn (DOT) com> wrote in message
news:1172109929.705338.176130 (AT) v45g2000cwv (DOT) googlegroups.com...

On Feb 21, 5:20 pm, "JM" <j... (AT) ns (DOT) sympatico.ca.ns> wrote:
Indeed, a rich mixture at idle can cause the hesitation. Check the
behavior of the oxygen sensor at idle vice at speed e.g. 2000-2500
RPM. If the O2 sensor is getting tired it may switch rapidly, which
is normal, at speed when it is hot but then switch slowly at idle when
it is cooler. That slow switching rate in turn causes the mixture to
be wrong at idle and results in a hesitation when you initially rev up
the engine from idle. Good luck. Al


Well, it is a new sensor, but I did notice that the sensor seems to be
switching slowly at idle vs. at 2000 rpm or so. I seem to remember reading
that the system is only designed to work above that speed anyway though, so
maybe that's normal. Or maybe there's an issue with the O2 sensor heater
causing the sensor to cool at idle speeds?

One thing I forgot to mention that may fit with the rich mixture theory is
the fact that the car seems to be slow to warm up, compared the '94 Altima I
had before this one. In the winter I can be driving for 30 minutes or more
before the needle reaches the normal point on the gauge, or before it even
comes off the cold mark on really cold days (-20 C). The rad hose to the
thermostat on these days is usually barely warm. I replaced the thermostat
last summer, but saw no real change. Maybe this is normal though.

Thanks for the input.


You did the coolant temp sensor but have you cleaned your throttle body?

It does wonders for low rpm smoothness if your TB is very coked up.

And, by the way, use throttle body cleaner that's safe for fuel injected
motors. Regular carburator cleaner will clean the throttle body, but it
will also strip away the fine film on the inside of the intake manifold
which helps to discourage carbon buildup.

CD

"Codifus" <codifus (AT) optonline (DOT) net> wrote

You did the coolant temp sensor but have you cleaned your throttle body?
It does wonders for low rpm smoothness if your TB is very coked up.

I have cleaned the TB, not for a while now, probably not too much more than
a year ago. The hesitation was present then, and didn't seem to improve
after the cleaning, unfortunately. I did use throttle body specific
cleaner.

Thanks for your input though!

Just to make sure:

Is it a hesitation or a miss? Is it rough for a while or momentarily off?

Did you check timing? Vacuum leaks?

JM wrote:

Hi all,

I'm still trying to figure out the cause of the slight hesitation off idle
in my 1999 Altima. I'll skip the history and the details of what I've tried
so far, but suffice it to say that all the regular tuneup items have been
replaced, EGR valve and passages have been cleaned, coolant temp sensor is
new, etc etc. I've posted in detail before if anyone wants to know more
background.

Anyway, I've got an Autotap interface for my laptop now so I'm able to
monitor a few of the OBD paramaters (when the software isn't crashing).
Everything looks as I would expect as far as sensor readings; the TPS, MAP,
MAF, coolant and intake air temp sensors all seem good.

I have a question about the short term fuel trim though. When I first
started the car with the scanner connected (it was warm, and had only been
off for maybe 5 minutes) the short term trim was reading around 0%, and the
hesitation was at its worst (as it always is when the car is just started).
When I blipped the throttle slightly, which is when the hesitation manifests
itself, the short term trim dropped to around -10% after the engine
recovered from the miss, and then after returning to idle, the short term
trim was back to about 0%.

After a few minutes, the short term trim worked its way down to around -8%
at idle, and would still drop a little lower when I blipped the throttle.
The hesitation at this stage was less apparent though, as it normally is
after a few minutes of running.

So, I'm wondering if the ECU is compensating for an over-rich mixture, and
that's over-richness is what is causing the hesitation.

Interestingly, the long term trim is actually close to + 10%. So is the
short term trim just a fine tuning either side of that long term value, in
which case maybe there is no problem here? Or is there something fishy
going on with the two values working in opposite directions?

Thanks in advance for any insight!

"AS" <donot (AT) spame (DOT) com> wrote

Just to make sure:
Is it a hesitation or a miss? Is it rough for a while or momentarily off?
Did you check timing? Vacuum leaks?

Well, it's more of a hesitation. I shouldn't have used the term "miss" I
suppose. What happens is there's about a 1/2 - 3/4 second pause when I step
into gas where the tach actually drops maybe one or two needle-widths
(highly precise measurement there) and then it responds. It *feels* like
it's starving for fuel when I open the throttle for a split second, but the
fuel pressure is ok, and I can hear the injectors respond when I snap the
throttle from under the hood...

I have a video I recorded to try and capture the effect that might help;
I'll upload it and post a URL if it might help anyone get a feel for what's
happening.

The timing is 20 deg BTDC at idle, on spec, and I haven't been able to find
any vacuum leaks.. the idle is very smooth and steady. When I first got
the car, the idle was high, close to 1000 rpm, and I had a seized idle
adjust screw, but since I replaced that part I was able to almost bring the
idle to around 750 rpm, and over time since then it seems to have dropped to
about the proper speed.

Thanks for your reply!

It's a little hard to see/hear it from the video (taken from my phone) but
here's a Quicktime recording of it:

http://www3.ns.sympatico.ca/pmason/altima_hes.3g2

I tried to use my finger over the bottom part of the lens to show when I was
pressing and releasing the throttle.