'98 V6 bad fuel economy / O2 sensor? IAC ?

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Old 25 Oct 2004, 12:54 am   #1 (permalink)
Jay
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Default '98 V6 bad fuel economy / O2 sensor? IAC ?

Hey all,

I've noticed that the fuel economy on my '98 Camry V6 (99k mi) has been
bad recently. I usually get 24-26+ and I'm seeing 20-22. Work is 1
mile city, 7 miles Highway and then 1 more mile city (9 total). I drive
65-70 mph on the highway and ~45 MPH in the city. I always use 91
octane Chevron gas and try to fill at the same station.

My thinking is the car is running too rich for some reason.

Some background checks I did:

I pulled the dip-stick (it's about 3K miles since last oil change,
Mobil-1), color is good but I can definitely smell gas.

I pulled a spark plug on the exposed bank and while the tip isn't
charcoal black, it is darker than the outter edges of the bent
electrodes. The base area of the spark plug, at the very end of the
thread (closest to the electrode) IS completely black.

OBD2 testing:

I was able to borrow a friend's OBD2 code reader and I noticed a few
things about the Long Term Fuel Trim settings:

At idle/foot off the gas LT FT #1 is usually -8 to -4% while LT FT #2 is
-2% to 0. Under any decent amount of throttle/speed (35 MPH+) LT FT #1
and LT FT #2 are quickly adjusted (almost snapped) to the same, usually
-3%.

Standing still (neutral or park), if I steadily increase the RPM from
2000 to 3500, I can see the LT FT #1 start to go more positive
approaching the value of LT FT #2.

Does this sound normal? At idle/low-speed should one bank be
considerably trimmed more than the other?

Since my driving style is to let off the gas to slow down and then
gently re-apply to speed up, I think the ECU is running banks the
majority of the time with the idle/low-speed Fuel Trim settings, which
seem highly mismatched.

The OBD2 code reader I have doesn't graph data, but looking at the
sensor values at speed (45 MPH+), it looks like the O2 sensor after the
cat (Bank 1, #2) is relatively slowly varying, indicating (I think) that
the O2/feedback system is working correctly (at least at highway
speeds).

Is there any easy way to probe the O2 sensors with a oscilliscope or is
the OBD2 method the only way? I opened up "DLC1" in the engine bay but
the pins for the OX hook-ups are missing (there are just a few terminals
with metal contacts).

Finally, I notice if I let off the throttle completely while moving (i.e
if I'm going 40 MPH or revving the engine to 2500 RPM then back off the
throttle), the OBD2 indicates for a short period the FUEL SYS#1 and FUEL
SYS#2 are OPEN (loop) then switch back to closed loop.

Is this also normal?

Suggestions and commentary appreciated.

Jay.






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Old 25 Oct 2004, 11:11 am   #2 (permalink)
Philip
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Default Re: '98 V6 bad fuel economy / O2 sensor? IAC ?

Jay wrote:
> Hey all,
>
> I've noticed that the fuel economy on my '98 Camry V6 (99k mi) has
> been bad recently. I usually get 24-26+ and I'm seeing 20-22. Work
> is 1 mile city, 7 miles Highway and then 1 more mile city (9 total).
> I drive 65-70 mph on the highway and ~45 MPH in the city. I always
> use 91 octane Chevron gas and try to fill at the same station.
>
> My thinking is the car is running too rich for some reason.
>
> Some background checks I did:
>
> I pulled the dip-stick (it's about 3K miles since last oil change,
> Mobil-1), color is good but I can definitely smell gas.


I really doubt you smell gasoline in the oil while still getting MPG in the
low twenties.


> I pulled a spark plug on the exposed bank and while the tip isn't
> charcoal black, it is darker than the outter edges of the bent
> electrodes. The base area of the spark plug, at the very end of the
> thread (closest to the electrode) IS completely black.


The tip is supposed to be white and the metal housing that is exposed to
combustion gases will always be black on a used plug.


> OBD2 testing:
>
> I was able to borrow a friend's OBD2 code reader and I noticed a few
> things about the Long Term Fuel Trim settings:
>
> At idle/foot off the gas LT FT #1 is usually -8 to -4% while LT FT #2
> is
> -2% to 0. Under any decent amount of throttle/speed (35 MPH+) LT FT
> #1 and LT FT #2 are quickly adjusted (almost snapped) to the same,
> usually
> -3%.
>
> Standing still (neutral or park), if I steadily increase the RPM from
> 2000 to 3500, I can see the LT FT #1 start to go more positive
> approaching the value of LT FT #2.
>
> Does this sound normal? At idle/low-speed should one bank be
> considerably trimmed more than the other?


Yes, normal. What you are seeing is the ECU adjusting for differences in
the header pipe length (the forward pipe being longer than the rear bank
which creates a little more backpressure).


> Since my driving style is to let off the gas to slow down and then
> gently re-apply to speed up, I think the ECU is running banks the
> majority of the time with the idle/low-speed Fuel Trim settings, which
> seem highly mismatched.


Well DUH. Same as a carburetor would do .... idle to transition circuit and
back.


> The OBD2 code reader I have doesn't graph data, but looking at the
> sensor values at speed (45 MPH+), it looks like the O2 sensor after
> the cat (Bank 1, #2) is relatively slowly varying, indicating (I
> think) that the O2/feedback system is working correctly (at least at
> highway speeds).
>
> Is there any easy way to probe the O2 sensors with a oscilliscope or
> is the OBD2 method the only way? I opened up "DLC1" in the engine bay
> but the pins for the OX hook-ups are missing (there are just a few
> terminals with metal contacts).
>
> Finally, I notice if I let off the throttle completely while moving
> (i.e if I'm going 40 MPH or revving the engine to 2500 RPM then back
> off the throttle), the OBD2 indicates for a short period the FUEL
> SYS#1 and FUEL SYS#2 are OPEN (loop) then switch back to closed loop.
>
> Is this also normal?


Yes. Electronic fuel injection shuts off fuel delivery during coast until
about 1500 rpms or ... if the coasting condition lasts for an excessive
amount of time, the delivery may be restarted to keep the catalyst from
overcooling.


> Suggestions and commentary appreciated.
>
> Jay.


Jay, you didn't mention where you live. Seasonal changes in air temperature
and region changes in gasoline formulation account for a high degree of MPG
loss about the time of year.
--

-Philip


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Old 25 Oct 2004, 01:54 pm   #3 (permalink)
Jason James
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Default Re: '98 V6 bad fuel economy / O2 sensor? IAC ?


"Jay" <127.0.0.1@garbage.gar> wrote in message
news:my0fd.27346$Kl3.25350@twister.socal.rr.com...
> Hey all,
>
>
> Is there any easy way to probe the O2 sensors with a oscilliscope or is
> the OBD2 method the only way?> Suggestions and commentary appreciated.
>
> Jay.


A CRO switched to DC coupling will act as an excellent high impedance DC
voltmeter. Just move the timebase with ground selected at Y input (vertical
deflection) to a convenient reference point on the screen,..then switch to
DC (not AC) while the probe is connected to the O2 sensor and the earth clip
is grounded on the chassis.. The trace will shift upwards proportional to
the DC output of the O2 sensor. Multiply shift in CMs by the input
attenuation millivolts per CM setting.

I haven't tried this, and its possible the EHT or spark generation will be
picked up by the CRO as well,.....worth a try tho as CROs are usually in the
region of 10meg-ohm input loading and will not damage or load down the
sensor. AFAIK you can do the measurement with the sensor off line to the ECU
or on line. The latter may produce some reading offset due to ECU
backfeeding some of its rail.



Jason


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Old 26 Oct 2004, 04:59 pm   #4 (permalink)
m Ransley
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Default Re: '98 V6 bad fuel economy / O2 sensor? IAC ?

Is your timing belt old, they stretch retarding timing, reducing engine
temp and mileage and power. You probably need a tune up anyway

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