Sometimes runs way too rich - Computer or oxygen sensor?

[Vanguard]

Vehicle info: '92 Subaru Legacy L wagon 4WD non-turbo

Sometimes when I start my car, the exhaust is far too rich and detonates
back in the catalytic converter. When it happens seems sporadic. The
weather could be cold or hot, humid or dry, and the car may have been
sitting for several days or only for 4 to 8 hours since the last time it
ran. I can compensate by shifting manually to keep the RPMs way up. When
the problem arises, I have to keep the RPM up to around 2500 when at a
stoplight (so it doesn't peter out). When I take off, I'll put it in first
gear and shift manually to keep the RPMs up high; otherwise, it will
hesitate on acceleration and I hear the detonation in the exhaust system.
Even after the temperature guage shows normal level, it is only after it has
been at normal for about 15 minutes before the over-rich problem fades away.
It only runs overly rich occasionally.

The other problem is that the "Check Engine" light comes on sometimes. I've
had it in the shop 4 times to have them read the codes. The first 3 times,
they got no error code. On the 4th time, they got no error code but when I
started my car to leave then the "check engine" light came on and they
hooked up while the car was running. I don't remember what was the code but
it didn't make sense at the time and the mechanic said it was wrong. When
the "check engine" light comes on is *not* when I have the first-startup
overly rich mixture problem. They seem to be independent symptoms but maybe
not. I suppose the "Check Engine" light might not be coming on but the
computer is still screwing up the mixture when the car is started.

So my choices (that I can figure out) is to replace the engine computer
and/or the oxygen sensor. I replaced my exhaust system about 4 to 5 years
ago but they reused the original oxygen sensor (they were going to replace
it but couldn't get one at that time). As I recall, this sensor check how
much oxygen is in the exhaust and recirculates a portion of the exhaust if
the oxygen is too low. Well, if it is running overly rich then wouldn't the
oxygen level would be low?

Any opinions if it is more likely the computer or the oxygen sensor screwing
up the mixture (so it is sometimes too rich on startup)? Might it be
something else? Besides the 4 times to take it into the shop to see if they
can nail the cause of the "check engine" light coming on (but never finding
anything), I've even left my car at their shop over the weekend to let them
start it cold to see if the over-rich problem crops up, but it never failed
when they had my car.

 

[Rick Courtright]

Sometimes when I start my car, the exhaust is far too rich and detonates
back in the catalytic converter. When it happens seems sporadic. The

Hi,

Seems odd the error codes weren't stored so you could do an easier job
of troubleshooting!

I don't know about the '92, but my '90 doesn't throw an error code on a
bad O2 sensor. Since it sounds like you're still on the original (did I
read that right?), I'd certainly investigate installing a new one.
Aftermarket units aren't that expensive (around $35 for mine vs $130 at
the dealer) and seem to work well as long as you follow the instructions
to keep overall wire length the same, etc. I don't know what the factory
recommendation for changing that sensor is, but aftermarket units
suggest changing as often as 15k miles. That's perhaps a bit
often--maybe 30k would be reasonable? For reference, my last factory
unit went south at around 75-80k miles.

If the O2 sensor's letting things run way too rich, you also run the
risk of trashing your cat: they're a bit sensitive to continued
over-rich conditions. The sensor's WAY cheaper than a new cat!

Other things to check: disconnect each sensor you can find, squirt the
connections clean with some electronic component cleaner (better auto
parts stores stock it), then put a little dielectric grease (same
source) inside the connector before snapping it back together. Sometimes
just a little dirt and/or corrosion can cause some screwy voltage
signals to the ECU and it gets confused. There was a Subaru TSB out some
years back concerning this.

WARNING: DON'T use any grease, sealer, etc. on any connections involving
the O2 sensor. I've read where some of them rely on "hollow" insulation
on the wire as a "vent path" for the wire, and clogging that will warm
things up, also sending bad signals to the ECU. Don't know if Subie uses
that style or not.

Is there any other "clue" that might help you narrow things down, like
recently changing the air filter to a new brand or anything? Plugs,
wires, cap, rotor are all good? I'm assuming you've already been down
that path, but sometimes an otherwise "obvious" answer is overlooked.

Good luck!

Rick

 

[Vanguard]

Only 15K (or even 30K) on an oxygen sensor? Geez, that's low. I was
looking around and the price I got for a replacement O2 sensor was $82 (plus
shipping) for a Bosch unit. Not real bad (but more than you paid).

Yep, the O2 sensor that is in there is the same one that has been in there
since 1992 (i.e., it is the original one). I had the exhaust system
replaced about 5 years ago. They were going to replace the O2 sensor but
couldn't get one on order at the time (i.e., it would have to be
backordered). So they put back in the old one and I've never went back to
get it replaced. Geez, their maintenance schedule
(http://www.cars101.com/subaru/subaru_maintenance.html#90-99maintenance)
says I don't need to replace spark plugs until 60K intervals, and the O2
sensor is half of that?

My recollection when I read the error codes (using the flashing lights
method) was that once in 6 times tried that the code pointed at the O2
sensor. As far as other parts, I haven't looked or changed out the spark
plugs, spark cables, rotor, or any of that type of stuff. When the problem
arises, which is only occassionaly, and once the car gets hot (and has been
hot for about 15 to 20 minutes), then the over-rich problem goes away. It
won't reappear if I stop the car, like at the store, and then start again.
Only if the car has sat for many hours might the problem reappear. I doubt
spark plugs & cables, rotors, etc., are temperature sensitive. If, for
example, the spark cable had degenerated over time, its resistance would be
too high regardless of temperature fluctuation. While the oil inside the
coil would heat up, I don't see how the coil's performance would alter
depending on the temperature of its oil use for cooling. I'll probably be
replacing the plugs, cables, rotor, and normal stuff for tuning and
maintenance but I'd like to solve this over-rich problem first (big stuff
first, little stuff last).

I can try the dry-and-grease method you mention regarding the electrical
connectors for the sensors. In fact, I should still have some dielectric
grease around from last time (I use it to avoid corrosion of the contacts
within a connector that I take apart and put back together if it is outside
the passenger compartment). I'll have to get some more contact cleaner
spray, though.

I suspect that getting rid of the problem will end up being a piecemeal
guess method: keep replacing possible components involved in the fuel
injection system until I happen to replace the culprit.

 

[Rick Courtright]

Only 15K (or even 30K) on an oxygen sensor? Geez, that's low. I was

Yeah, seems low, but we've gotta remember the vendors want to sell
sensors! Since US emissions requirements usually spec a minimum 50k
miles for emissions related equipment warranties, perhaps 60k w/ new
plugs and such sounds better? As I said, I don't recall seeing a
replacement interval in the books I've checked...

Yep, the O2 sensor that is in there is the same one that has been in there
since 1992 (i.e., it is the original one). I had the exhaust system

Combining age and the fact your error codes tended to point toward that
sensor, I'd probably put it in No. 1 position as a potential culprit!

The other stuff I mentioned (plugs, cap, etc.) shouldn't be too
temperature sensitive, as you suggested. The backfiring you mentioned
was what triggered that thought: spent a day once being totally baffled
by a VW engine that would backfire like your Subie. It was rather
intermittent... a better mechanic than I am found a carbon track in the
distributor cap. It was a black cap (as opposed to the stock "orange"
Bosch units) which made the track almost invisible.

Also, you might want to try resetting your ECU by disconnecting your
battery for 10-30 minutes. That will clear any error codes, put the ECU
into its default settings, and MAY help find other problems easier?

Best of luck!

Rick

 

[ed]

Did you check the engine temperature sensor? If this gives a bad signal, the
mixture will be lean or rich, depending on the reading it's giving to the
engine computer. If it is not off its specs, it won't cause an error code,
but could still be giving the computer a cold (or cool) engine temp when it
is actually hot and not needing the rich mix. Then the O2 sensor would read
a rich mix, and when the computer can't correct the mix, it gives the error
code for bad O2 sensor. If you have the manual, you can check the resistance
the temp sensor is supposed to have at various engine temperatures to see if
they are within spec.
Ed B

 

[CompUser]

I've
had it in the shop 4 times to have them read the codes. The first 3 times,
they got no error code. On the 4th time, they got no error code but when I
started my car to leave then the "check engine" light came on and they
hooked up while the car was running. I don't remember what was the code but
it didn't make sense at the time and the mechanic said it was wrong. When
the "check engine" light comes on is *not* when I have the first-startup
overly rich mixture problem. They seem to be independent symptoms but maybe
not. I suppose the "Check Engine" light might not be coming on but the
computer is still screwing up the mixture when the car is started.

I've even left my car at their shop over the weekend to let them
start it cold to see if the over-rich problem crops up, but it never failed
when they had my car.

The info above suggests yrou shop mechanic is
either seriously inept at diagnosing engine
malfunction, or milking you shamelessly.

 

[mulder]

Vehicle info: '92 Subaru Legacy L wagon 4WD non-turbo

Sometimes when I start my car, the exhaust is far too rich and detonates
back in the catalytic converter. When it happens seems sporadic. The
weather could be cold or hot, humid or dry, and the car may have been
sitting for several days or only for 4 to 8 hours since the last time it
ran. I can compensate by shifting manually to keep the RPMs way up. When
the problem arises, I have to keep the RPM up to around 2500 when at a
stoplight (so it doesn't peter out). When I take off, I'll put it in first
gear and shift manually to keep the RPMs up high; otherwise, it will
hesitate on acceleration and I hear the detonation in the exhaust system.
Even after the temperature guage shows normal level, it is only after it has
been at normal for about 15 minutes before the over-rich problem fades away.
It only runs overly rich occasionally.

The other problem is that the "Check Engine" light comes on sometimes. I've
had it in the shop 4 times to have them read the codes. The first 3 times,
they got no error code. On the 4th time, they got no error code but when I
started my car to leave then the "check engine" light came on and they
hooked up while the car was running. I don't remember what was the code but
it didn't make sense at the time and the mechanic said it was wrong. When
the "check engine" light comes on is *not* when I have the first-startup
overly rich mixture problem. They seem to be independent symptoms but maybe
not. I suppose the "Check Engine" light might not be coming on but the
computer is still screwing up the mixture when the car is started.

So my choices (that I can figure out) is to replace the engine computer
and/or the oxygen sensor. I replaced my exhaust system about 4 to 5 years
ago but they reused the original oxygen sensor (they were going to replace
it but couldn't get one at that time). As I recall, this sensor check how
much oxygen is in the exhaust and recirculates a portion of the exhaust if
the oxygen is too low. Well, if it is running overly rich then wouldn't the
oxygen level would be low?

Any opinions if it is more likely the computer or the oxygen sensor screwing
up the mixture (so it is sometimes too rich on startup)? Might it be
something else? Besides the 4 times to take it into the shop to see if they
can nail the cause of the "check engine" light coming on (but never finding
anything), I've even left my car at their shop over the weekend to let them
start it cold to see if the over-rich problem crops up, but it never failed
when they had my car.

If this happens right at startup it isn't the 02 sensor. When the
engine is first started cold it runs in "open loop" mode and the 02
sensor is not used until the engine is at operating temperature, then
it goes into "closed loop" where the 02 is used for regulating the A/F
mixture.
A more likely culprit, and a rather common one, is a bad coolant
temperature sensor. These tend to fail over time as their resistance
value shifts so the ECU will receive faulty temperature information,
this can result in an improper mixture and symptoms like you have
reported.
A bad coolant temp sensor may or may not set a code, depending on
whether it still produces readings that the ECU considers "valid" even
though they are actually out of range. This is an inexpensive part and
you can replace it yourself.

 

[Carl 1 Lucky Texan]

JUst to be clear, is this the sensor with 2 wires that reports engine
temp to the ECU? Not the one with 1 wire for the coolant temp gauge right?

Carl

 

[mulder]

I'm not familiar with the exact setup on your car, but I'd say yes. If
you have two coolant sensors the only one that would cause engine
running problems is the ECU sensor, not the gauge sensor.

 

[Rick Courtright]

JUst to be clear, is this the sensor with 2 wires that reports engine
temp to the ECU? Not the one with 1 wire for the coolant temp gauge right?

Hi, Carl

On my '90 it's a two wire sensor with a green connector. It's mounted
horizontally below the t-stat housing pointing to the right side
(looking forward) of the engine. ISTR one of the books showing a
slightly different location, but can't swear to that.

I don't know if I was lucky or not: when mine failed it just died, threw
a code and triggered the CEL. It MAY have already been failing slowly as
Mulder describes, but I'd just gotten the car and hadn't learned its
little quirks. Failure mode is "full rich" so I noticed an immediate
improvement in fuel economy (2 mpg or so?) when I installed the new one.

Rick

 

[mulder]

Hi, Carl

On my '90 it's a two wire sensor with a green connector. It's mounted
horizontally below the t-stat housing pointing to the right side
(looking forward) of the engine. ISTR one of the books showing a
slightly different location, but can't swear to that.

I don't know if I was lucky or not: when mine failed it just died, threw
a code and triggered the CEL. It MAY have already been failing slowly as
Mulder describes, but I'd just gotten the car and hadn't learned its
little quirks. Failure mode is "full rich" so I noticed an immediate
improvement in fuel economy (2 mpg or so?) when I installed the new one.

Rick

Failure mode can go either way, depending on whether the sensor shorts
or opens, or in less extreme cases whether the resistance shifts up or
down.

 

[Rick Courtright]

Failure mode can go either way, depending on whether the sensor shorts
or opens, or in less extreme cases whether the resistance shifts up or
down.

Hi,

I'm assuming we're defining things the same way: shorted is zero
resistance across the terminals, open is infinity (or at least WAY
high?) As I mentioned, it's been a few years but IIRC mine shorted--I
know it failed "full rich."

Rick

 

[Vanguard]

Did you check the engine temperature sensor? If this gives a bad signal,
the
mixture will be lean or rich, depending on the reading it's giving to the
engine computer. If it is not off its specs, it won't cause an error code,
but could still be giving the computer a cold (or cool) engine temp when
it
is actually hot and not needing the rich mix. Then the O2 sensor would
read
a rich mix, and when the computer can't correct the mix, it gives the
error
code for bad O2 sensor. If you have the manual, you can check the
resistance
the temp sensor is supposed to have at various engine temperatures to see
if
they are within spec.

The manual I have doesn't detail specs, like resistance. I forget the
publisher but it's like one of those Chilton manuals. I tried hitting the
local library because they have the AllDataDIY automative database that is
supposed to be very thorough, but all the kids had the computers tied up at
noon, so I'll have to try again this week (and ask the librarian to bump any
kid that didn't actually sign-in to use a computer). Hopefully it will show
the location for the temperature sensor and what resistances I should see at
various temperatures.

 

[Vanguard]

Actually neither my favorite car shop or the dealer charged me when they
couldn't see any error codes. The only cost incurred so far is the wasted
time in having them try to get codes out of the ECU. I guess depends on
your location and who you deal with as to whether or not they want to nickel
and dime you to death. My recollection is that I had the car shop test the
ECU three times but each time the "check engine" light did not come on for
them and they got no error codes. I then decided to try the dealer figuring
that maybe the car shop wasn't doing it correctly, but the dealer didn't get
any codes, either. Between each attempt, I had cleared the ECU and then
waited until the "check engine" light came on again. Each time, I was
prepared to pay something for their trouble but they never charged me.

Because the ECU is turning on the "check engine" light but reports no codes
is why it also might be screwing up the mixture. It already appears
something is defective with the ECU. That doesn't preclude another
component also involved in the problem but the ECU is already acting weird.
If it is screwing up one thing, it could also be screwing up something else.

 

[Vanguard]

On Thu, 11 Aug 2005 12:03:48 -0500, "Vanguard"


If this happens right at startup it isn't the 02 sensor. When the
engine is first started cold it runs in "open loop" mode and the 02
sensor is not used until the engine is at operating temperature, then
it goes into "closed loop" where the 02 is used for regulating the A/F
mixture.

Yep, it only occurs when the engine is cold, like when I start it up in the
morning to go to work or it has been sitting over the weekend unused. It
has, but less often, also occurred after driving the car to work but then
sat in the parking lot for 8 hours. At one time, I thought it was more
prevalent when it was cool and damp but that must've been when I first
started noticing the problem.

When I first start the car, it is way too rich and want to die at idle, so I
have to rev it up to around 2500 RPM. Although it is an automatic, I have
to start out in first gear and manually shift up as I takeoff from a stop in
order to keep the revs high. It takes around 15 minutes before the problem
goes away. The temperature guage has reached the normal mid-point position
but it won't be until about 10 minutes after that until the problem goes
away (i.e., temperature must be normal hot but be normal hot for awhile).

A more likely culprit, and a rather common one, is a bad coolant
temperature sensor. These tend to fail over time as their resistance
value shifts so the ECU will receive faulty temperature information,
this can result in an improper mixture and symptoms like you have
reported.
A bad coolant temp sensor may or may not set a code, depending on
whether it still produces readings that the ECU considers "valid" even
though they are actually out of range. This is an inexpensive part and
you can replace it yourself.

I didn't get to recheck the ECU for codes this weekend. Got too busy. This
week I'll be checking the AllDataDIY automative database at the library to
see if it shows where is the temperature sensors (so I can differentiate
between the guage sensor and the engine sensor) and what resistances it
should have.

It doesn't help to take the car to the dealer for an appointment because the
engine is hot and has been hot long enough for the over-rich problem to
disappear. I've left my car on their lot over the weekend and told them
that they need to check it right on startup for the problem but it did not
occur when they tested (it is sporadic as to when it will show up).

I would think that if a sensor were out-of-range, high or low but within
range, or open, short, or broke, that the problem would be consistent (i.e.,
it would be over-rich everytime I started the engine when it was cold).

 

[Carl 1 Lucky Texan]

Ya know - this does seem a little backwards doesn't it? I mean, if the
sensor thought the engine were cold all the time - wouldn't it start
easily but get poor mileage as if the 'choke' were still on? And if the
sensor indicated to the ECU that the engine were aleays warm - you'd get
'lean' on start up(maybe a difficult start in cold weather) THEN near
normal A/F ration for warmed-up driving.

I dunno, maybe one of the O2 sensors instead. Does it actually seem over
rich when you start - smell gasoline, any black smoke?

Carl

 

[Vanguard]

Ya know - this does seem a little backwards doesn't it? I mean, if the
sensor thought the engine were cold all the time - wouldn't it start
easily but get poor mileage as if the 'choke' were still on? And if
the sensor indicated to the ECU that the engine were aleays warm -
you'd get 'lean' on start up(maybe a difficult start in cold weather)
THEN near normal A/F ration for warmed-up driving.

I dunno, maybe one of the O2 sensors instead. Does it actually seem
over rich when you start - smell gasoline, any black smoke?

There is no black smoke. There is a light blue tinge on cold start
(when the over-rich problem occurs) and you can smell gasoline. I have
started when cold when the problem appeared, drove 3 miles to a
restaurant, and restarted it about 40 minutes later and the problem was
still there. Only after the car has gotten hot (normal range) AND been
hot for 10 minutes, or more, does the over-rich problem go away (when it
happens). It's so sporadic that I haven't found any consistent
condition under which I can reliably recreate the problem. It's like
shooting at a thin flock of birds flying around in haphazard paths in
the air by using a bow and arrow. So far, I haven't manage to get a hit
when anyone is trying to read the codes. Argh! Although I'll be
testing for the codes again this week, I surely won't be surprised if it
says there aren't any error codes. However, the check engine light does
come on now more often then it did before, so the flock that I'm
shooting that arrows at is getting denser.

That's why I suspect the only way to cure the problem is to start
replacing parts in piecemeal fashion. However, I won't know if the
problem got solved until after sometime later after seeing the check
engine light doesn't come on anymore and the over-rich problem goes
away. I'm just not sure which order I should follow in replacing parts,
from most likely to least likely (with the most expensive part last but
not necessarily in price order before that). PCV, oxygen sensor, engine
temperature sensor, knock sensor, purge valve (EGR), and ECU (engine
computer) seems about the right order. However, the ECU comes in LHD
and RHD versions, and an in-state salvage yard lists a RHD unit for only
$30 (plus shipping). There might be mounting problems (mine is a LHD
unit) but a perforated strapping iron strip could be used to affix the
box to the body. They want $100 for the LHD unit, like mine.

I read the AllDataDIY automotive database regarding the O2 sensor.
After all the hype that I heard for AllDataDIY, I was disappointed that
it didn't describe remove/install procedures for many parts and the
pictures were difficult to read drawings instead of actual pictures. Of
course, they refer to a Subaru tool part number for removing the O2
sensor. Does it really take a special tool, or will a deep-well socket
or box wrench work? They really didn't describe the location very well
and the picture is poor (very few references to the objects in the
picture). It looks to be atop either the exhaust pipe before the front
catalytic converter or maybe even atop the front catalytic converter
itself. Hopefully I don't need a special tool since the cost of which
would obviate any savings from me buying the part and replacing it
myself compared to having the shop do it.

 

[Rick Courtright]

course, they refer to a Subaru tool part number for removing the O2
sensor. Does it really take a special tool, or will a deep-well socket
or box wrench work? They really didn't describe the location very well

Hi,

Problem w/ using regular wrenches/sockets on O2 sensors is the sensor's
often buried in sheet metal so you can't get to the flats easily. And I
think you may find it easier to disconnect the Y-pipe at the heads and
downstream so you can drop it a bit to get at the location with your
wrench. Many auto parts sell a socket to do the job--it's a deep set
socket with a slot up the side so you can move the wire out of the way
while installing/removing. If your parts shop handles K-D tools, they
make one. Shouldn't be too expensive... seems it was in the $10-$14
range when I looked after fighting and finally getting the sensor in w/
regular wrenches.

Rick